Coupled hydromechanical modeling of rock mass response to hydraulic fracturing and hydro-shearing: Outcomes related to permanent enhancement of fracture permeability

by Giona Preisig1, Erik Eberhardt1 & Armin Hosseinian1

1Geological Engineering, EOAS, The University of British Columbia, Vancouver, BC, Canada. gpreisig@eos.ubc.ca

ABSTRACT

Injection of pressurized fluid for hydraulic fracturing and hydro-shearing treatments is rapidly becoming a vital method for the unconventional oil & gas, deep mining and enhanced geothermal system (EGS) sectors. Such techniques will also play a key role in the development of carbon capture and storage (CCS). However, the optimal design of hydraulic injections is often limited by continuum treatments of the rock mass without consideration of the influence of the natural fractures present. These may interact with the hydraulic fracture to promote offsets or arrest, or may cause the natural fractures to undergo shear and dilation. Given the limited access to the rock afforded by deep boreholes common to oil & gas and geothermal projects, access and direct monitoring through mine-back experiments of hydraulic treatments is a necessary step for producing critical data sets to constrain and calibrate advanced numerical models.

In this work, we first present a suite of forward coupled hydromechanical distinct-element models with UDEC (Itasca 2013) used to aid in the design of a multi-disciplinary mine-back experiment conducted in a deep mine in New South Wales, Australia, in June 2014. Then, field data from the experiment are integrated in the modeling framework and used to constrain the models. Based on model results, the enhancement of rock mass permeability is assessed during and after separate hydraulic fracture and hydro-shearing treatments. Results include 3-D scanning of a natural fracture to correlate surface roughness effects to dilation as a function of shearing. With this, detailed fracture flow models are computed at different time steps for different fracture geometries.

Based on the intact rock properties, discrete fracture network characteristics, and measured in-situ stresses, the forward hydromechanical analysis was used to test different fluid injection scenarios providing important information, related to: (1) rock failure mode and type of failure events, (2) growth of fractures and stimulated volume, (3) pore pressure time responses, (4) rock mass deformation, and (5) stress changes (Fig.1). Modeling results indicate that the opening of a tensile hydraulic fracture is accompanied by millimetre-scale shear displacements associated with slip and wedging between adjacent blocks (Fig. 2). This leads to a small mismatch between the upper and lower walls of the fracture, which may result in permanent dilation, aperture change and permeability if the fracture asperities are irregular and strong enough. In effect, offset asperities may prevent a fully elastic closure when the hydraulic fracture is depressurized.

Future work will continue on the back-analysis of the mine-back experiment data. This will consist of integrating measured microseismic event locations, magnitudes and source mechanisms, together with tiltmeters deformation, stress change and pore pressure data associated with the generation of newly formed fractures and stimulated rock volume.

Figure 1 – Simulated changes in (a) horizontal, (b) shear and (c) vertical components of the stress tensor during fluid injection for hydraulic fracturing (modified after Preisig et al. in review)

Figure 2 – Simulated shear displacement on fracture during (1) a hydro-fracturing and (2) a hydro-shearing treatment (modified after Preisig et al. in review)

KEYWORDS

Hydro-fracturing, Hydro-shearing, Distinct-element modeling, Fracture permeability, Mine-back experiments

BYBLIOGRAPHY

Itasca (2013) UDEC 5.0 Universal Distinct Element Code, Itasca Consulting Group Inc, Minneapolis, USA.

Preisig G, Eberhardt E, Gischig V, Roche V, van der Baan M, Valley B, Kaiser PK, Duff D & Lowther R (2014). Development of connected rock mass permeability through hydraulic fracture propagation and shearing accompanying fluid injection. Geofluids: In Review.