1 spe distinguished lecturer program primary funding is provided by the spe foundation through...
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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 Programwww.spe.org/dl
Core Analysis: A Guide to Maximising Added Value
Colin McPhee
Senergy (GB) Limited
Society of Petroleum Engineers Distinguished Lecturer Programwww.spe.org/dl
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Why core matters…
• 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”
Bob Harrison, JPT Technology Focus, August 2009
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Why core analysis matters - volumetrics
Oil initially in place OIP
Gross rock volume GRV
Net to Gross N/G
Porosity
Water saturation Sw
Formation volume factor Bo
0
11
BSw
G
NGRVOIP
Logs, welltests, CORE
Logs, CORE
Logs, CORE
PVT
Geophysicist Geologist Reservoir EngineerPetrophysicist
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Why core analysis matters – reserves
• Recovery factor depends on technical and economic factors
• Recovery factor is partly defined by formation’s relative permeability– from CORE
o
w
rw
row
kk
f
.1
1
Welge fractional flow equation
RFOIPRESERVES *
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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
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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
n
m Rt
RwaSw
1
tortuosity constanta=1 unless core says otherwise
formation water resistivity
saturation exponentfrom core
true formation resistivityfrom logs
porosity exponentfrom core
porosityLogs – calibrated by core
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Porosity error – excess brine
• Correct for excess brine in annulus between core and coreholder test sleeve
• Otherwise … porosity too low
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Porosity errors - impact
Lab B: -7% error in log
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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
wR
RF 0
mF
1
0R
RI t
nSwI
1
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Correcting for excess brine
• Formation factor (F) tests at stress on tight sand
mw
true
R
RF
10
mean - 30% error in ambient ‘m’
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Correcting for excess brine
• Resistivity index (I) tests at ambient
true
ttrue R
RI
0
)log(
)log(
0true
t
R
Rn +15% error in ambient ‘n’
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Grain loss– material balance
- =
Before test After test
Grain loss
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Grain loss correction
1
10
100
0.01 0.1 1
Water Saturation (Fraction)
Res
isti
vity
In
dex
(R
atio
)
Corrected
Uncorrected
Porosity: 16%Water Density: 1.05 g/ccInitial Dry Weight: 140.6 g
Total Grain Loss: 2.7 g (2%)
Grain loss correction required
Water saturation (v/v)0.01 0.1 1
1
100
Res
istiv
ity I
ndex
-20 saturation unit error in Sw
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Impact of errors on OIP
• Uncertainty analysis– North Sea reservoir– 20% and 20% Sw– 100 MMbbl OIP– +20% error in input data
• Largest impact , m and n (core)
-30 -20 -10 0 10 20 30
phi
m
n
Rw
Rt
MMBBL
n
m Rt
RwaSw
1
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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