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Petr 2510 Lectures 2
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Transcript of Petr 2510 Lectures 2
1
Slide 1Dr Elena Pasternak
Petroleum Engineering Fundamentals
PETR2510
Exploration methods
Slide 2Dr Elena Pasternak
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
Slide 3Dr Elena Pasternak
Plan
Aim
Geophysics
Borehole geophysics (logs)
(Pictures, if not indicated otherwise, are copied from Selley, 1998 )
Slide 4Dr Elena Pasternak
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
3
Slide 5Dr Elena Pasternak
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
Slide 6Dr Elena Pasternak
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.
4
Slide 7Dr Elena Pasternak
Timing of different exploration methods
Slide 8Dr Elena Pasternak
Geophysics
Gravity surveys
Magnetic surveys
Seismic surveys
3D and 4D surveys
5
Slide 9Dr Elena Pasternak
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
Slide 10Dr Elena Pasternak
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
6
Slide 11Dr Elena Pasternak
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)
Slide 12Dr Elena Pasternak
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
7
Slide 13Dr Elena Pasternak
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.
Slide 14Dr Elena Pasternak
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
8
Slide 15Dr Elena Pasternak
Example of magnetic survey
http://www.ga.gov.au/education/minerals/magsurv.html
Slide 16Dr Elena Pasternak
Example (cont)
9
Slide 17Dr Elena Pasternak
Catalogue of typical
situations
Calculate the gravity and magnetic signatures of typical geological structures
Slide 18Dr Elena Pasternak
Seismic surveys Basics
» Types of stress waves
» Reflection of wave from the boundary
Surveys» Land
» Sea
Examples
10
Slide 19Dr Elena Pasternak
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
Slide 20Dr Elena Pasternak
Basics
Types of stress waves» P-wave (pressure wave)
» S-wave (shear wave)
» Rayleigh wave (surface wave)
11
Slide 21Dr Elena Pasternak
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
Slide 22Dr Elena Pasternak
Non-planar wave front
Locally the front can be replaced with a tangent plane
Plane waves
x
z
y
uxuz
uy
Wave front
Direction of wave propagation
12
Slide 23Dr Elena Pasternak
Rayleigh wavesWaves near the boundary of a semi-
space
Direction of wave propagation
Direction of particle motion
Dec
ays
expo
nent
iall
y
Velocity
Slide 24Dr Elena Pasternak
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
13
Slide 25Dr Elena Pasternak
Velocity-density relationship for sedimentary rocks
Slide 26Dr Elena Pasternak
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)
14
Slide 27Dr Elena Pasternak
Land (Hyne, 2001)
GeophoneVibrator truck
A geophone is a small, cheap instrument for measuring ground motion.
Slide 28Dr Elena Pasternak
Sea
6 km
Hydrophones on cable
(Hyne, 2001)
15
Slide 29Dr Elena Pasternak
Marine Seismic Surveys
Slide 30Dr Elena Pasternak
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
16
Slide 31Dr Elena Pasternak
Wave Source, Receiver and Recording Equipment
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
Slide 32Dr Elena Pasternak
Wave Source, Receiver and Recording Equipment
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
17
Slide 33Dr Elena Pasternak
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
Slide 34Dr Elena Pasternak
Time Structure Map LEGENDRE
18
Slide 35Dr Elena Pasternak
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
Slide 36Dr Elena Pasternak
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
Slide 37Dr Elena Pasternak
Depth Structure Map LEGENDRE
Slide 38Dr Elena Pasternak
LEGENDRE
20
Slide 39Dr Elena Pasternak
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.
Slide 40Dr Elena Pasternak
Velocity determination
S
Velocity
21 Sv
22
v
DZ
21
Slide 41Dr Elena Pasternak
Example
(Hyne, 2001)
Slide 42Dr Elena Pasternak
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)
22
Slide 43Dr Elena Pasternak
Borehole geophysics
Boreholes» Exploration
» Production
Logging» Measurements in borehole
Slide 44Dr Elena Pasternak
Production boreholes
23
Slide 45Dr Elena Pasternak
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
Slide 46Dr Elena Pasternak
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
24
Slide 47Dr Elena Pasternak
Typical wave velocities
Slide 48Dr Elena Pasternak
Electric logs
Spontaneous Potential (self potential, SP) logs» To delineate permeability
zones
Resistivity logs» Quantification of
hydrocarbon saturation
25
Slide 49Dr Elena Pasternak
Basic arrangement for the SP log
Slide 50Dr Elena Pasternak
Spontaneous Potential (self potential, SP) logs
26
Slide 51Dr Elena Pasternak
Resistivity logs
Slide 52Dr Elena Pasternak
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)
27
Slide 53Dr Elena Pasternak
Natural radioactivity of rocks
Slide 54Dr Elena Pasternak
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
28
Slide 55Dr Elena Pasternak
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)
Slide 56Dr Elena Pasternak
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.)
29
Slide 57Dr Elena Pasternak
Formation MicroImager
Unrolled format Cylindrical format
Slide 58Dr Elena Pasternak
Mud Logs
Drilling rate» Information about lithology
» Qualitative indication of porosity
Investigation of cuttings lifted with mud» Traces of hydrocarbons
Gas detector
30
Slide 59Dr Elena Pasternak
Mud logs
(Jahn et al., 2007)
Slide 60Dr Elena Pasternak
Summary of exploration objectives and methods
(Jahn et al., 2007)
Exploration requires integration of different techniques and disciplines
31
Slide 61Dr Elena Pasternak
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