1. PETROPHYSICS: ROCK/LOG/SEISMIC CALIBRATION
John T. Kulha
Petrophysical Consultant
July 2016

2. Petrophysical Evaluation

3. Petrophysics: data acquisition
Acquire digital log files in LAS format
lithology: spontaneous potential, gamma ray
resistivity: normal, lateral, induction
porosity: density, neutron, compressional and shear acoustic
others: caliper, microlog, cased-hole gamma ray/neutron, LWD/MWD, images, NMR
Review paper copy log data
verification of digital data
additions to existing digital data base (microlog, etc)
Acquire paper/digital copy of hydrocarbon logs
ROP, TG and CG curves; description of shows (fluorescence, cut, odor, etc); lithology
Rock data from multiple sources; petrophysical measurments

4. Petrophysics: data preparation
Build continuous digital file of all log curves in measured depth
LAS files (LIS, DLIS, ASCII)
digitize or scan to infill any missing data
Enter log header and log run information
mud parameters (weight, resistivities)
maximum temperature
Enter paleontological, geological correlation markers, perforations, test and core intervals
annotations of test results, production, core description, etc
Enter core data into digital log file
porosity, permeability, grain density, fluid saturations, etc

5. Petrophysics: data preparation
Perform selected edits of invalid data
“hand” edits (between runs, casing points, obvious bad data)
edits using similar response curves (conductivity)
edits using synthetic curves (acoustic or density)
Depth merge log curves to resistivity
porosity (neutron, bulk density, sonic)
lithology (gamma ray, spontaneous potential)
depth correlation between neutron and bulk density!
Enter directional information and perform TVD calculations (if applicable)

6. Petrophysics: data preparation
Perform environmental corrections and conversions
borehole and mud weight to gamma ray
matrix conversion to neutron
bulk density/density porosity
shift spontaneous potential to a constant shale baseline
resistivity invasion
Normalization of key log curves
neutron, bulk density and acoustic, gamma ray
only if reasonable data coverage and distribution exists
consistent “normalization” lithology interval in all wells
review quality of resistivity (“strange profiles”)
consistency in regional shales and/or other lithologies

7. Petrophysics: model development
Determine shale volume for clastic and carbonate reservoirs using single curve and x-plot indicators
SP, gamma ray, neutron, bulk density, resistivity
neutron-density, neutron-sonic and sonic-density crossplots
average value for final shale volume
minimum value for final shale volume
calibrate to X-ray diffraction or other core-derived clay measurements
compare final composite shale volume curve to individual components
gamma ray and/or neutron-bulk density better in carbonates
calibrate with geological model
Use to calculate “effective” from total porosity

8. Petrophysics: model development
Determine total porosity
matrix porosity (macro- and micro-porosity)
secondary porosity: fracture, vugular
correct to effective porosity
Multiple porosity tools
compensated neutron log and bulk density (PHIND)
compensated neutron log and acoustic (PHINS)
compare PHIND with PHINS
compare acoustic and PHIND for secondary porosity
Single or two porosity tools
compensated neutron log and the bulk density (PHIND)
acoustic or density (PHIS, PHID)

9. Petrophysics: model development
Parameter selection (Rw, m, n, CEC, Qv, Rsh)
Formation water resistivity (Rw)
produced water: wireline, DST’s, intial tests or production
offset wells
water salinity catalogues
analysis of spontaneous potential, need mud filtrate resistivity value (Rmf)
Rw and cementation exponent (m): Pickett crossplot
total porosity versus resistivity (log-log crossplot)
apply in wet reservoirs; hydrocarbons won’t define Rw

10. Petrophysics: model development
Special core analysis for cementation (m) and saturation (n) exponents
rock catalogues with analogous rock types
empirical relationships
estimated from cuttings
Clay electrical properties for cation-exchange capacity water saturation model
clay type, amount and mode of distribution
analytical measurements
Log-derived shale properties for shale volume water saturation model
clay type and mode of distribution
average log values in adjacent or internal shales

11. Petrophysics: model development
Water saturation calculation (Archie model)
resistivity from deepest investigating curve (invasion?)
total porosity
formation water resistivity, Rw
cementation, m, and saturation, n, exponents
Water saturation calculation (shale volume models)
Modified Simandoux, Indonesian, others
effective porosity (shale values RHOB, NPHI, DT)
shale volume, porosity and resistivity

12. Petrophysics: model development
Water saturation calculation (cation-exchange capacity models)
Waxman-Smits, Dual Water (rigorous or simplified)
resistivity from deepest investigating curve (invasion?)
total porosity
cation exchange capacity
equivalent conductance
specific clay area
bound water resistivity
formation water resistivity
cementation, m (m*), and saturation, n (n*), exponents
shale volume and porosity

13. Petrophysics: model development
Pay distribution and reservoir properties are defined by cutoffs of key log curves
porosity
water saturation
shale volume
other curves, as needed (hydrocarbon pore volume, perm)
Cutoffs should be calibrated
producing intervals in wells
intervals that produce water
analogue data
consistent with permeability estimation
Reservoir property summation used in geological model, simulation, reserves and completion design

14. Petrophysics: example evaluation

15. Rock/Log/Seismic Calibration

16. Rock/Log/Seismic Calibration-1
Relationships between “rocks/logs/seismic”
rocks/logs/seismic must be calibrated to provide a complimentary and realistic geoscience and engineering evaluation
petrophysics and “petro-geophysics”
Rocks and logs
logs calibrated to rock type/reservoir quality
standard petrophysical relationships
porosity, water saturation, reservoir quality, permeability, pay
Rocks and seismic
seismic calibrated to rock type/reservoir quality
elastic variables
velocity, bulk density, acoustic impedance
Logs and seismics
seismic “calibrated” to corrected logs (rock type/reservoir quality)
synthetic elastic logs
lithology and fluid models
synthetic seismograms

17. Rock/Log/Seismic Calibration-2
ROCKS
PETROPHYSICAL
RELATIONSHIPS
ELASTIC
VARIABLES
VARIABLES
LOGS
SEISMIC
SYNTHESIZED
ELASTIC LOGS
SYNTHETIC
SEISMOGRAMS
SYNTHESIZED
LOG TRANSFORM
CALIBRATION
MODEL ROCK/FLUID
DISTRIBUTION
TIME/DEPTH SEISMIC SIGNATURE AVO
CALIBRATION

18. Rock/Log/Seismic Calibration-3
Recorded acoustic and bulk density logs
inaccurate: miscalibration, erroneous tool response, service company problems
incomplete: intervals not logged, data not recorded, data lost
unavailable: logs not run, data lost, old wells (pre-sonic and bulk density)
Synthetic acoustic and bulk density logs
generated from resistivity/conductivity (primary independent variable)
generated from shale volume (secondary independent variable)
generated as a function of depth, rock type, correlation interval, geologic age, pressure regime (tertiary independent variables)
applied a common regression algorithm to a maximum number of wells within a project area
Problems resolved by synthetic logs
borehole rugosity and erroneous measurements
effects of borehole alteration due to drilling fluid reaction and stress relief
different service companies’ tools and associated tool response variations
tool miscalibration; wrong log scales (paper logs)
incomplete or unavailable logs

19. Rock/Log/Seismic Calibration-4
Synthetic logs for in-situ conditions
effects of lithology and rock type change
wet reservoir response
hydrocarbon response
identify intervals of questionable measured data
Synthetic logs for modeled conditions
changes in lithology or reservoir quality
hydrocarbon fluid substitution
variations in reservoir quality and fluid content
Synthetic seismograms from synthetic logs
consistent character within a project area
provide usable well/seismic tie
provide synthetics where log data in not available or unusable

20. Rock/Log/Seismic Calibration-5
Correlate intervals of similar rock properties
rock properties related to wireline log responses
guided by regional depositional model and environments
guided by regional seismic correlation with preliminary log/seismic tie
incorporate mudlog lithology and/or independent cuttings descriptions
“overlook” intervals of questionable or missing log data
Create shale volume curve
lithology related wireline logs
gamma ray, spontaneous potential, neutron/density, resistivity
defined by either reservoir scale or seismic scale
flexible to accommodate changes in lithology
Build calibration data set
select intervals of acceptable measured log data
“experience” driven and supplemented with other discipline input
representative of all various independent variables
inclusive of multiple well data sets

21. Rock/Log/Seismic Calibration-6
Perform multivariable regression and calculate synthetic logs
shale volume (VSH) log
resistivity (RT) or conductivity (COND) log
Other log curves, NPHI, PE, mudlog curves(?)
depth, geologic age, depositional environment, pressure regime
regression coefficients, A1, A2, A3, A4
RHOB=A1+A2*(VSH)+A3*(LOG (RT), COND)+A4*(DEPT)
DT=A1+A2*(LF, VSH)+A3*(LOG (RT), COND)+A4*(DEPT)
may simplify to only two variables
Compare synthetic logs with calibration data set
modify calibration intervals, other independent variables
repeat process until correlation with measured data or tie to seismic is acceptable
Perform fluid substitution or calibrate directly
calibrate directly to in-situ fluid conditions
calculate wet reservoir condition and then substitute hydrocarbons
selected intervals from representative wells

22. Example well: 4 intervals
2950’- 4000’
4400’- 5400’
T1: SP-blue; GR-black, VSH-red
T2: RD-red; RM-green
T3: RHOB-black; RHOB-ED, red; CALI-green
T4: DT-black; DT-ED-red
Yellow shading indicates amount of error between measured and synthetic (corrected) log curve
6100’- 7600’
8100’- 9750’