Petr 2510 Lectures 2

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Slide 1Dr Elena Pasternak

Petroleum Engineering Fundamentals

PETR2510

Exploration methods


Literature Selley, R.C. 1998. Elements of Petroleum

Geology, Academic press Jahn, F., M. Cook & M. Graham, 2007.

Hydrocarbon Exploration and Production. Elsevier

Hyne, N.J. 2001. Nontechnical Guide to Petroleum Geology, Exploration, and Production. Penn Well Corporation

2


Plan

Aim

Geophysics

Borehole geophysics (logs)

(Pictures, if not indicated otherwise, are copied from Selley, 1998 )


Aim Objective is to find new volumes of

hydrocarbons (at a low cost and in short period of time)

Two types of methods» Geophysics (exploration from the surface)» Borehole geophysics (logging)

Geochemistry» Detects surface anomalies caused by hydrocarbon

accumulation

Field studies (outcrops)» Vertical and lateral relationship of the different rock

types of a reservoir

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Definition of a good reservoir

Now: How to find a reservoir (“good” or otherwise!)

» Find a basin (coarse scale)

– gravity surveys, magnetic surveys, macro-geology (in many cases today this data can be available in public domain)

» Within the basin (medium level of coarseness)

– coarse seismic 2D grid, covers a wide area to show a potential accumulation (plus regional geology)

» With prospects identified

– Fine scale seismic

– drill an exploration well (only drilling of an exploration well proves the validity of the concept)

Exploration Methods & Techniques


Exploration activities are potentially damaging to the environment.» eg, cutting down of trees in preparation for an

onshore seismic survey may result in soil erosion in the future

» Offshore, fragile ecological systems (eg, reefs) can be permanently damaged by spills of crude or mud chemichals.

» Responsible companies have to carry out an Environmental Impact Assessment before the activity planning and draw up contingency plan in the case of an accident happening.

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Timing of different exploration methods


Geophysics

Gravity surveys

Magnetic surveys

Seismic surveys

3D and 4D surveys

5


Gravity surveys

r

F

M

m

Newton’s law

222

2

311

2

,,

10670.6

zyxr

zyx

skg

mG

rr

mMG

r

r

rF

Acceleration of mrr

MG

m

rFa

2

1

x

y

z


Direct reconstruction of the density distribution

x

V

y

V

dVG 3

)()(

xy

xyyxa y

Solution of this integral equation is sensitive to errors –Difficult to solve

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Corrections

Correction for latitude» Gravity varies with latitude

Free-air correction» Gravity varies with altitude

Bouguer anomaly correction» Influence of the mass of rock between the

survey station and a reference datum (usually sea level)


Gravimeter (Gravity Meter)

Accuracy within 1 µGal.

L&R Aliod G MeterMass and spring gravity meterhttp://www.earthsci.unimelb.edu.au/ES304/MODULES/GRAV/NOTES/spring.html

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The gravity method measures small (~10-6g) variations of the Earth’s gravity field caused by density variations in geological structures.

The sensing element is a spring balance. Variations in the Earth’s gravity field cause changes in the length of the spring which is measured (gravimeter).

The gal, sometimes called Galileo (Gal) is a unit of acceleration for a gravitational field. 1Gal=1cm/sec2=10-2m/sec2.


Magnetic surveys Measure changes in the Earth magnetic field caused by

variations in the magnetic properties of rocks» Airborne - Both fluxgate and proton precession magnetometers

can be mounted within or towed behind aircraft, including helicopters. These so-called aeromagnetic surveys are rapid and cost effective. When relatively large areas are involved, the cost of acquiring 1 km of data from an aeromagnetic survey is about 40% less than the cost of acquiring the same data on the ground. In addition, data can be obtained from areas that are otherwise inaccessible. GPS is used to fix the positions of the aircraft.

» Shipborne - Magnetic surveys can also be completed over water by towing a magnetometer behind a ship. Marine magnetic surveying is slower than airborne surveying. Efficient together with other geophysical methods from the same ship.

» Ground Based - Like gravity surveys, magnetic surveys are also commonly conducted on foot or with a vehicle. Ground-based surveys may be necessary when the target of interest requires more closely-spaced readings than are possible to acquire from the air.

http://www.earthsci.unimelb.edu.au/ES304/MODULES/MAG/NOTES/fieldmodes.html

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Example of magnetic survey

http://www.ga.gov.au/education/minerals/magsurv.html


Example (cont)

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Catalogue of typical

situations

Calculate the gravity and magnetic signatures of typical geological structures


Seismic surveys Basics

» Types of stress waves

» Reflection of wave from the boundary

Surveys» Land

» Sea

Examples

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Seismic surveys involve the generation of artificial shock waves which propagates through ‘overburden’rocks to the reservoir and beyond, reflects back to the receivers.

Receivers register waves as a pressure pulse (hydrophones – offshore) or as acceleration(geophones – onshore).

The objective of seismic surveying is to produce an (acoustic) image of the subsurface with as much resolution is possible, with all the reflections correctly positioned and the image is as close to the real geological picture as possible.» in exploration for determining structures and traps

to be drilled» in field appraisal and development for estimation

of reserves and plans of field development» during production for observing movement of

contacts, distribution of reservoir fluids and changes in pressures


Basics

Types of stress waves» P-wave (pressure wave)

» S-wave (shear wave)

» Rayleigh wave (surface wave)

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Wave equations. Planar wave

xz

y

uxuz

uy

Wave front

Direction of wave propagation

P-waveS-waves0

1

01

01

2

2

22

2

2

2

22

2

2

2

22

2

t

u

cx

u

t

u

cx

u

t

u

cx

u

z

S

z

y

S

y

x

P

x

∑/∑y=∑/∑z0


Non-planar wave front

Locally the front can be replaced with a tangent plane

Plane waves

x

z

y

uxuz

uy

Wave front


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Rayleigh wavesWaves near the boundary of a semi-

space


Direction of particle motion

Dec

ays

expo

nent

iall

y

Velocity


Reflection of wave from the boundary

Depth(geophone at the source location)

21tv

D

v1 – average wave velocity in rockst – two-way (there and back) travel time

III

II

v2

v1

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Velocity-density relationship for sedimentary rocks


Arrays Bundles of geophones on a streamer

Energy source» Land

– Explosives

– Dropping heavy weight

– Vibrating plate

» Sea– Electric sparker (implosion, shallow depths)

– Air gun (bubble of compressed air, up to 5km)

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Land (Hyne, 2001)

GeophoneVibrator truck

A geophone is a small, cheap instrument for measuring ground motion.


Sea

6 km

Hydrophones on cable

(Hyne, 2001)

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Marine Seismic Surveys


Wave Source, Receiver and Recording Equipment

Survey vessel» Tows 12 to 16, 3000 to 8000 m long

hydrophone streamers spaced 50 to 100 m apart

» Survey speed 5 knots

» 68 crew members

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Airguns use compressed air to generate bubbles which emit sound as they expand

They are often used in an array to create the required amplitude and frequency range



Hydrophone arrays» Each streamer is made up of hundreds

of groups of 12 to 24 hydrophones» Streamers are towed at a depth of 6 to

10m

Streamer SteererStreamer Spool Hydrophone

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Time Structure Map

Seismic Survey Lines and Times

X 1.205

X 1.205

X 1.205

X 1.205

X 1.205

X 1.505

X 1.505

X 1.485

X 1.485

X 1.215

X 1.215

X 1.215

X 1.215

X 1.215

X 1.225

X 1.225

X 1.225

X 1.225

X 1.225

X 1.225

X 1.505

X 1.485

Equal times are joined by contours

X 1.205

X 1.205

Time Structure Map

H

- Fault Edge

X 1.215


Time Structure Map LEGENDRE

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Time-Depth conversion

From the Time Structure map it is possible to create a Depth Structure map by utilising the velocity profile which was created during the data processing procedures


Depth Structure Map

Time Structure Map

H

+ Velocity Profile

Increasin

g

Velocity

2.0 km/s

2.5 km/s

3.0 km/s

3.5 km/s

4.0 km/s

= Depth Structure Map

H

19


Depth Structure Map LEGENDRE


LEGENDRE

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Hydrophone

A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change.


Velocity determination

S

Velocity

21 Sv

22

v

DZ

21


Example

(Hyne, 2001)


3D and 4D surveys

3D surveys» Combinations of 2D surveys into a 3D

picture

4D surveys» Succession of 2D and 3D surveys at

intervals of time during which it is expected that the variations in wave propagation can occur (eg, due to production)

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Borehole geophysics

Boreholes» Exploration

» Production

Logging» Measurements in borehole


Production boreholes

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Logs

Sonic (acoustic) log

Electric logs

Radioactivity logs

Porosity logs in combination

Dielectric log » Dielectric constant

» Porosity and saturation

Dipmeter log and borehole imaging

Mud logs


Sonic (acoustic) log

Used in open uncased boreholes

Determines rock porosity, , by measuring the wave velocities

maf

ma

tt

tt

log

Interval transit time from the log

Interval transit time of the rock

Interval transit time of the fluid

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Typical wave velocities


Electric logs

Spontaneous Potential (self potential, SP) logs» To delineate permeability

zones

Resistivity logs» Quantification of

hydrocarbon saturation

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Basic arrangement for the SP log


Spontaneous Potential (self potential, SP) logs

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Resistivity logs


Radioactivity logs Gamma-Ray log

» Uses scintillation counter to measure natural radioactivity of rocks (in API units)

» Lithological identification Neutron log

» Rock is bombarded by neutrons» As a result, gamma-rays are emitted in proportion to

hydrocarbon content (HC+neutron → -ray) Density log (gamma-gamma tool)

» Tool emits gamma-rays and measures gamma-rays returned from formation. This depends upon density. Knowing density of dry rock and density of fluid, the porosity can be recovered (HC+ -ray → attenuation of -rays)

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Natural radioactivity of rocks


Porosity logs in combination Sonic (acoustic) log → porosity Electric logs → porosity Radioactivity logs → porosity Dielectric logs (electromagnetic wave propagation,

salty water – bad dielectric, dielectric constant in salty water < than in fresh water < HC; cf. resistivity of salty water is low, higher in fresh water and HC)→porosity

Combination» The three types of porosity measurements are differently

influenced by factors:– Lithology– Clay content– Presence of gas

» Combination increases accuracy

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Dipmeter log and borehole imaging

Dipmeter» Multi-arm micro-resistivity log» Measures direction of dip of beds adjacent

to borehole

Formation MicroImager» Large numbers of micro-resistivity probes » Imaging through statistical analysis

(synthesises an image of lithology of a borehole face by using dipmeter log)


Dipmeter

4 pad 4 track dipmeter

Locations of a, b, c, d –peaks on resistivitycurves give location of bedding plane (boundary between different rocks. Boundary does not conduct electricity well – high resistivity.)

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Formation MicroImager

Unrolled format Cylindrical format


Mud Logs

Drilling rate» Information about lithology

» Qualitative indication of porosity

Investigation of cuttings lifted with mud» Traces of hydrocarbons

Gas detector

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Mud logs

(Jahn et al., 2007)


Summary of exploration objectives and methods

(Jahn et al., 2007)

Exploration requires integration of different techniques and disciplines

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Exploration Methods & Techniques

Definition of a good reservoir Now: How to find a reservoir (“good” or

otherwise!!)» Find a basin (coarse scale)

– gravity surveys, magnetic surveys, macro-geology

» Within the basin (medium level of coarseness)

– coarse seismic

» Now, With prospects identified– drill an exploration well

drill an exploration wellmake well-bore measurements

Documents

Petr 2510 Lectures 2