SPE Distinguished Lecturer Program
Primary funding is provided by
The SPE Foundation through member donations
and a contribution from Offshore Europe
The Society is grateful to those companies that allow their
professionals to serve as lecturers
Additional support provided by AIME
Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dl
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Core Analysis: A Guide to
Maximising Added Value
Colin McPhee
Senergy (GB) Limited
Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dl
Why core matters…
Bob Harrison, JPT Technology Focus, August 2009
• Core….
–
–
–
–
“confirms lithology and mineralogy
calibrates estimates of fundamental rock properties
shows how fluids occupy and flow in pore space
supplies mechanical properties for faster & safer
drilling and better completions”
• “Logs cannot characterize a reservoir if
knowledge of the rock is absent”
• “a struggle to convince management that the
project benefits from the knowledge gained”
3
Why core analysis matters - volumetrics
N
1
OIP  GRV     1  Sw 
G
B0
Geophysicist
Geologist
Petrophysicist Reservoir Engineer
Oil initially in place
OIP
Gross rock volume
GRV
Net to Gross
N/G
Logs, welltests, CORE
Porosity

Logs, CORE
Water saturation
Sw
Logs, CORE
Formation volume factor Bo
PVT
4
Why core analysis matters – reserves
RESERVES  OIP * RF
• Recovery factor depends on technical and
economic factors
• Recovery factor is partly defined by
formation’s relative permeability
– from CORE
fw 
1
k ro  w
1
.
k rw  o
Welge fractional flow equation
5
Core data – do we get value?
• The “ground truth” for formation evaluation
• But….
• Lab
– variable lab data quality and method sensitivity
– poor lab reporting standards
• End user
– inadequate planning and inappropriate design
• Have undermined value from core analysis
6
Core data – do we get value?
• Review of > 20,000 SCAL measurements
• 70% of legacy data is unfit for purpose
• ~ $10,000,000 data redundancy cost
• Examples of unreported lab artifacts
– porosity, Sw, and capillary pressure
• Impact on hydrocarbons in place
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Example - Archie water saturation, Sw
formation water resistivity
tortuosity constant
a=1 unless core says otherwise
 a Rw 
Sw   m

 Rt 
saturation exponent
from core
true formation resistivity
porosity
Logs – calibrated by core
1
n
from logs
porosity exponent
from core
8
Porosity error – excess brine
• Correct for excess brine in annulus
between core and coreholder test sleeve
• Otherwise … porosity too low
9
Porosity errors - impact
Lab B: -7% error in log 
10
Excess brine – resistivity tests
• Ambient ‘m’ and ‘n’
– core must be fully saturated
– excess brine on plug surface
– Formation factor (F):
• resistivity (R0) too low
– ‘m’ too low
– Resistivity index (I)
• Rt unaffected
– ‘n’ too high
R0
F
Rw
F
Rt
I
R0
1
I n
Sw
1
m
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Correcting for excess brine
• Formation factor (F) tests at stress on tight sand
mean - 30% error in ambient ‘m’
R0 true 1
F
 m
Rw

12
Correcting for excess brine
• Resistivity index (I) tests at ambient
Rt
I true 
R0true
n
log( Rt )
log( R0true )
+15% error in ambient ‘n’
13
Grain loss– material balance
Before test
After test
Grain loss
-
=
14
Grain loss correction
100
100
Resistivity Index (Ratio)
Resistivity Index
Grain
loss correction
-20
saturation
unitrequired
error in Sw
Corrected
Uncorrected
10
1
1
0.01
0.01
Porosity: 16%
Water Density: 1.05 g/cc
Initial Dry Weight: 140.6 g
Total Grain Loss: 2.7 g (2%)
0.1
0.1
Saturation (Fraction)
Water Water
saturation
(v/v)
11
15
Impact of errors on OIP
Effect of 20% Errors in input
Parameters to Archie Equation on
Model STOIIP of 100 MMBBL
• Uncertainty analysis
– North Sea reservoir
– 20%  and 20% Sw
– 100 MMbbl OIP
– +20% error in input data
• Largest impact
– , m and n (core)
 a Rw 
Sw   m


Rt


1
n
MMBBL
-30
-20
-10
0
10
20
30
phi
m
n
Rw
Rt
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Pc curve distortion
•
•
•
•
Mercury injection capillary pressure (MICP)
Pre-1994: tests on 50 – 80 ml plugs
Now: most tests on < 10 ml “chips”/end trims
Pc curve (Sw versus Pc) problems
– Use Hg-filled pore volume (> 20,000 psi)
– clay destruction in small pores
– distorted Pc curves
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