Australian School of Petroleum The University of Adelaide ... · Petroleum Production and Exploration Association Journal, 42, 151 – 166 Paper 2: Hunt, S. P., Camac, B. A. and Boult,

AN INTEGRATED GEOMECHANICAL EVALUATION OF CAP AND FAULT-

SEAL FOR RISKING PETROLEUM TRAP INTEGRITY USING DISTINCT

ELEMENT AND BOUNDARY ELEMENT NUMERICAL METHODS

Bronwyn Anne Camac, B. App. Sc. Geology

Australian School of Petroleum

The University of Adelaide

This thesis is submitted in fulfilment of the

requirements for the degree of Doctor of Philosophy in the

Faculty of Engineering, Computer and Mathematical Sciences

November 2009

2 ��

ABSTRACT

This thesis comprises nine published papers on an integrated geomechanical evaluation of

cap and fault-seal for risking petroleum trap integrity using distinct element and boundary

element numerical methods. Paper 1 provides back-ground information and an introduction

to the body of research presented in this thesis. In some parts of the Penola Trough, South

Australia, the seal lithotype is fractured providing structural permeability and thereby

compromising seal competency. This work inferred that existing geomechanical techniques,

which only considered stresses on the fault plane, had limited application in the prediction

of fracture generation within the country rock away from the well-bore. It also suggested

that computational stress modelling techniques may provide a useful tool in this area and

similar tectonic provinces.

An important stage of the modelling workflow is analysing the sensitivity of the numerical

models to various input parameters. Papers 2 and 3 show that the models are particularly

sensitive to fault parameters such as friction angle ( ) and cohesion (C). However, fault rock

properties are not well understood in petroleum exploration due to depths of investigation

and the expense of acquiring core samples.

This thesis develops a new technique, using widely available dipmeter data for the entire

borehole. In this, rotations in borehole breakouts caused by discontinuities, in the vertical

sense, are used to give qualitative indications of fault rock behaviour (Paper 3). These

observations were used to make decisions about fault rock input parameters into the

numerical stress models. Paper 8 showed the results of varying fault rock stiffness moduli

3 ��

and fault zone width on the predicted stress within the surrounding rock mass. Where the

prevailing stress conditions border between stress regimes, a new and unconventional

technique whereby is used to increase confidence in understanding the stress regimes

active at a particular depth and/or location (Paper 7). A comparative study of a single fault

using three different methods of stress modelling, the distinct / discrete element (DEM),

boundary element (BEM) and finite difference (FDM) methods (Paper 7) showed that the

DEM underestimated the stress perturbation relative to the other models. Therefore where

a clear variation is shown by DEM, there is increased confidence that it does exist and will

be enhanced using other codes. Where there is the requirement to model multiple faults,

DEM is preferred as the other methods trialled either have restrictions to the number of

faults incorporated into the models (FDM) or does not account for full fault interaction and

possible moment rotations between fault blocks, such as in the case of BEM.

The application of computational stress modelling offers a new workflow to fully integrate

stress studies, cap-seal analysis, fault-seal analysis and structural interpretation to

improve the understanding of hydrocarbon leakage risk at the prospect and play scales and

was illustrated by way of multiple case study examples (Papers 4, 5, 6, 7 & 8).

The research presented in this thesis has development new concepts and additional

workflows which add to the ‘tool-box’ that may be used by those researchers and

consultants working in the field of petroleum geomechanics (Paper 9).

4 ��

TABLE OF CONTENTS �ABSTRACT ................................................................................................................... 2 TABLE OF CONTENTS ............................................................................................. 4 1. DECLARATION ................................................................................................... 6 2. ACKNOWLEDGEMENTS ................................................................................. 10 3. CONTEXTUAL STATEMENT .......................................................................... 12 4. INTRODUCTION ............................................................................................... 15 5. LITERATURE REVIEW .................................................................................... 21

5.1 Computational Stress Modelling�� 5.1.1 The Modelling Technique�� 5.1.2 An overview of the various methods of numerical methods used in rock mechanics�� 5.1.3 Distinct or Discrete Element Method�� 5.1.4 Constitutive laws and failure criterion of faults��

5.2 Conventional fault seal and cap seal analysis�� 5.2.1 Fault-seal��

5.3 Intact rock-mass properties�� 5.4 Fault-rock properties�� 5.5 In situ stress field determination�� 5.6 Borehole Breakout Analysis��

5.6.1 SHmax Rotations�� 6. CRITICAL ASSESSMENT OF THE MODELLING APPROACH .............. 36 �� ! �� "��#��$�� %��#��&��#��'��(��)�� *&��+��

7. CONCLUSIONS .................................................................................................. 44 8. REFERENCES .................................................................................................... 47 9. Conferences Papers/Posters Presented ....................................................... 54 10. Awards .............................................................................................................. 54 STATEMENT OF AUTHORSHIP ........................................................................... 55

PAPER 1�� Summary of Authorship��

Signatures of Co-authors: ....................................................................................... 56 PAPER 2��

Summary of Authorship�� Signatures of Co-authors: ....................................................................................... 58



5 ��

Summary of Authorship�� Signatures of Co-authors: ....................................................................................... 62



Summary of Authorship�� Signatures of Co-authors: ....................................................................................... 66 STATEMENT OF AUTHORSHIP ........................................................................... 67


Signatures of Co-authors: ....................................................................................... 68 STATEMENT OF AUTHORSHIP ........................................................................... 69


Signatures of Co-authors: ....................................................................................... 70 STATEMENT OF AUTHORSHIP ........................................................................... 71


Signatures of Co-authors ........................................................................................ 72 CHAPTER 1................................................................................................................ 73 CHAPTER 2................................................................................................................ 74 CHAPTER 3................................................................................................................ 75 CHAPTER 4................................................................................................................ 76 CHAPTER 5................................................................................................................ 77 CHAPTER 6................................................................................................................ 78 CHAPTER 7................................................................................................................ 79 CHAPTER 8................................................................................................................ 80 CHAPTER 9................................................................................................................ 81

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1. DECLARATION

This work contains no material which has been accepted for the award of any other degree

or diploma in any other university or other tertiary institution and, to the best of my

knowledge and belief, contains no material previously published or written by another

person, except where due reference has been made in the text.

I give consent to this copy of my thesis when deposited in the University Library, being

made available for loan and photocopying, subject to the provisions of the Copyright Act

1968.

The author acknowledges that copyright of published works contained within this thesis (as

listed below) resides with the copyright holder(s) of these works.

-------------------------------------------------------

Bronwyn Anne Camac

7 ��

Papers offered for examination for the award of Doctor of Philosophy (Engineering Science)

are as follows:

Paper 1: Boult, P.J., Camac, B.A. and Davids, A.W. 2002. 3D Fault Modelling and Assessment of

Top Seal Structural Permeability – Penola Trough, Onshore Otway Basin. Australian

Petroleum Production and Exploration Association Journal, 42, 151 – 166

Paper 2: Hunt, S. P., Camac, B. A. and Boult, P.J. 2003. A parametric analysis and applications of the Discrete Element Method for Stress modelling: eNZ Conference Proceedings of the

Rock Mechanics Association of Australia and New Zealand. Paper 3:

Camac, B. A., Hunt, S. P. and Boult, P.J. 2004. Fault and top seal integrity at relays and intersections using 3D Distinct Element Code. Australian Petroleum Production

and Exploration Association Journal, 44, 481 - 496

Paper 4: Camac, B. A., Hunt, S. P. and Bailey, W. R. 2005. Distinct element stress modelling for

top seal appraisal in the Pyrenees-Macedon oil and gas fields, Exmouth Sub-basin, Australian North West shelf. Alaska Rocks 2005, The 40th U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure

Development in the Northern Regions, Anchorage, Alaska, June 25-29, 2005. Paper 5: Camac, B. A. and Hunt, S. P. 2004. Applications of Stress Field Modelling Using the

Distinct Element Method for Petroleum Production: APOGCE Conference Proceedings, SPE Paper # 88473

8 ��

Paper 6:

Camac, B. A., Hunt, S. P and Boult, P. J. 2006. Local rotations in borehole breakouts – Observed and modelled stress field rotations and their implications for the petroleum industry. International Journal of Geomechanics, 6 (6) 399-410

Paper 7: Camac, B. A., Hunt, S. P., Boult, P. J. and Dillon, M. 2006. Unconventional borehole

breakout rotation analysis provides a QC for stress models. Australian Petroleum

Production and Exploration Association Journal, 46, 307-327

Paper 8: Camac, B. A., Hunt, S. P. and Boult, P. J. 2009, Predicting brittle cap-seal failure of petroleum traps: An application of 2-D and 3-D distinct element method. Petroleum

Geoscience, 15, 75-89

Paper 9: Hunt, S.P., Camac, B. A. and Boult, P.J. 2006. A new geomechanical tool for the

evaluation of hydrocarbon trap integrity. Golden Rocks 2006, The 41st U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development, Conference Proceedings

9 ��

Other publications and/or presentations submitted during the course of the

PhD but not offered up for examination:

Camac, B. A., Hunt, S. P., Gilbert, C. E. and Anthony, D. P. 2005. Using 3D Distinct

Element Method to predict stress distribution – Taranaki Basin, New Zealand. This extended abstract was prepared for presentation at the EAGE 67th Conference & Exhibition — Madrid, Spain, 13 - 16 June 2005. (Not Reviewed)

Boult, P. J., Lyon, P., Camac, B. A., Edwards, D. and McKirdy, D. A. 2004. Subsurface plumbing of the Penola Trough: Otway Basin. Eastern Australasian Basins Symposium

(EABS) II Conference Proceedings. Fully Reviewed Australian Conference Paper.

Hunt, S. P., Boult, P. J. and Camac, B. A. 2004. Discrete Element Stress Modelling in the Timor Sea and Otway Basin, Australia: EAGE Fault and Top Seals: What do we

know and where do we go? Proceedings of the EAGE Conference, Montpellier, France. Paper-22 (Not Reviewed)

Dewhurst, D.N., Mildren, S. D., Camac, B. A., Boult, P. J. and Hennig, A. 2003. Top Seal Fracturing in the Carnarvon and Otway Basins, Australia. AAPG International Conference & Exhibition September 21-24, 2003 • Barcelona, Spain – Poster Presentation (Not Reviewed)

10 ��

2. ACKNOWLEDGEMENTS

I would like to offer the greatest of appreciation to Drs. Suzanne Hunt and Peter Boult for

their creative ideas, supervision, technical expertise and support during the course of my

research.

My gratitude goes to the Commonwealth Scientific Industry Research Organisation

(CSIRO) Petroleum Division and the University of Adelaide for providing financial support

during the research phase of this study. The University of Adelaide provided a

commonwealth funded scholarship under the RTS scheme (2003-2005) and a Research

Abroad Scholarship (2003) which assisted me to attend two major international

conferences; whilst the CSIRO provided a PhD Top-up Scholarship (2004-2005) which

enabled me to travel and present my research at three major international conferences.

I would like to acknowledge the work of post-graduate students Dmitri Koupriantchik and

Michael Dillon who added to the increasing understanding of numerical stress modelling

and Paul Lyon who during the course of his study increased the structural understanding of

my major study area, the Penola Trough, onshore Otway Basin, South Australia.

I would also like to thank Mike Coulthard from ITASCA for his general assistance in

applying the ITASCA codes.

My gratitude also extends to the following companies for providing data and local expertise

in the specific case study areas presented in this thesis.

11 ��

Origin Energy Resources Ltd, particularly Dr. Richard Suttill for the Penola Trough,

Otway Basin case study and Greg Pass, Chris Gilbert, Kym Mills and David

Anthony, for the Kupe Field Study, New Zealand.

Santos Ltd, particularly Tom Herries, Richard Osbon and Tony Lake, for the Big

Lake Field Study (Cooper Basin, South Australia).

Beach Petroleum for their support during the writing phase of the thesis.

CSIRO, particularly Dr. David Dewhurst, for his invaluable assistance with increasing my understanding of the geomechanical properties of fault rock and his critical review and invaluable help with my last paper. Also Wayne Baily, now with Woodside Petroleum, for his help with the Pyrenees-Macedon fields study in the

NW-Shelf, Australia.

PIRSA (Primary Industry Resources South Australia) for providing funding to purchase 3DEC (Itasca), the 3D discrete element code used in the majority of the studies presented in this body of work.

Faultseal Pty Ltd, specifically Titus Murray for his generosity in providing a probabilistic fault seal analysis on the Balnaves fault. Titus’ ideas and feedback are

truly appreciated. Many thanks must go to the many researchers in fault seal studies, particularly those that

have worked extensively in the Penola Trough and all those who have reviewed my papers and offered sincere and positive feedback.

An finally to my family, Adrian, Ashley, Nicholas and Madeleine for your love and support and tolerating my late nights and sometimes lengthy absences – Thank you.

12 ��

3. CONTEXTUAL STATEMENT Previous research has shown the importance of understanding the relationship between

fault geometry and current applied tectonic stresses in the prediction of critically stressed

faults and their propensity for fluid flow via generated fracture networks along and/or

around the fault plane. This thesis consisting of nine published papers attempts to increase

this understanding, by applying the Distinct Element Method (DEM) to several case

studies. For example, data collected from the Penola Trough, onshore Otway Basin, South

Australia, shows that a more complex 3D failure mechanism may be active, whereby the

cap-seal may fracture preferentially to fault failure (Paper 1).

In Paper 2, computational modelling techniques, using distinct / discrete element method

(DEM) were applied to the problem of integrating cap and fault seal studies to determine a

more comprehensive risking strategy for petroleum exploration. Paper 2 presents the

results of a sensitivity study showing the effect of a single displacing fault on the applied

regional stress. Input parameters such as the angle between the fault strike and the

maximum horizontal stress direction (θ); the ratio of magnitudes of maximum and

minimum stresses (k= 1 / 3); and friction angle were varied in this modelling

experiment. This sensitivity study showed the models are highly sensitive to fault

parameters, in particular the fault friction angle ( ).

Paper 3 addressed the importance of fault parameters by using dipmeter data to obtain

borehole breakout information. Innovation involved plotting borehole break-out data with

depth which was then used to identify possible fault intersections with the well-bore and

observe rotations of regional stress trajectories. A rotation of the maximum principal

13 ��

horizontal stress perpendicular into the fault plane is indicative of a fault rock with a

higher stiffness moduli than the surrounding rock mass. In the absence of rock strength

data from laboratory tests, this technique offers a new procedure for making decisions

about fault parameter inputs into numerical models.

The knowledge gained from the sensitivity studies and application of the new borehole

break-out technique was applied in Papers 4, 5 and 6, to various case study examples

within Australia and New Zealand. These case studies included the Penola Trough,

(onshore Otway Basin); Pyrenees-Macedon fields, Exmouth Sub-basin (Northwest Shelf);

Kupe Field, Taranaki Basin (New Zealand); Big Lake Field, Cooper Basin (South

Australia); and the Jabiru-Tancred area, Timor Sea. In each of these studies DEM code was

used in either two-dimensions (2-D UDEC) or three-dimensions (3-D 3DEC) and shows

clear correlations between higher-than-regional predicted differential (2-D) or deviatoric

stress (3-D) and brittle failure of the rock mass observed in petrophysical logs, core or from

drilling information. Predicted mean and minimum stress from DEM models was also used

to predict regions of preferred hydrocarbon pooling (Papers 6 and 7).

Paper 7 also offered a comparison of distinct / discrete element (DEM — 3DEC), finite

difference (FDM — FLAC3D) and boundary element (BEM — MAP3D) methods for a single

fault. In this comparative study, it was found that each of the methods gave similar results

for a single fault. As a general trend, however, the BEM resulted in a larger perturbation

effect, DEM indicated the least perturbation of differential stress, and the FDM yields a

response that is of an intermediate order when compared to BEM and DEM. The

differences were mostly attributable to variations in the way the fault geometry was treated

in the three methods, that is, the specific discretisation scheme in each code; and the output

14 ��

gridding and display data created by the software. We know that the DEM method can

account for multiple faults whereas there are restrictions with the FDM, and the BEM

method does not account for full fault interaction and possible moment rotations between

fault blocks, so where multiple fault interaction is required, the DEM is preferred.

Paper 8 explained an emerging application for DEM modelling, at the play and prospect

level respectively, from the Penola Trough. Sensitivity studies at the prospect scale showed

how (1) fault rock strength; (2) fault zone width; and (3) the interaction of two fault sets;

generates local perturbations in the regional stress field. At the play-scale, the depth to

which a younger active fault set propagates was explained by the distribution of stress

within the rock mass generated by the present day far field stress acting on older regionally

significant faults. This paper offered a workflow and highlighted the importance of

understanding the effect of not only critically stressed and displacing faults but also that of

fault rock that has higher cohesion, stiffness or frictional strength than the surrounding

rock mass.

Finally, Paper 9 presented the results of the above research by way of a summary paper.

This paper also gave an overview of three case studies presented in previous papers, the

Penola Trough, Otway Basin, South Australia; Kupe Field, Taranaki Basin, New Zealand;

and Pyrenees-Macedon fields, Northwest Shelf, Australia, highlighting the applicability

and value of the developed techniques for the petroleum industry.

15 ��

4. INTRODUCTION

It has been shown previously by Barton & Zoback (1994), Wiprut & Zoback (2000), Castillo

& Moos (2000), Castillo et al. (2000), Wiprut & Zoback (2002), Reynolds et al. (2003) and

Dee et al. (2007a) that careful construction of a geomechanical model may reduce the risk of

encountering breached reservoirs in fault–dependent structures in petroleum exploration.

Their models resolve stresses on the fault plane, but do not include locally perturbed

stresses generated in the surrounding intact rock mass. Many studies have been published

on the effect of fault movement on the generation of secondary faults or fractures. King et

al. (1994) describe a method using the Coulomb failure criterion to explain the factors

critical in triggering earthquakes or aftershocks resulting from movement along a proximal

fault or faults. Kattenhorn et al. (2000) showed that movement along faults can perturb

the regional stress field in a manner that controls the orientations of induced secondary

structures. Their work was confirmed by an example of normal faulting from a North Sea

hydrocarbon reservoir where the variability in secondary fault orientations was attributed

to stress perturbation that developed around the larger faults during a single phase of

extension (Maerten et al. 2002). Dee et al. (2007b) used elastic dislocation theory to predict

the distribution, orientation and mode of fractures. More recently, Mutlu & Pollard (2008)

used the 2D Displacement Discontinuity Method (DDM) to model the formation and

patterns of wing cracks resulting from stress concentration generated by sliding along a

fault. Tamagawa & Pollard (2008) used a combination of elastic dislocation theory and

stress compliancy to evaluate flow properties in a fractured basement reservoir. The

majority of studies in the prediction of fractures, relevant to the petroleum industry, focus

on fracture characterisation within reservoirs. Boult et al (2008) have used a minimum

strain approach to QC fault interpretation from 2D seismic data.

16 ��

This research differs from those preceding it in two ways; (1) it is directed towards

providing a workflow and technique to understand seal breach risk in the cap–rock of

prospects and leads due to the formation of fracture networks providing structural

permeability; and (2) faults, not currently seismically active under the current stress

conditions, may generate local perturbations in the regional stress field due to cohesive and

frictional strength contrasts of fault rock and the surrounding rock mass.

Rock stress can only be measured at local points in space, however the state of stress in

rock masses is affected by heterogeneities within, most significantly faults and rock density

contrasts at horizon boundaries. Field or outcrop observations have shown that faults and

horizon boundaries can greatly affect the magnitude and orientation of the rock stress

components. Thus modelling large scale structures validated against small scale

observations in wells can be used to risk trap integrity. Various numerical techniques exist

to model heterogeneities in a rock mass. These methods have most commonly included the

finite element method (FEM), coupled FEM and boundary element method (BEM) and the

finite difference method (FDM). The characteristics of these methods make them difficult to

apply when there are numerous fractures. The distinct or discrete element method (DEM)

was developed to be specifically applicable for solving rock mass fracture related problems

(Cundall, 1971).

The DEM method was developed into a 2D and 3D code UDEC and 3DEC respectively,

which have recently been used for a number of geomechanical purposes. More specifically,

they have been used for stress field perturbation caused by fractures and horizon

boundaries. These studies, summarised briefly here, were undertaken with UDEC or FEM

17 ��

software and can be grouped into five main disciplines (1) tectonics; (2) structural geology;

(3) geotechnical and mining engineering; (4) mineral exploration; and (5) petroleum

geomechanics.

FEM models and more recently UDEC have been used to successfully match present day

maximum principal stress (σ1) orientation patterns in the Cenozoic of the mid-Norwegian

margin and northern North Sea with stress orientations from borehole breakouts, focal

mechanisms and in-situ measurements (Pascal & Gabrielsen 2001). Homberg et al. (1997)

used UDEC to model the Morez fault zone in the Jura mountains and observed that the

palaeostress trajectories obtained for the Mio-Pliocene compression are influenced by the

perturbation effect of this fault zone.

Prior to the studies undertaken and presented as part of this PhD, very little work had

been done to date on the numerical modelling of stress specifically within hydrocarbon cap–

seals and especially relating the predicted stress results to trap integrity.

Previous work on seal integrity for the Penola Trough had focussed on:

fault–seal potential encompassing juxtaposition, the build up of clay smear or shale

gouge on the fault plane and reactivation risk (Jones et al. 2000, Dewhurst et al.

2002, Lyon et al. 2005); and

cap–seal integrity involving mercury injection capillary pressure (MICP) data to

determine the ability of a seal to hold back a hydrocarbon column (Kaldi et al. 1999)

and the determination of a brittleness index of the cap seal (Kivior et al. 2001).

18 ��

In the Penola Trough, the possibility of fault reactivation was considered as a major risk to

trap integrity. In 2001, two fault-dependent structures were drilled, Balnaves–1 and

Limestone Ridge–1. The bounding faults of both prospects were analysed using existing

conventional geomechanical modelling techniques. These studies indicated that neither

fault was critically stressed under current stress conditions and subsequently were

considered to have a low pre-drill risk of reactivation. However, both wells intersected

palaeo-columns (Paper 1). Open, hydraulically conductive fractures were interpreted

throughout the brittle cap–seal in Balnaves–1 from the Formation Micro Image Log (FMI)

and a vertical rotation of the stress field was observed from borehole breakouts (Paper 1).

Subsequent studies attempted to integrate the geomechanics of fault reactivation and cap

seal brittleness (Dewhurst et al. 2003, Mildren et al. 2002) and provide a methodology for

risking prospects. In late 2002, Suzanne Hunt presented the results of a pilot 2D DEM

study where areas of predicted high shear stress correlated well with palaeo-columns in

the Penola Trough. This work was later published (Hunt & Boult 2005). The ideas resulting

from this pilot study led to the research presented in this thesis. Other ideas pursued

during the course of this research, many tested using DEM code, include:

(1) The selection of modelling parameters is critical to the success of model outcomes.

Much of this research has been directed towards developing tools to aid in the

selection of fault parameters, such as friction angle, strength / stiffness and

orientation as input parameters to the models.

(2) The observation of rotations in borehole breakouts and their use in calibrating 2D

and 3D DEM stress models has been developed during the course of this study.

19 ��

(3) The application of DEM models to determine between strike-slip and reverse stress

regimes

The developed techniques have been applied to other areas within Australia and New

Zealand, such as the Big Lake Field in the Cooper Basin SA; Pyrenees – Macedon fields in

the Northwest Shelf and the Kupe South Field in New Zealand. These modelling techniques

have direct application to risk assessment in other highly faulted regions throughout

Australia and internationally.

The main objectives addressed by this work were as follows:

To develop a technique to predict local present day stress perturbations away from

the well-bore using commercially available software or code.

By way of case studies from varying tectonic provinces, illustrate the application of

the technique and show meaningful correlations between the modelled and field

data.

Integrate past studies on stress fields, fault seal analysis and structural

interpretation to indicate hydrocarbon leakage with the geomechanical models.

A comparison of the distinct (discrete) element, boundary element and finite

difference methods.

Generate new understanding in the use of borehole breakout information in the

vertical sense as well as horizontal whereby that information is used actively to

quality control the numerical models and provide confidence in parameters when

conventional methods are not sufficient.

20 ��

Provide a new understanding on the effect of fault rock which has higher cohesive

strength, stiffness or frictional strength than the surrounding rock-mass, on the

applied stress field.

Provide a workflow for improved risking of cap rock and fault seal in undrilled

prospects.

Increase exploration confidence in the Penola Trough, South Australian Otway

Basin and other similar tectonic provinces.

This introduction provides the overall context for the body of work comprising this thesis. A

review of the relevant literature is given, highlighting the specific knowledge gaps that

existed prior to this research. A summary of the key findings of each the papers and the

linkages between each are stated. The significance of the findings of all the papers, in light

of the current knowledge gaps on stress modelling both within the Penola Trough and in

the wider global context, are provided in the conclusions section.

21 ��

5. LITERATURE REVIEW

A literature review of published methods and techniques and parameters relevant to this

thesis is presented below. This section is structured as follows:

Computational stress modelling methodology

Conventional cap—seal / fault—seal analysis

Intact rock mass properties

Fault rock properties

In situ stress boundary conditions

Borehole breakout analysis

5.1 Computational Stress Modelling

5.1.1 The Modelling Technique

The importance of developing a sound modelling technique cannot be understated and a

disciplined systematic methodology must be maintained. When attempting to solve

geological problems significant to petroleum exploration and development, it is rare to have

all information available, due to the depths of investigation and the associated costs to

acquire such data. Many assumptions must be made in the absence of measured field data.

A common problem when attempting to model rock systems is to build complex models,

incorporating as many different factors as possible. Added complexity often leads to

confusing results. A suggested approach is to build simple models and sometime many

different simple models to incorporate the variables to determine the effect of that variable

on the model, particularly where data availability is an issue. Starfield & Cundall (1988)

22 ��

outlined a number of guidelines that can be implemented to assist with the best outcomes

from the modelling process.

Be clear about what is to be achieved by the model

Use the model at the earliest stage of the study. Don’t wait for all data to be

acquired. A good conceptual model can aid is the design of field tests and save

money.

Ascertain the likely modes of deformation

Use models to rule out inconsistent data and help to consolidate ideas or hypotheses

Start with the simplest model and progress to more complexity as the results

become a meaningful fit to expectations and available data

Be critical

Be prepared to run numerous models to gain the greatest understanding of the

geological problem.

They conclude by saying ‘plan the modelling exercise in the same way as you would plan a

laboratory experiment’.

5.1.2 An overview of the various methods of numerical methods used in rock

mechanics

Jing & Hudson (2002) and Jing (2003) give an excellent overview of the various methods

currently available to model rock mechanics. These methods have all been developed into

commercially available code or software. They include:

The finite element method (FEM), which is probably the most widely used method in

science and engineering. It is a continuum model and as such is not able to

23 ��

incorporate large scale detachments and sliding blocks in the models. Much work

has been undertaken to develop the ability to incorporate joint/fracture elements

into the code but its treatment of fractures remains its limiting factor.

The finite difference method (FDM) / finite volume method (FVM) which uses the

discretisation of the governing partial differential equations by replacing the partial

derivatives with difference defined at neighbouring grid points. This method is

inflexible when dealing with fractures, complex boundary conditions and material

heterogeneity (Jing & Hudson 2002). FVM has improved FEM by incorporating the

ability to discretise with irregular meshes to fill the space. An example of

commercially available software is Flac2D and 3D, developed by Itasca.

Boundary element method (BEM). The main advantage of this method is its ability

to use a simpler mesh generation technique and hence data input as compared with

FEM and FDM. Its main disadvantage is it’s not as efficient at FEM in simulating

non-linear behaviour (Jing & Hudson 2002). Examples of BEM code is Map3D and

Poly3D.

Distinct or Discrete element method (DEM) was developed and has specific

applications in rock mechanics, soil mechanics, structural analysis, granular

material, material processing, fluid mechanics, multi-body systems, robot

simulation, computer animation and is one of the most rapidly developing areas of

computational modelling methods (Jing & Hudson 2002). Its advantage over other

methods is its ability to incorporate and provide a good representation of fractures

and discontinuities into the models. The best known commercially available codes

include UDEC (2D) and 3DEC (3D) and PFC (particle flow code) for use with

24 ��

granular materials. An more comprehensive explanation of this method can be found

below.

Lattice models assumes a linear elastic material constitutive relation and has been

applied in simulating fracture initiation and propagation in rocks (Mora & Place,

1993, van Mier, 1995)

Discrete Fracture method (DFN) is a specially developed discrete method that

incorporates fluid flow in fractured rock masses via a system of connected fractures.

Examples of commercially available code are FRACMAN/MAFIC and NAPSAC.

Hybrid models include BEM/FEM; DEM/FEM and DEM/BEM models.

Neural networks provide descriptive and predictive capabilities.

More recently, Bäckström et al. (2008) checked the validity of four different numerical

methods, Elasto-Plastic cellular automation (EPCA); Particle Flow Code (PFC);

Displacement Discontinuity Method (DDM) and Finite Element Method (FEM) by

modelling granite to check if they adequately represented the rock reality in simple

experiments.

5.1.3 Distinct or Discrete Element Method

Distinct Element Method (DEM) was developed and later incorporated into two-

dimensional (2D - UDEC) and three-dimensional (3D - 3DEC) numerical modelling

programs or codes, which are used to simulate progressive movements in blocky rock

systems. DEM was primarily intended for analysis in rock engineering projects, ranging

from studies of the progressive failure of rock slopes to evaluations of the influence of rock

joints, faults, bedding planes and any other discontinuity. DEM simulates the response of

discontinuous media subjected to either static or dynamic loading. In the DEM method, a

solid is represented as an assembly of discrete blocks. Joints or faults are modelled as

25 ��

interfaces between distinct bodies. The contact forces and displacements at the interfaces of

a stressed assembly of blocks are found through a series of calculations, which trace the

movements of the blocks. At all contacts, either rigid or deformable blocks are connected by

spring like joints with normal and shear stiffness kn and ks respectively (Fig. 1). Similar to

finite element method (FEM), the unknowns in DEM are also the nodal displacements and

rotations of the blocks. However, unlike FEM, DEM is a dynamic process and the

unknowns are solved by the equations of motion. The speed of propagation depends on the

physical properties of the discrete system. The solution scheme used by DEM is the explicit

time marching and finite contact stiffness. Block displacements are calculated from out of

balance moment and forces applied to the centre of gravity of each block. Resultant forces F

include boundary forces applied to the edges of the block and gravity.

Figure 1: Continuum and discontinuum elements in Discrete Element Method (DEM) (Cundall 1971).

a1172507

Text Box

NOTE: This figure is included on page 25 of the print copy of the thesis held in the University of Adelaide Library.

26 ��

Newton’s second law of motion is applied for each block:

(1)

where u is velocity, m is mass, and t is time.

Following the central difference integration scheme, Equation 1 can be transformed into:

(2)

For blocks in two dimensions where we assume several forces acting on the block (include

gravity load), the velocity (Eq. 2) can be re-arranged to include angular velocity of block:

(3)

where = angular velocity of block about centroid; � = moment of inertia of block; =

velocity components of block centroid; ��, = total moment acting on the block; �� =

total force acting on the block; and � = components of gravitational acceleration.

Assuming velocities are stored at the half-time step point, the new velocities in Equation 3

can be used to determine the new block location:

27 ��

(4)

where θ = rotation of block about centroid; and �� = coordinates of block centroid.

The new position of the block induces new conditions at block boundaries and thus new

contact forces. Resultant forces and moments are used to calculate linear and angular

accelerations of each block. The calculation scheme summarised above by Equation 1 to

Equation 4 is repeated until a satisfactory state of equilibrium or continuing failure is

reached for each block. It should be noted that time has no real physical meaning if a static

analysis is performed. Damping can be utilised in the above equations if a dynamic analysis

is performed. Cundall (1971) first applied this method to problems in rock mechanics. In

this paper, he also described the solution scheme overviewed above based on the equations

of motion; Cundall & Hart (1985) describe the two-dimensional formulation of DEM into

UDEC and Hart et al. (1988) describe the three-dimensional formulation of DEM into

3DEC.

DEM has three main differences from other numerical methods mentioned previously.

1. Blocks undergo large rotations and displacements

2. As a block changes shape it affects the forces applied to neighbouring bodies.

3. The solution scheme is explicit in time.

5.1.4 Constitutive laws and failure criterion of faults

The numerically modelled examples presented in this paper were performed with the

numerical code UDEC (2D) and 3DEC (3D). In this code, the faults are represented

28 ��

numerically as contact surfaces formed between two block edges. The constitutive laws

applied to the contacts are:

(5)

(6)

where � and �� are the normal and shear stiffness of the contact, Δσ and -.� are the

effective normal and shear stress increments, and Δ� and Δ�� are the normal and shear

displacement increments. Finally, stresses calculated at grid points located along contacts

are submitted to the selected failure criterion. For faults allowed to fail under a strike-slip

stress regime, the Coulomb friction is formulated:

(7)

where ��is the cohesion and �is the friction angle.

There is a limiting tensile strength �� for the joint or fault. If the tensile strength is

exceeded, then / = 0.

29 ��

5.2 Conventional fault seal and cap seal analysis

Previous work on seal integrity has focused on:

fault seal potential encompassing juxtaposition of a sealing unit against the

reservoir; the build up of clay smear or shale gouge on the fault plane; reactivation

risk; and

cap seal integrity involving mercury injection capillary pressure (MICP) data to

determine the ability of a seal to hold back a hydrocarbon column (Kaldi et al. 1999),

the determination of a brittleness index of the cap seal (Kivior et al. 2001) and the

geomechanical modelling of top seal integrity using finite element, continuum

approach (Lewis et al. 2002)

5.2.1 Fault-seal

In hydrocarbon provinces such as the Otway Basin, many if not all prospects are fault

bounded. For this reason, fault-seal is a major determining factor whether the prospect is

capable of maintaining an economic hydrocarbon column. Over the last 20 years, research

into fault seal has progressed and can be delineated into the following topics:

Juxtaposition: It is important to ascertain whether potential reservoir rock is

juxtaposed against a sealing lithology on the opposite side of the fault. If reservoir is

juxtaposed against reservoir, then some other sealing process is required for the

fault to seal and retain a hydrocarbon column (Watts 1987, Allan 1989, Knipe 1992,

Knipe 1997, James et al. 2004).

Fault zone processes forming a permeability barrier: In addition or instead of fault

juxtaposition, impermeable material such as shale or mineral development can be

30 ��

incorporated into the fault plane and/or associated fracture networks, providing a

permeability barrier and in some cases a sealing mechanism (Aydin 1978, Aydin &

Johnson 1983, Fulljames et al. 1997, Fisher & Knipe 1998, Gibson 1998, Yielding

1997, Yielding 2002, Bretan et al. 2003).

Fault reactivation risk is the likelihood of breach on the fault plane, in the current

stress field or any palaeo-stress field occurring subsequent to hydrocarbon charge

(Barton & Zoback, 1994, Barton et al. 1995, Barton et al. 1997) This technique

assesses the shear and normal stress acting on discretised sections of the fault

plane. Castillo et al. (2000), Wiprut & Zoback (2002), Jones & Hillis (2003) have

used this method in the Timor Sea, North Sea and Northwest Shelf (Australia) to

assess trap integrity. The proximity of these values to an approximate fault failure

envelope indicates the likelihood of failure of the fault plane (Mildren et al. 2002).

5.3 Intact rock-mass properties

From well-bore intersections, the Laira Formation (seal) is between 300 and 800 metres

thick in the Penola Trough. It comprises inter-bedded siltstone, claystone and fine grained

sandstone and forms the main sealing horizon to commercial gas fields located within the

Katnook Graben in the South Australia portion of the Penola Trough. The sealing lithology

(Laira) exhibits a high degree of heterogeneity over a large distance which adds difficulty

when attempting to ascertain its rock properties for input to the stress models. Only one

core has been obtained from the lower Laira Formation in Zema-1, located west of the

Katnook Graben. A point load test was undertaken on this core to establish an approximate

uniaxial compressive strength (UCS). The actual strength of the core could not be

31 ��

established as the rock failed before a load could be applied. Recommendations from the

International Society for Rock Mechanics (ISRM) (Edelbro 2003) suggests that point load

tests are to be thought of as an unreliable method for obtaining strength measurements for

weak and very weak rock (based on work by Brown 1981). Furthermore, heterogeneity is

reported to be the most important factor when up-scaling rock measurements from core or

rock samples. An increase in heterogeneity results in decreased UCS (Cunha 1990). Edelbro

(2003) adds that this scale effect exists as long as the rock is intact. As a result from these

previous studies, the Laira intact rock was inferred to have a very low UCS, perhaps less

than 20 MPa.

The properties required by the DEM model, for the intact rock, are as follows: bulk modulus

(K), shear modulus (G) and density ( ). Unfortunately neither Young’s modulus nor

Poisson’s ratio have been measured for the cored rock from Zema-1 and as such we assumed

an appropriate analogue for the cap-seal lithotype is weak micaceous shale. The analogue

rock properties assigned to the model for the intact Laira have been derived from static

laboratory tests (Lama & Vutukuri 1978) as K: 8.8 GPa G: 4.3 GPa density: 2.56 g/cm3. It

must be noted that a major impediment in the ability to predict the strength of the cap-seal

rock in this area is the lack of available core.

5.4 Fault-rock properties

3DEC, describes the fault deformation, using the Coulomb slip model (eq. 7). Below the

frictional limit, only elastic deformation occurs. At a constant magnitude of applied

differential stress the resultant elastic deformation is also constant, governed by Young’s

32 ��

modulus (and is independent of the Coulomb parameters) once the fault reaches its

frictional limit or shear stress limit as defined by the strength envelope, shear displacement

occurs along the fault. Fault properties are conventionally derived from laboratory testing

(e.g. tri-axial and direct shear tests). These tests can produce physical properties for joint

friction angle, cohesion, dilation angle, and tensile strength, as well as joint normal and

shear stiffness. The joint cohesion and friction angle correspond to the parameters in the

Coulomb strength criterion. Values for normal and shear stiffness for rock joints typically

range from around 10 to 100 MPa/m for joints with soft clay in-filling to over 100GPa/m for

tight joints in granite and basalt (Kulhawy 1975, Rosso 1976, Bandis et al. 1983). Published

strength properties for joints are more readily available than stiffness properties. Friction

angles can vary from less than 10° for smooth joints in weak rocks, such as tuff, to over 50°

for rough joints in hard rock, such as granite (Jaeger and Cook 1969, Kulhawy 1975, Barton

1976). Byerlee (1978) indicates that for almost all rock types, for common temperatures, the

angle of friction is between 30° and 40°, whether these figures can be extrapolated to the

larger scale study of joints in the field is unknown, although some results are proposed by

Kulhawy (1975). Bandis et al. (1981) suggests the effect of scale on joints is that peak shear

strength decreases as the sample size increases. Axen (2008) states that low angle normal

faults appear to seem to slip at much lower friction angles than those measured in the

laboratory. He states that rock friction angles appear to be understood very well at an

engineering level but may not be acting the same way at geological scales.

Minerals dragged into the fault plane as clay gouge or created by fault processes such as

cataclasis can have a significant effect on the frictional and cohesive strength of the fault

rock material. Typical gouge materials with a sheeted-like structure are characterized by

33 ��

low frictional strength = 26o (Morrow et al. 2000) and low cohesive strength (Dewhurst et

al. 2002). The introduction of water adsorbed onto the surface of platy or sheeted minerals

such as kaolinite, muscovite, chlorite, montmorillinite and chrysotile can further reduce the

frictional strength of fault rock by 33-65% ( = 6-11o) (Morrow et al. 2000, Moore et al.

2004). These friction angles agree well with the sensitivity studies performed in the course

of the research presented in this thesis. Often slip along faults is not achieved in the simple

numerical (DEM) models when friction angles are equal or above 25o (Papers 2 & 3).

Diagenetic changes within the fault rock such as the deposition of quartz, calcite and some

cases pyrite can substantially increase the frictional and cohesive strength of the fault rock

relative to the surrounding rock (Dewhurst et al. 2002). 3D framework grains such as

quartz, calcite, laumonite, zeolite, albite are relatively strong with friction angles between

31 - 40o. The frictional strength of these minerals is only slightly affected by the

introduction of water (Morrow et al. 2000). Fracture cohesion can range from zero cohesion

to values approaching the compressive strength of the surrounding rock. Lama and

Vutukuri (1978) suggest a range between 0–75 MPa for clastic sandstones. In this study, for

simplicity cohesion is held at zero, only the friction angle is varied, this is an adequate

approach for the sensitivity analysis performed because increasing the friction angle has a

similar effect.

5.5 In situ stress field determination The best determination of the complete in situ stress state of a petroleum province is very

important to the modelling process, as these data provide the boundary conditions applied

to the model. The determination of the in situ stress field is common practice in the

petroleum industry, particularly in tectonically active regions. There are many published

34 ��

studies from various geological settings throughout the world, which describe the

methodology to constrain the stress tensor from well-bore data (Zoback et al. 1989, Bell

1990, Adams & Bell 1991, Müller et al. 1992, Clauss et al. 1989, Spann et al. 1991). The

orientation of the principal stress can be obtained using earthquake focal mechanisms;

borehole breakouts or drilling induced fractures. Prensky (1992) offers an excellent review

of previous work on the subject of borehole breakouts with application to in situ stress

determination.

5.6 Borehole Breakout Analysis The observation of breakouts and drilling induced tensile fractures (DITFs) in boreholes is

an important step to constrain the stress tensor. In a vertical borehole, breakouts result

from the two orthogonal horizontal stresses, maximum horizontal stress (SHmax) and

minimum horizontal stress (Shmin), acting on the void space of the bore. Compressive stress

is concentrated in the direction of Shmin within the borehole. Where the compressive stress

around the borehole exceeds the compressive strength of the rock mass, shear failure occurs

with subsequent fracturing and spalling of the borehole. Drilling induced tensile fractures

(DITFs) result from tensile failure of the surrounding rock mass and are located

perpendicular to the breakout region of the borehole or in the direction of SHmax. Thus, the

orientation of these breakouts and DITFs are used to determine the orientation of SHmax and

Shmin with depth. The breakouts and DITFs can be interpreted from the dipmeter-caliper

and FMI logs respectively, with a good level of accuracy if care is taken to isolate

enlargements due only to in situ stresses acting on the well bore. FMI logs offer greater

confidence in the interpretation of breakouts as they can be visualised from the resistivity

data, removing one level of abstraction from the process (Springer 1987, Bell 1990). Many

35 ��

other factors influence the validity of the interpreted breakouts. These include deviation of

the well-bore, length of the breakout, differentiation between well-bore breakouts and well-

bore instability leading to washout and the identification of key seating. A full explanation

of the criteria required to interpret Shmin from borehole breakouts can be found in Plumb

and Hickman (1985), Zoback et al. (1985), Springer (1987), Zajac & Stock (1997) and Hillis

et al. (1995). Once the valid breakouts are extracted, it is common practice to combine all

data for the well into a single point for regional analysis. The mean and standard deviation

of the extracted azimuth value is then graded according to the quality of the source data,

with A-C quality data providing the best estimate of the interpreted orientation of SHmax.

This system was developed by Zoback et al. (1985) and continues to be used to produce the

World Stress Map (Zoback 1992). These combined breakout orientations can then be length

weighted (i.e. normalized for the length of the breakout), and eccentricity weighted (i.e.

normalized for the difference between the two caliper readings (C1-C2) from the dipmeter

log). The resulting values are then reported as azimuths of Shmin or SHmax (Shmin + 90o) and

subsequently used in further geomechanical studies.

5.6.1 SHmax Rotations In the petroleum industry, it is common to identify a variation in the orientation of SHmax,

particularly as discontinuities are approached and intersected in the borehole (Nielson et

al. 1988, Allison 1990, Zhang et al. 1994, Bell 1996, Hardebeck & Hauksson 2000). These

discontinuities are usually faults or fault zones but may also be lithological boundaries

(Allison 2004), which have varying mechanical properties.

Major discontinuities within a rock mass have the ability to disturb the applied stress field

causing localized increases in differential stress and an associated change in the orientation

36 ��

of the stress trajectory (Flodin & Aydin 2003, Homberg et al. 1997, Kattenhorn et al. 2000,

Maerten et al. 2002). In 1967 Ramsay showed how faults might affect stress trajectories

and that SHmax tends to be focused along faults with low cohesion or stiffness and become

orthogonal to faults with high cohesion or stiffness. Many workers have corroborated this

work since that time (Zhang et al. 1994, Hardebeck & Hauksson 2000, Pourjavad et al.

1998, Bell 1996, Yale et al. 1994). Authors refer to different fault mechanical parameters,

friction angle (Su & Stephansson 1999, Hunt & Boult 2005); stiffness (Pourjavad et al.

1998); or cohesion (Ron et al. 2000, Ramsay 1967).

6. CRITICAL ASSESSMENT OF THE MODELLING APPROACH In this section the strengths and weaknesses of the DEM approach to the geological

problems presented in this thesis are outlined. Also discussed are the differences between

reality and the models and their importance. Also discussed is how the modelling approach

used in this research approach, has dealt with these limitations and introduced further

limitations in the models and lastly, further work that could be done to improve confidence

in the modelling process.

6.1 Str engths of the methodology:

The results of the pilot study (Boult et al. 2002) which initiated this research showed that a

modelling approach could be applied to the problems of fracture prediction within a

shale/siltstone lithology compromising its sealing competency at a regional scale. Whilst the

distinct element method is not unlike other computational methods, it was chosen to apply

to the regional problem presented in this thesis because of its unique ability to: (1)

Incorporate multiple fault blocks into the model; (2) Allow geological bodies or fault blocks

37 ��

to undergo large rotations and displacements relative to one another; and (3) Allow the

interaction of forces between the fault blocks to affect neighbouring fault blocks, as they do

in nature. 0&��1��2��!�,��&��#3��#4&��$��#�&��#��#�(��

�#�#��5&#�#4�#&��2*#�&��,��6��#�� #�(�&�� (�� #�#�� #�#��

�#��(��+#��#��#��4�(�&��$��#��#��4��+��'��6��

�� '�� #�� #�� (�� 7�#4#�� (�'�4#�#�#�� #�� #��

*#�&�� 8� 4&�� $�� 4�� &��#(� (��(�� (�#�� #��(�#�� #(8�

#�(�&�#��#�#��#��#��(�#��4��(��

a1172507

Text Box

NOTE: This figure is included on page 37 of the print copy of the thesis held in the University of Adelaide Library.

38 ��

Figure 2: Shows the attributes of the four classes of distinct element method and the limit equilibrium method (Cundall & Hart 1989)

6.2 Weaknesses of the DEM methodology:

The DEM method has been developed for small scale problems in mining and civil

engineering. In these disciplines, the rock mechanical problems that can be applied to DEM

are usually located near surface and are at a relatively small scale (hundreds of metres, not

thousands as in my project). Subsequently information about the rock mass and

fault/fractures can be directly measured by sampling, observation and measurement. Prior

to this work, DEM had not been applied to large scale, regional problems regarding the

prediction of hydrocarbon seal fracturing between 2-3 kilometres beneath the earth’s

surface. The main limitations of DEM are as follows:

1. Computing power sensitive: Whilst the software has been designed to incorporate

hundreds of discontinuities, the number of resultant fault blocks that can be

modelled is limited by the processing power of the computer. I found that when

dealing with regional studies it was necessary to limit the number of modelled

faults. In a study area of 1250 square kilometres, thousands of seismically resolvable

faults exist in the rock mass and many more below seismic resolution. To overcome

this limitation, I grouped the mapped faults into genetic sets and only the major

critical faults from each of these fault sets were incorporated into the model. This

resulted in a smaller number of modelled fault blocks which were subsequently

zoned or discretized and could be handled by the computing power available at the

time. This was a project assumption and practical limitation but the author believes

is acceptable because the validation data primarily drill results was also at this

scale.

39 ��

2. Scale effects: The major limitation in the methodology is the requirement to down-

scale the large regional model to a size that can be handled by DEM and the

available computing power. In doing so, the number of faults that can be

incorporated into the model is limited. Jing & Hudson (2002) suggest that modelling

success can depend almost entirely on the quality of the characterisation of the

fracture system geometry, the physical behaviour of the individual fractures and the

interaction between the fractures. By simplifying and omitting most of the

faults/fractures from the model without having a sound understanding of their

character, genetic history and the critical interaction distance, may have a

detrimental effect on the results so every effort was made to understand in detail the

geological evolution of each basin prior to the model build.

3. Rigid vs Deformable Blocks:

DEM allows the incorporation of both rigid and deformable blocks within the same

model. "#�#��4��(��#��#�(��4��(��&��''�#��#��6��

�� &�� +�� #�� #�� 4�� #�(��#�&#�#�� +�#(�� #(�

'��'��#��#��(��(��(��4��#�� (��#��+��#��#��&��&��#��+��

��#��$��#��(��+��4#�#��#��+��#��

�� +� �''�#�� #��#�� 4�� 4��(�� '��#�� #�� #��

��(��4��(��4��4��(��#�(��#3��#��#��&��3��

6��(��'��7#��#��#��&�4��3��4��(��(��4��#$#��

6�� $�� &�4�� 4�� 4��(�� +�� #�� #�� #�� 4��

�$�#��4�� "�� 9((�� 2"9 ,� �� (��'&�� :�� (�8� �� "9 �

�$�#��4��8��4��4��(��(��4��#�(��'��#��

�

40 ��

6.3 Reality vs Models

Any geological or geomechanical model is a gross simplification of 'real' geology. One can

not hope to capture all the nuances and subtleties that exist in nature but can only attempt

to capture the most critical and influential of these parameters in the modelling process.

Geological modelling and hence geomechanical modelling at any depths of investigation

that can not be directly measured or observed, gives rise to a plethora of uncertainty in the

input parameters to the models. That is, nearly all information used to model at these

depths has been obtained by some level of remote detection. These methods include seismic

data interpretation resulting in an understanding of the gross structure of an area, however

the ‘real’ structural geometry can only be determined if the data has been processed with no

errors and as interpreters, we understand all of the complexities surrounding velocities

within the modelled rock mass. Other properties can be inferred from seismic data and

these include the density and elastic properties of the rock. Wire-line electric logs obtained

within a single well-bore can offer some insight into the various physical properties of a

lithotype, such as porosity, permeability, density, strength, elastic properties and

mineralogy. The closest we come to obtaining a direct measurement is when a core sample

can be cut at depth and tested in the laboratory for various rock properties including

compressive strength. This core is still a very small sample of the rock mass that is being

investigated, in the range of tens of metres within a volume of modelled rock which is in the

order of 106 times larger. The lack of directly measured data can not be underestimated and

the modeller must always be mindful of the shortcomings in the parameters used in the

models.

41 ��

Sensitivity analyses should always be performed to assist in the understanding of the

variability in the modelling parameters. Critical parameters in this research project were

identified as the careful interpretation of seismically resolvable faults and their allocation

to genetic fault sets; boundary stress conditions; fault zone width; fault-rock properties;

friction angles of both fault planes and the in-tact rock mass; gross rock mass strength; and

the effect of critical fault sets on each other within the regional model. Much time was

spent in an attempt to understand the sensitivities of these parameters. Papers 2 & 3 show

the results of sensitivity analyses performed on fault friction angle, ratio of 1 to 3 and the

angle of 1 as applied as a boundary condition in 2D and 3D. Paper 7 outlines a

methodology whereby the models can be used to provide sensitivity checks on the in situ

stress field where there was great uncertainty. Paper 8 showed the methodology and

results of sensitivity analysis as pertaining to fault zone width; fault-rock properties; and

the interaction of fault sets.

The final modelled results must always be viewed as an estimation of the ‘real’ geology of

an area. Correlation with observed data assists in gaining confidence that the input

parameters and the modelling process are offering a reasonable estimation of the actual

geology at depth (Papers 2, 3, 4, 6 & 8).

6.4 Getting the most out of the modelling process:

Confidence in the modelling process can be increased by:

Incorporating as much data from the geological model as possible. Working very

closely with the geologist and gleaning as much data as possible from the literature.

Critically review all input data, such as in situ stress conditions, fault rock

parameters, in-tact rock mass.

42 ��

Obtaining the most powerful computer possible. The amount of RAM in the

computer is the most critical with this technique. I used a computer with only

500MB RAM, the complexity of the models can be increased with greater computing

power.

Perform sensitivity analyses on all input parameters prior to running the models.

Treat the modelling process like a laboratory experiment. Start simple and add

complexity, understanding the changes to the results as the process progresses. It is

important to only add or change one parameter at a time so its effect can be

recognised and understood.

6.5 Further work

Increasingly geomechanics is becoming a mainstream discipline in the petroleum industry.

Its use spans not only the search for permeability in fractured clastic reservoirs, fault seal

potential and the generation of fractures within a seal, but also well-bore stability issues,

sand production and fracture stimulation design for both conventional and unconventional

resources.

My personal view is that I would like to see some research incorporating a probabilistic

range of parameters. The results of the models can then be treated probabilistically and

incorporated into the geologist’s tool box whenever a geomechanical problem needs to be

addressed in both the exploration and development of hydrocarbons. Although technology is

improving and will continue to do so, we will never be able to directly measure the

parameters required to input to the models within the petroleum industry. Also these

parameters change with time, the earth is dynamic and the stresses are naturally

43 ��

changing, as we produce from a hydrocarbon field the stresses can change and like-wise

when a fracture stimulation program is undertaken, we artificially introduce changes to our

environment. They may be minor but they are there, none the less. A probabilistic approach

can mitigate this risk by incorporating the possibilities and then assigning a risk to the

outcome.

44 ��

7. CONCLUSIONS

This thesis has identified that existing geomechanical techniques used to determine

whether a fault is critically stressed or not in the prevailing stress conditions, may not have

application when dealing with fault rock that is stronger than the surrounding rock—mass.

Paper 1 identifies this potential problem and suggests that a local rotation of the stress

field caused by a ‘strong’ or stiff fault results in the formation of natural fractures. This

paper also suggests that numerical stress modelling may be a useful tool to predict these

perturbations.

This thesis presents the results of sensitivity studies of various input parameters into the

numerical model (Paper 2) and showed that the models are particularly sensitive to fault

parameters such as friction angle ( ) and cohesion (C).

This thesis presents a new workflow using dipmeter data for the entire borehole whereby

rotations in borehole breakouts caused by discontinuities, in the vertical sense, can be used

to give qualitative indications of fault rock behaviour (Paper 3). These observations can

then used to make decisions about fault rock input parameters into the numerical stress

models.

This thesis also presents a new and unconventional technique whereby retaining the

variability in borehole breakout rotation with depth can be used to control the quality (QC)

45 ��

or calibrate numerical stress models to increase confidence in their results (Paper 7).

This thesis has applied the techniques developed in the course of the research to various

case study examples within Australia and New Zealand, such as the Big Lake Field in the

Cooper Basin SA; Pyrenees – Macedon fields in the North West Shelf and the Kupe South

Field in New Zealand. These modelling techniques have direct application to risk

assessment in other highly faulted regions throughout Australia and internationally

(Papers 4, 5, 6, 7, 9).

This thesis also presents a comparative study of a single fault using three different methods

of stress modelling, the distinct (discrete) element, boundary element and finite difference

methods (Paper 7).

This thesis shows the effect of fault-rock which has higher cohesive strength, stiffness or

frictional strength than the surrounding rock-mass, on the applied stress field (Paper 8).

This thesis shows how the application of stress modelling technique offers new workflow to

fully integrate stress studies, cap-seal analysis, fault-seal analysis and structural

interpretation to improve the understanding of hydrocarbon leakage risk at the prospect

and play scales (Paper 4, 5, 8).

The research presented in this thesis has provided additional workflows and added to the

‘tool-box’ used by those researchers and consultants working in the field of petroleum

geomechanics. Examples of how this modelling workflow has been implemented by

46 ��

companies are (1) Schlumberger now promoting stress modelling using WIPS Terratek; and

(2) Badley’s provide a stress modelling package using elastic dislocation theory in

Traptester.

Prospective further work consequential to the research presented in this thesis can be

suggested as follows:

(1) 4D modelling of fault behaviour, looking at historical stresses with respect to present

day stresses;

(2) Attempting to correlate stress models with seismic attributes and incorporating seismic

anisotropy into the models.

(3) Developing a probabilistic approach to the modelling process to mitigate risk in the

input parameters and increase confidence in the modelled outcome.

47 ��

8. REFERENCES Allan, U.S. 1989. Model for hydrocarbon migration and entrapment within faulted structures. AAPG Bulletin, 73, 803-811 Adams, J. and Bell, J.S. 1991. Crustal stress in Canada, Chapter 20. In Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Blackwell, D.D., eds., Neotectonics of North America: Geological Society of America, Decade Map Volume 1, 367-386. Allison, M.L. 1990. Remote detection of active faults using borehole breakouts in the Heber geothermal field, Imperial Valley, California: Geothermal Resources Council Transactions, 14, 2, 1359-1364 Allison, M.L. 2004. Variations of in situ stresses as indicators of active fractures and faults. AAPG Conference Abstract, Annual Meeting Dallas, Texas, USA. Axen, G. 2008. http://www.nmt.edu/mainpage/news/2008/12june01.html Aydin, A., 1978. Small faults formed as deformation bands in sandstone. Pure and Applied Geophysics, 116, 913-930. Aydin, A., and Johnson, A.M. 1983. Analysis of faulting in porous sandstone. Journal of Structural Geology, 5, 19-31. Bäckström, A., Antikainen, J., Backers, T., Feng, X., Jing, L., Kobayashi, A., Koyama, T., Pan, P., Rinne, M., Shen, B. and Hudson, J.A. 2008. Numerical modelling of uniaxial compressive failure of granite with and without saline porewater. International Journal of Rock Mechanics and Mining Sciences, 45, 7, 1126-1142 Bandis, S.C., Barton, N. and Lumsden, A. 1981. Experimental studies of scale effects on shear behaviour of rock joints. International Journal of Rock Mechanics and Mining Sciences, 18, 1-21 Bandis, S. C., A. C. Lumsden and N. R. Barton. 1983. Fundamentals of Rock Joint Deformation. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts,. 20, 6, 249-268. Barton, C.A. and Zoback, M.D. 1994. Stress perturbations associated with active faults penetrated by boreholes: Possible evidence for near-complete stress drop and a new technique for stress magnitude measurement. Journal of Geophysical Research, 99, B5, 9373 – 9390 Barton, C.A., Zoback, M.D. and Moos, D. 1995. Fluid flow along potentially active faults in crystalline rock, Geology, 23, 8, 683–686

48 ��

Barton, C.A., Moos, D. and Zoback, M.D. 1997. In situ stress measurements can help define local variations in fracture hydraulic conductivity at shallow depth, The Leading Edge, 1,653–1,656. Barton, N. 1976. The Shear Strength of Rock and Rock Joints. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 13, 255-279. Bell, J.S. 1990. Investigating stress regimes in sedimentary basins using information from oil industry wireline logs and drilling records. In Hurst, A., Lovell, M.A., and Morton, A.C., eds, Geological Applications of Wireline logs: Geological Society of London Special Publication, 48, 305-325. Bell, J.S. 1996. In situ stresses in sedimentary rocks (Part 2): Applications of stress measurements. Geoscience Canada, 23, 3, 135-153 Boult, P.J., Freeman, B., Yielding, G., Menpes, S. and Diekman, L.J. 2008. A minimum-strain approach to reducing the structural uncertainty in poor 2D seismic data, Gambier Embayment, Otway Basin, Australia. PESA Eastern Australasian Basins Symposium III, 109 - 118 Bretan, P., Yielding, G. and Jones, H. 2003. Using calibrated shale gouge ratio to estimate hydrocarbon column heights, AAPG Bulletin, 87, 3, 397-413 Brown, E.T. (Ed) 1981. Rock characterisation testing and monitoring, ISRM suggested methods. Pergamon Press. Byerlee, J.D., 1978. Friction of rocks. Pure and Applied Geophysics, 116, 615–26. Castillo, D.A., Bishop, D.J., Donaldson, I., Kuek, D., De Ruig, M., Trupp, M. and Shuster, M.W., 2000. Trap integrity in the Laminaria High—Nancar Trough Region, Timor Sea: Prediction of fault seal failure using well-constrained stress tensors and fault surfaces interpreted from 3D Seismic. APPEA Journal, 40, 151-173 Castillo, D.A. and Moos, D. 2000. Reservoir geomechanics applied to drilling and completions programmes in challenging formations: Northwest Shelf, Timor Sea, North Sea and Colombia. APPEA Journal, 40, 509-521 Clauss, B., Marquart, G., and Fuchs, K. 1989. Stress orientations in the North Sea and Fennoscandia, a comparison to the central European stress field. In Gregersen, S., and Basham, P.W. eds, Earthquakes at North Atlantic passive margins – neotectonics and post glacial rebound, Kluwer Academic Publishers, Dordrecht, 277-287 Cundall P.A. 1971. A computer model for simulating progressive large scale movements in block rock systems. In: Proceedings of the Symposium International Society of Rock Mechanics, Nancy, France, vol.1. paper II-8. Cundall, P.A. and Hart, R.D. 1985. Development of generalised 2-D and 3-D distinct element programs for modelling jointed rock. Itasca Consulting Group; Misc. Paper SL-85-1, US Army Corps of Engineers.

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Cunha, A.P. 1990. Scale effects in rock mechanics. In: Proceedings of the first international workshop on scale effects in rock masses, Loen, 7 - 8 June, 3-31 Dee, S.J., Yielding, G., Freeman, B. and Bretan, P. 2007a. A comparison between deterministic and stochastic fault seal techniques. Geological Society London, Special Publications, 292, 259-270 Dee, S. J., Yielding, G., Freeman, B., Healy, D., Kusznir, H.J., Grant, N. and Ellis, P. 2007b. Elastic dislocation modelling for prediction of small scale fault and fracture network characteristics. From: Fractured Reservoirs, Lonergan, L., Jolly, R.J.H., Rawnsley, K. and Sanderson, D.J. (eds). Geological Society London, Special Publications, 270, 130-155 Dewhurst, D.N., Jones R.M., Hillis, R.R. and Mildren, S.D. 2002. Microstructural and geomechanical characterization of fault rocks from the Carnarvon and Otway Basin. APPEA Journal, 42, 167 – 186 Dewhurst, D.N., Mildren, S.D., Camac, B.A., Boult, P.J. and Hennig, A.L. 2003. Top Seal Fracturing in the Carnarvon and Otway Basins, Australia. AAPG International Conference, Barcelona, Poster Session. Edelbro, C. 2003. Rock Mass Strength - A Review. Technical Report for the University of Technology, Lulea, Department of Civil Engineering, Division of Rock Mechanics Fisher, Q.J. and Knipe, R.J. 1998. Fault sealing processes in siliclastic sediments. In Jones, G., Fisher., Q.J., and Knipe., R.J., (eds). Faults, fault sealing and fluid flow in hydrocarbon reservoirs. Geological Society of London, Special Publication, 147, 117-134 Flodin, E.A. and Aydin, A. 2003. Evolution of a strike-slip fault network, Valley of Fire State Park, southern Nevada. Geological Society of America Bulletin, 116, 1, 42-59 Fulljames, J.R., Zijerveld, L.J.J. and Franssen, R.C.M.W., 1997. Fault seal processes: systematic analyses of fault seals over geological and production time scales. In: P. Møller-Pedersen and A.G. Koestler (Editors), Hydrocarbon Seals: Importance for Exploration and Production. Norwegian Petroleum Society (NPF), Special Publication 7. Elsevier, Amsterdam, 51–59. Gibson, R.G., 1998. Physical character and fluid flow properties of sandstone derived fault zones. In: Coward, M.P. Daltaban, T.S. Johnson, H. (eds), Structural geology in reservoir characterisation, Geological Society of London, Special Publication, 127, 83-97 Hardebeck, J.L. and Hauksson, E. 2000. The San Andreas Fault in Southern California: A weak fault in a weak crust. 3rd Conference on Tectonic Problems of the San Andreas Fault System, Stanford, California. Hart, R, Cundall, P.A. and Lemos, J. 1988. Formulation of a three-dimensional element model-Part II. Mechanical calculations for motion and interaction of a system composed of many polyhedral blocks. International Journal of Rock Mechanics Mining Science and Geomechanics Abstracts, 25, 3, 117-125

50 ��

Hillis, R.R., Monte, S.A., Tan, C.P. and Willoughby, D.R. 1995. The contemporary stress field of the Otway Basin, South Australia: Implications for hydrocarbon exploration and production. APEA Journal, 35, 1, 494-506 Homberg, C., Hu, J.C., Angelier, J., Bergerat, F., and Lacombe, O. 1997. Characterisation of stress perturbations near major fault zones: insights from 2-D distinct-element numerical modelling and field studies (Jura mountains). Journal of Structural Geology, 19, 5, 703-718 Hunt, S.P. and Boult, P.J. 2005. Discrete element stress modelling in the Penola Trough, Otway Basin, South Australia. AAPG Hedberg Series, v.1 Jaeger, J. C., and Cook, N. G. W. 1969. Fundamentals of Rock Mechanics., 2nd Ed. London: Chapman and Hall. James, W.R., Fairchild, L.H., Nakayama, G.P., Hippler, S.J. and Vrolijk, P.J. 2004. Fault-seal analysis using a stochastic multifault approach. American Association of Petroleum Geologists Bulletin, 88, 7, 885-904 Jing, L. 2003. A review of techniques, advances and outstanding issues in numerical modelling for rock mechanics and rock engineering. International Journal of Rock Mechanics and Mining Sciences, 40, 283-353. Jing, L. and Hudson, J.A. 2002. Numerical methods in rock mechanics. International Journal of Rock Mechanics and Mining Sciences, 39, 409-427. Jones, R.M., Boult, P.J., Hillis, R.R., Mildren, S.D. and Kaldi, J. 2000. Integrated hydrocarbon seal evaluation in the Penola Trough, Otway Basin, APPEA Journal, 40, 1, 194–212. Jones, R.M. and Hillis, R.R. 2003. An integrated, quantitative approach to assessing fault-seal risk. American Association of Petroleum Geologists Bulletin, 87, 3, 507-524 Kaldi, J.G., O’Brien, G.W. and Kivior, T. 1999. Seal capacity and hydrocarbon accumulation history in dynamic petroleum systems: the East Java Basin, Indonesia and the Timor Sea Region, Australia. APPEA Journal, 39, 1, 73–86. Kattenhorn S.A., Aydin, A., Pollard, D.D. 2000. Joints at high angles to normal fault strike: an explanation using 3-D numerical models of fault perturbed stress fields. Journal of Structural Geology, 22, 1-23. King, G.C.P, Stein, R.S. and Lin, J. 1994. Static stress changes and the triggering of earthquakes. Bulletin of the Seismological Society of America, 84, 3, 935-953 Kivior, T., Kaldi, J.G. and Lang, S.C., 2001. Seal potential in Cretaceous and Late Jurassic rocks of the Vulcan Sub-basin, North West Shelf, Australia. AAPG Hedberg research conference: Evaluating the hydrocarbon sealing potential of faults and caprocks, December 1–5, 2002, Barossa Valley, SA.

51 ��

Knipe R.J. 1992. Faulting processes and fault seal. In: Larsen, R. M.; Brekke, H.; Larsen, B. T.; Talleraas, E. (eds) Structural and Tectonic Modelling and its Application to Petroleum Geology. NPF Special Publication 1, 325-342 Knipe, R.J., 1997. Juxtaposition and seal diagrams to help analyze fault seals in hydrocarbon reservoirs. American Association Petroleum Geologists, Bulletin, 81, 187–195. Kulhawy, F.H. 1975. Stress deformation properties of rock and rock discontinuities. Engineering Geology, 9, 327-350. Lewis, H, Olden, P, and Couples, G.D. 2002. Geomechanical simulations of top seal integrity. Hydrocarbon Seal Quantification, edited by A.G. Koestler and R. Huntsdale, NPF Special Publication, 11, 75-87, (Elsevier Science, Amsterdam, publishers) Lama R.D. and Vutukuri, V.S. 1978. Handbook on mechanical properties of rocks. Serieson Rock and Soil Mechanics. Lyon, P., Boult, P.J., Hillis, R.R. and Mildren, S.D. 2005. Sealing by shale gouge and subsequent seal breach by reactivation: A case study of the Zema Prospect, Otway Basin. AAPG Hedberg Series, v. 1. Maerten L., Gilespie, P., and Pollard, D.P. 2002. Effects of local stress perturbations on secondary fault development. Journal of Structural Geology, 24, 145-153. Mildren, S.D., Hillis, R.R. and Kaldi, J. 2002. Calibrating predictions of fault seal reactivation in the Timor Sea. APPEA Journal, 42, 1, 187–202. Moore, D.E., Lockner, D.A., Tanaka, H and Iwata, K. 2004. The co-efficient of friction of crysotile gouge at seismogenic depths. International Geological Review, 46, 5, 385-398 Mora, P. and Place, D. 1993. A Lattice Solid Model for the Nonlinear Dynamics of Earthquakes. International Journal of Modern Physics 4, 6, 1059-1074 Morrow, C.A., Moore, D.E. and Lockner, D.A. 2000. The effect of mineral bond strength and adsorbed water on fault gouge frictional strength. Geophysical Research Letters, 27, 6, 815 - 818 Müller, B., Zoback, M., Fuchs, K., Mastin, L., Gregersen, S., Pavoni, N., Stephanson, O., and Ljunggren, C. 1992. Regional patterns of tectonic stress in Europe. Journal of Geophysical Research, 97, B8, 11783-11804 Mutlu, O., and Pollard, D.D., 2008. On the patterns of wing cracks along an outcrop scale flaw: A numerical modeling approach using complementarity. Journal of Geophysical Research, 113, B06403 Nielson, D.L., Hulen, J.B., Allison, M.L. 1988. Fracture systematics in high temperature geothermal fields: Roles of inherited structures and stress field re-orientation. International Symposium on Geothermal Energy, Kumamoto and Beppu, Japan (abstract).

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Pascal, C., and Gabrielson, R.H. 2001. Numerical modelling of Cenozoic stress patterns in the mid-Norwegian margin and North Sea. Tectonics, 20, 4, 585-599 Plumb, R.A. and Hickman, S.H. 1985. Stress induced borehole elongation – A comparison between the four arm dipmeter and the borehole televiewer in the Auburn geothermal well. Journal of Geophysical. Research, 90, B7, 5513-5521 Pourjavad, M., Bell, J.S., Bratli, R.K. 1998. Stress trajectories in the neighbourhood of fault zones. SPE – EUROC Conference Proceedings, Trondheim, Norway. Prensky, S. 1992. Borehole breakouts and in situ rock stress – A Review. The Log Analyst, 33, 3, 304-312 Ramsay, J.G. 1967. The folding and fracturing of rocks McGraw-Hill, New York Reynolds, S., Hillis, R.R. and Paraschiviou, E., 2003. In-situ stress field, fault reactivation and seal integrity in the Bight Basin, S.A., ASEG Conference, Adelaide. Ron, H, Beroza, G., Nur, A. 2000. A mechanical explanation for multiple-fault rupture in the Mojave. Proceedings of the 3rd conference on tectonic problems of the San Andreas Fault System. Rosso, R. S. 1976. A Comparison of Joint Stiffness Measurements in Direct Shear, Triaxial Compression, and In Situ. International Journal of Rock Mechanics and Mining Sciences. & Geomechanics Abstracts, 13, 167-172. Spann, H., Brudy, M. and Fuchs, K. 1991. Stress evaluation in offshore regions of Norway. Terra Nova, 3, 2, 148-152 Springer, J.E. 1987 Stress orientation from well bore breakouts in the Coalinga region. Tectonics, 6, 5, 667-676 Starfield, A.M. and Cundall, P.A. 1988. Towards a methodology for rock mechanics modelling. International Journal of Rock Mechanics Mining Science and Geomechanics Abstracts, 25, 3, 99-106 Su, S. and Stephansson, O. 1999. Effect of a fault on in situ stresses studied by the distinct element method. International Journal of Rock Mechanics and Mining Sciences, 36, 1051-1056 Tamagawa, T. and Pollard, D.D. 2008. Fracture Permeability created by perturbed stress fields around active faults in a fractured basement reservoir. AAPG Bulletin, 92, 6, 743 - 764

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van Mier, J. G. M., Schlangen, E and Vervuurt, A. 1995. Lattice Type Fracture Models for Concrete. Continuum Models for Materials with Microstructure. H. B. Muhlhaus, John Wiley & Sons: 341-377. Watts N.L. 1987. Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns. Marine and Petroleum Geology, 4, 274 – 307 Wiprut, D. and Zoback, M.D. 2000. Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea. Geology, 28, 7, 595-598 Wiprut, D. and Zoback, M.D. 2002. Fault reactivation, leakage potential, and hydrocarbon column heights in the northern North Sea. In: Koestler, A.G. and Hunsdale, R. (ed) Hydrocarbon Seal Quantification. Norwegian Petroleum Society, Special Publications, 11, 17-35. Yale, D.P, Rodriguez, J.M., Mercer, T.B and Blaisedell, D.W. 1994. In situ stress orientation and the effects of local structure – Scott Field, North Sea . Eurorock, Balkema, Rotterdam. Yielding, G., Freeman, B., and Needham, D.T. 1997. Quantitative fault seal prediction, AAPG Bulletin, 81, 6, 897-917 Yielding, G. 2002. Shale Gouge Ratio - calibration by geohistory. In: A.G. Koestler and R. Huntsdale (eds.), Hydrocarbon Seal Quantification, Norwegian Petroleum Society (NPF), Special Publication, 11, 1-15 Zajac, B.J. and Stock, J.M. 1997. Using Borehole Breakouts to Constrain the Complete Stress Tensor: Results from the Sijan Deep Drilling Project and Offshore Santa Maria Basin California. Journal of Geophysical Research, 102, 10083-10100. Zhang Y-Z., Dusseault, M.B. and Yassir, N.A. 1994. Effects of rock anisotropy and heterogeneity on stress distributions at selected sites in North America. Engineering Geology, 37, 3-4, 181-197 Zoback M.D., Moos, D., Mastin, L. and Anderson, R.N. 1985. Well bore breakouts and in situ stress. Journal of Geophysical. Research, 90, 5523-5530 Zoback, M.L., Zoback, M.D., Adams, J., Assumpcao, M., Bell S., Bergman, E.A., Blümling, P., Brereton, N.R., Denham, D., Ding, J., Fuchs, K., Gay, N., Gregersen, S., Gupta, H.K., Gvishiani, A., Jacob, K., Klein, R., Knoll, P., Magee, M., Mercier, J.L., Müller, B., Paquin, C., Rajendran, K., Stephansson, O., Suarez, G., Suter, M., Udias, A., Xu, Z.H., Zhizhin, M. 1989. Global patterns of tectonic stress. Nature, 341, 291-298. Zoback, M. L., 1992. First and second-order patterns of stress in the lithosphere: The World Stress Map Project. Journal of Geophysical. Research, 97, B8, 11703-11728

54 ��

9. Conferences Papers/Posters Presented

2008 - AAPG Hedberg Conference on The Geologic Occurrence and Hydraulic Significance of Fractures in Reservoirs, Casper, Wyoming

2006 - APPEA Conference (Gold Coast, Australia) – Paper and Presentation

2005 - Alaska Rocks, American Rock Mechanics Association Annual Conference (Anchorage, Alaska USA) – Paper and Presentation

2005 - EAGE Conference (Madrid, Spain) Presentation and Extended Abstract

2004 - APPEA Conference (Canberra, Australia) – Paper and Presentation

2004 - Seals 1 Consortium – Presentation and Extended Abstract

2004 - AAPG Annual Convention (Dallas, USA) – Paper and Presentation

2004 - SPE Asia Pacific Oil and Gas Conference and Exhibition – Poster and Paper

2003 - AAPG International Conference (Barcelona, Spain) – 2 Posters

2003 - EAGE Fault and Top Seals Conference (Montpellier, France) – Paper and

Poster

2002 - APPEA Conference (Adelaide, Australia) – Paper and Presentation

10. Awards

2005 American Association of Petroleum Geologists, Grants in Aid Award (Gustavus E Archie International Memorial Grant) 2003 Midland Valley International Structural Geology Prize: 2nd Place

2003 CSIRO PhD Top Up Scholarship 2003 University of Adelaide: Research Abroad Scholarship

An Integrated Geomechanical Evaluation of Cap and Fault--seal for Risking Petroleum Trap Integrity – Using Distinct Element and Boundary Element

Numerical Methods

Bronwyn Anne Camac

CHAPTER 1

3D FAULT MODELLING AND ASSESSMENT OF TOP SEAL

STRUCTURAL PERMEABILITY – PENOLA TROUGH,

ONSHORE OTWAY BASIN

P.J. Boult, B. A. Camac, A. W. Davids

Ginko ENPGNG, Adelaide, S.A. Australia

Faculty of Engineering, Mathematics and Computer Science, Australian School of Petroleum, The University of Adelaide, Adelaide, S.A. 5005, Australia

Woodside Petroleum, PO Box D188 GPO Perth, W.A. 6840

Australian Petroleum Production Exploration Association Journal (2002) 151 – 166

STATEMENT OF AUTHORSHIP

PAPER 1

Boult, P.J., B. A. Camac and A. W. Davids, 2002. 3D Fault Modelling and Assessment

of Top Seal Structural Permeability – Penola Trough, Onshore Otway Basin: Australian

Petroleum Production Exploration Association Journal, 151 – 166

Summary of Authorship Bronwyn Camac (estimated % of contribution: 10%) Performed the interpretation of the FMI data set Wrote approximately 10% of the paper Peter Boult (estimated % of contribution: 80%) Suggested the possibility of local stress rotations Wrote approximately 80% of the paper Andrew Davids (estimated % of contribution: 10%) Interpreted the seismic data set for the Balnaves Structure Wrote approximately 10% of the paper

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A NOTE: This publication is included in the print copy of the thesis held in the University of Adelaide Library.

An Integrated Geomechanical Evaluation of Cap and Fault--seal for Risking Petroleum Trap Integrity – Using Distinct Element and Boundary

Element Numerical Methods

Bronwyn Anne Camac

CHAPTER 2

A PARAMETRIC ANALYSIS AND APPLICATIONS OF THE

DISCRETE ELEMENT METHOD FOR STRESS MODELLING

S.P. Hunt, B. A. Camac and P. J. Boult

Santos Ltd, Flinders Street, Adelaide, S.A. 5005, Australia



The 9th Australian New Zealand Conference on Geomechanics (2003)


PAPER 2

Hunt, S. P., B. A. Camac and P. J. Boult, 2003. A Parametric Analysis and Applications of

the Discrete Element Method for Stress modelling: The 9th Australian New Zealand

Conference on Geomechanics

Summary of Authorship Bronwyn Camac (estimated % of contribution: 45%)

Wrote approximately 50% of the paper Performed the stress modelling study on the Balnaves structure in 3-dimensions using 3DEC

Suzanne P. Hunt (estimated % of contribution: 50%)

Performed the sensitivity study for a single fault in 2-dimensions Wrote approximately 50% of the paper Provided training in the use of the 2D and 3D numerical stress codes (UDEC & 3DEC)

Peter Boult (estimated % of contribution: 5%)

Provided geological advice Suggested the original idea of locally perturbed stress fields around faults

A Parametric Analysis and Applications of the Discrete Element Method For Stress Modelling

Hunt*1, S P, Camac*, B A. and Boult P�.*School Of Petroleum Engineering and Management, Adelaide University, SA 5005 Australia

�Chief Geoscientist, Primary Industry and Resources, SA 5005, Australia.

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An Integrated Geomechanical Evaluation of Cap and Fault--seal for

Risking Petroleum Trap Integrity – Using Distinct Element and Boundary Element Numerical Methods

Bronwyn Anne Camac

CHAPTER 3

FAULT AND TOP SEAL INTEGRITY AT RELAYS AND

INTERSECTIONS USING A 3D DISTINCT ELEMENT CODE

B. A. Camac, S. P. Hunt and P.J. Boult




Australian Petroleum Production Exploration Association Journal (2004) 481-496


PAPER 3

Camac, B. A., S. P. Hunt and P.J. Boult, 2004. Fault and Top Seal Integrity at Relays

and Intersections using a 3D Distinct Element Code: Australian Petroleum Production

Exploration Association Journal, 481-496


Wrote the paper Performed the stress modelling study on the Penola Trough Study area in 3-dimensions using 3DEC Performed the stress modelling in 2-dimensions for the Jabiru-Tancred Terrace


Provided supervision and suggestions Assisted with the data gathering for the 2D study Performed the 3D stress modelling on the generic structures Edited manuscript

Peter Boult (estimated % of contribution: 5%)

Edited manuscript

APPEA JOURNAL 2004—1

FAULT AND TOP SEAL INTEGRITY AT RELAYS AND INTERSECTIONS USING A 3D

DISTINCT ELEMENT CODE

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Bronwyn Anne Camac

CHAPTER 4

DISTINCT ELEMENT STRESS MODELLING FOR TOP SEAL

APPRAISAL IN THE PYRENEES-MACEDON OIL AND GAS

FIELDS, EXMOUTH SUB-BASIN, AUSTRALIAN NORTH

WEST SHELF

B. A. Camac, S. P. Hunt and W. R. Bailey



Woodside Petroleum, PO Box D188 GPO Perth, W.A. 6840

The 40th U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development in the Northern Regions, held in Anchorage,

Alaska, June 25-29, 2005


PAPER 4

Camac, B. A., S. P. Hunt, and W. R. Bailey, 2005. Distinct element stress modelling for

top seal appraisal in the Pyrenees-Macedon oil and gas fields, Exmouth Sub-basin,

Australian North West shelf. This paper was prepared for presentation at Alaska Rocks

2005, The 40th U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for

Energy, Mineral and Infrastructure Development in the Northern Regions, held in

Anchorage, Alaska, June 25-29, 2005.

Status: International Conference Paper


Wrote the paper Performed the stress modelling study on Pyrenees-Macedon area in 3-dimensions using 3DEC Generated ideas on how the stress modelling can predict seal breach Recognised the observation of HRDZs (previously published) can be used to fine tune the fault parameters used in the modelling study


Provided supervision and suggestions Edited manuscript

Wayne R. Bailey (estimated % of contribution: 10%)

Assisted with the geological understanding of the study area and prior fault seal studies Edited manuscript

ARMA/USRMS 05-706

Distinct element stress modelling for top seal appraisal in the Pyrenees-Macedon oil and gas fields, Exmouth Sub-basin, Australian North West shelf.

Camac1, B.A., Hunt1, S.P., and 1 Australian School of Petroleum, Engineering Discipline, University of Adelaide, Adelaide, South Australia, Australia

Bailey2, W.R. 2 CSIRO Petroleum, 26 Dick Perry Avenue, Technology Park, Kensington, Western Australia, Australia

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Bronwyn Anne Camac

CHAPTER 5

APPLICATIONS OF STRESS FIELD MODELLING USING

THE DISTINCT ELEMENT METHOD FOR PETROLEUM

PRODUCTION

B. A. Camac and S. P. Hunt



APOGCE Conference Proceedings (2004) SPE Paper # 88473


PAPER 5

Camac, B. A. and S. P. Hunt. 2004. Applications of Stress Field Modelling Using the

Distinct Element Method for Petroleum Production: APOGCE Conference Proceedings,

SPE Paper # 88473



Wrote the paper Performed the stress modelling study on the Big Lake Field in 3-dimensions using 3DEC Generated ideas and results on how the results of the stress modelling can be used to predict bypassed pay in a field and a location for hydrocarbon pooling.


Provided supervision and suggestions Suggested how the modelling outputs could be used to predict fluid migration through rock Undertook sensitivity modelling of a single fault in 3-dimensions using 3DEC Edited manuscript

SPE Paper Number # 88473

Applications of Stress Field Modelling Using the Distinct Element Method for Petroleum Production

Bronwyn A. Camac SPE (Australian School of Petroleum - University of Adelaide) and Suzanne P. Hunt (Australian School of Petroleum -University of Adelaide)

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Bronwyn Anne Camac

CHAPTER 6

LOCAL ROTATIONS IN BOREHOLE BREAKOUTS –

OBSERVED AND MODELLED STRESS FIELD ROTATIONS

AND THEIR IMPLICATIONS FOR THE PETROLEUM

INDUSTRY





International Journal of Geomechanics (2006) (6) 6, 399-410


PAPER 6

Camac, B. A., S. P. Hunt and P. J. Boult, 2006. Local Rotations in Borehole Breakouts

– Observed and Modelled stress field rotations and their Implications for the Petroleum

Industry. International Journal of Geomechanics, (6) 6, 399-410

Status: International Journal Paper


Wrote the paper Performed the stress modelling study on the Penola Trough area in 2-dimensions using UDEC Performed the borehole breakout analysis on each well presented Proposed an innovative methodology for incorporating all information gained from borehole breakouts particularly in the vertical dimension (with depth) Generated new ideas on how dipmeter data can be used to determine fault parameters for input into stress models Show how borehole breakout data can be used to indicate sub-seismic faulting which can not be seen in the seismic data.



Peter J. Boult (estimated % of contribution: 10%)

Suggested the original idea of looking at vertical stress rotations in breakout data Edited manuscript

Local Rotations in Borehole Breakouts—Observed andModeled Stress Field Rotations and Their Implications for

the Petroleum IndustryBronwyn A. Camac1; Suzanne P. Hunt2; and Peter J. Boult3

INTERNATIONAL JOURNAL OF GEOMECHANICS © ASCE / NOVEMBER/DECEMBER 2006 / 399

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A Camac, B.A., Hunt, S.P. & Boult, P.J. (2006) Local rotations in borehole breakouts - observed and modeled stress field rotations and their implications for the petroleum industry. International Journal of Geomechanics, v. 6(6), pp. 399-410.

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A NOTE: This publication is included in the print copy of the thesis held in the University of Adelaide Library. A It is also available online to authorised users at: A http://dx.doi.org/10.1061/(ASCE)1532-3641(2006)6:6(399) A

http://dx.doi.org/10.1061/(ASCE)1532-3641(2006)6:6(399)


Numerical Methods

Bronwyn Anne Camac

CHAPTER 7

UNCONVENTIONAL BOREHOLE BREAKOUT ROTATION

ANALYSIS PROVIDES A QC FOR STRESS MODELS

B. A. Camac, S. P. Hunt, M. Dillon and P.J. Boult



Faculty of Engineering, Mathematics and Computer Science, Australian School of

Petroleum, The University of Adelaide, Adelaide, S.A. 5005, Australia


Australian Petroleum Production Exploration Association Journal (2006) 307 - 328


PAPER 7

Camac, B. A., S. P. Hunt, P.J. Boult and M. Dillon, 2006. Unconventional Borehole

Breakout Rotation Analysis provides a QC for Stress Models, Australian Petroleum

Production Exploration Association Journal, 307 - 328

Status: Australian Journal Paper


Wrote the paper Performed the stress modelling study on the Kupe Field area in 3-dimensions using 3DEC Generated the idea and showed how borehole breakout data can be used to calibrate and quality control stress model input parameters



Michael Dillon (estimated % of contribution: 5%)

Undertook boundary element stress modelling on a single fault


Provided supervision

UNCONVENTIONAL BOREHOLE BREAKOUT ROTATIONANALYSIS PROVIDES A QC TOOL FOR STRESS MODELS

B.A. Camacl, S.P. Hunt', P.J.Boult2 and M. Dillon I

I Australian School of Petroleum, University of Adelaide,North Tee, Adelaide, SA 5005 Australia.2 Primary Industries and Resources, SA, 101 Grenfell Street,Adelaide, SA, 5000 Australia

APPEA Journal 2006-307

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A Camac, B.A., Hunt, S.P., Boult, P.J. & Dillon, M. (2006) Unconventional borehole breakout rotation analysis provides a QC tool for stress models. Australian Petroleum Production Exploration Association Journal, 2006, pp. 307-328

��


Numerical Methods

Bronwyn Anne Camac

CHAPTER 8

PREDICTING SEAL FAILURE: AN APPLICATION OF 2D

AND 3D DISTINCT ELEMENT METHOD IN PETROLEUM

GEOMECHANICS





Petroleum Geoscience (15) 75-89

��


PAPER 8

Camac, B. A., Hunt, S. P. and Boult, P. J. 2009, Predicting brittle cap-seal failure of

petroleum traps: An application of 2-D and 3-D distinct element method. Petroleum

Geoscience, 15, 75-89

Status: International Journal Paper


Wrote the paper Performed the stress modelling sensitivity study on the Balnaves Fault in 3-dimensions using 3DEC Performed the stress modelling study on the Penola Trough in 2-dimensions using UDEC Built on the original idea that basement terrain faults affect the limit of the main graben faults and in doing so affect the younger downward propagating fault sets. By using the sensitivity modelling of ‘strong’ faults and the results of the 2D model, a fracture zone and a potential new fault was predicted by the modelling aligned with the old basement fault. This fracture zone / potential new fault has never been observed nor predicted before.




Provided the original idea that basement terrain faults affect the orientation of younger fault sets in this area. Provided supervision and edited the manuscript

Predicting brittle cap-seal failure of petroleum traps: an application of2D and 3D distinct element method

Bronwyn A. Camac1,*, Suzanne P. Hunt2 and Peter J. Boult3

1Australian School of Petroleum (Engineering Discipline) University of Adelaide, Adelaide S.A. 5005, Australia(Present address: Beach Petroleum, 25 Conyngham Street, Glenside, S.A. 5065, Australia)

2Santos Ltd, 60, Flinders Street, Adelaide, S.A. 5000, Australia3GINKO ENPGNG Adelaide, Australia

*Corresponding author (e-mail: [email protected])

Petroleum Geoscience, Vol. 15 2009, pp. 75–89 1354-0793/09/$15.00 � 2009 EAGE/Geological Society of LondonDOI 10.1144/1354-079309-796

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A NOTE: This publication is included in the print copy of the thesis held in the University of Adelaide Library. A It is also available online to authorised users at: A http://dx.doi.org/10.1144/1354-079309-796 A

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A Camac, B.A., Hunt, S.P. & Boult, P.J. (2009) Predicting brittle cap-seal failure of petroleum traps: an application of 2D and 3D distinct element method. Petroleum Geoscience, v. 15 (1), pp. 75-89

http://dx.doi.org/10.1144/1354-079309-796


Numerical Methods

CHAPTER 9

A NEW GEOMECHANICAL TOOL FOR THE EVALUATION

OF HYDROCARBON TRAP INTEGRITY





Golden Rocks 2006, The 41st U.S. Symposium on Rock Mechanics (USRMS): Rock

Mechanics for Energy, Mineral and Infrastructure Development.


PAPER 9

Hunt, S.P., B.A. Camac, and P.J. Boult, 2006. A new geomechanical tool for the

evaluation of hydrocarbon trap integrity. Golden Rocks 2006, The 41st U.S. Symposium

on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure

Development.



Performed the stress modelling (DEM) study on a single fault plane in 3-dimensions using 3DEC Performed the stress modelling (DEM) study for the three case study areas presented, being the Otway Basin, South Australia; Pyrenees-Macedon fields, NW Shelf, Australia; and the Kupe Field, New Zealand. Edited the manuscript


Wrote the paper Provided supervision and suggestions


Provided the original idea of using stress models to predict local rotations in stress fields which led to this work

ARMA/USRMS 06-1064

A new geomechanical tool for the evaluation of hydrocarbon trap integrityHunt, S.P. Australian School of Petroleum, The University of Adelaide, South AustraliaCamac, B.A. Sigma One Geomechanics Pty Ltd , South Australia. Boult, P.J. Chief Petroleum Geologist, Primary Industries and Resources of South Australia.

Copyright 2005, ARMA, American Rock Mechanics Association

This paper was prepared for presentation at Golden Rockss 2006, The 41st U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development .

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A Hunt, S.P., Camac, B.A. & Boult, P.J. (2005) A new geomechanical tool for the evaluation of hydrocarbon trap integrity. 41st US Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development in the Northern Regions, Golden Rocks, 2006.

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Documents

Australian School of Petroleum The University of Adelaide ... · Petroleum Production and Exploration Association Journal, 42, 151 – 166 Paper 2: Hunt, S. P., Camac, B. A. and Boult,