1. TAD M. SMITH VERITAS EXPLORATION SERVICES
FLUID SUBSTITUTIONS AND FRAME PROPERTIES; EFFECTIVE USE OF A MODEL-BASED APPROACH
TAD M. SMITH
VERITAS EXPLORATION SERVICES

2. WELL MODELING
Water Depth: 3200 ft
Middle Miocene
Is this pay?

3. FLUID SUBSTITUTED CURVES

4. MODELED GATHERS INSITU (gas) OIL BRINE SAND 1 = BLUE SAND 2 = RED
TOP SAND 1
TOP SAND 2
INSITU (gas)
OIL
BRINE
SAND 1 = BLUE
SAND 2 = RED

5. GASSMANN’S EQUATION (Gwet = Gdry)
GASSMANN’S EQUATION RELATES THE SATURATED PROPERTIES OF ROCK TO ITS FRAME AND MATRIX PROPERTIES.
BEST AVAILABLE TOOL FOR PERFORMING FLUID REPLACEMENT MODELING.
(Gwet = Gdry)

6. TOPICS Uncertainties in application of Gassmann
Constraints on “dry frame” values
Using frame property models to solve (partially) difficult problems
shaley sands
tight gas sands
invasion corrections
porosity modeling
When do the models fail?

7. SOURCES OF UNCERTAINTY
Porosity
Fluid saturation in the formation (Sw)
Fluid properties in the formation
Matrix (i.e., lithology)
Fluid saturation in the invaded zone (Sxo)
Fluid distribution (homogeneous or “patchy”)
INCREASING UNCERTAINTY

8. OTHER SOURCES OF UNCERTAINTY
Quality of the measurements
Frequency dependence
Depth of investigation (convolving tools with different DOI’s and resolutions)
Well-bore condition
Circulation time
Tool configuration
Processing

9. CALCULATING THE “DRY” FRAME (Kfr)
Calculate
Algebraically eliminate
Empirical estimates/direct measurement*
*most core measurements for clean quartz sands, indicate that the ratio of Kfr/G  1 (PRfr = 0.125) 2
Two problems: 1) end up not calculating K*, which provides some valueable information, and 2) porosity is in the denominator.

10. CALCULATE
EXPLICIT
CRITICAL POROSITY (Nur, 1992)
Krief et al., 1990

11. CALCULATED FRAME PROPERTIES
Remember, for clean quartz sands, Kfr/G  1 (PRfr = 0.125)

12. CALCULATED FRAME PROPERTIES
Calculated using CP
Explicit calculation
Remember, for clean quartz sands, Kfr/G  1 (PRfr = 0.125)

13. RATIOS MOST THEORETICAL MODELS PREDICT SIMILAR RESULTS
EXPLICIT CALCULATION OF Kfr CAN BE PROBLEMATIC
DIRECT APPLICATION OF FRAME MODELS IS PROBLEMATIC
WHAT DO THE MODELS TELL US ABOUT RATIOS?
Krief et al. (1990)
Critical porosity
MOST THEORETICAL MODELS PREDICT SIMILAR RESULTS
(dry frame PR) IS INDEPENDENT OF POROSITY

14. VP calculated using Gassmann and pseudo Kfr
Pseudo Kfr (red curve)
VP calculated using Gassmann and pseudo Kfr
Kfr (calculated; green curve)
2 COMPONENT SYSTEM
VRH MIX OF END-MEMBERS

15. DO CORE MEASUREMENTS SUPPORT THIS MODEL?

16. PRfr = 0.25
PRfr = 0.125
Kfr (GPa)
PRfr = 0.08
G (GPa)

17. APPLICATIONS

18. FLUID SUBSTITUTION IN SHALEY SANDS

19. CORRECTING FOR NEGATIVE POISSON RATIOS
Negative PR values are commonly observed in tight gas sands
NEGATIVE PR VALUES DUE TO CRACKS IN THE ROCK MATRIX?
Effective medium modeling shows that Vp/Vs ratios in dry rock decrease as porosity becomes more “crack-like” (Katahara, 1999)

20. CORRECTING FOR NEGATIVE POISSON RATIOS

21. INVASION CORRECTIONS
DRY GAS SAND, DRILLED WITH OBM

22. UNDERSTANDING COMPLEX LITHOLOGY
NOTE THE LARGE VARIATION IN PRfr
Note that Kfr/G ratios vary from ~1 - >3

23. ASSUME A QUARTZ MATRIX MODELED VP LOWER THAN MEASURED VP
LOW Kfr/G RATIOS
< 1 FOR “CLEAN” ZONES

24. THIS MIGHT EXPLAIN THE WIDE RANGE IN PR*
Feldspar?
Quartz sand?
CROSS-PLOTTING AND MULTIMINERAL ANALYSIS INDICATES THAT FELDSPARS MIGHT BE COMMON.
THIS MIGHT EXPLAIN THE WIDE RANGE IN PR*

25. SCENARIO 2 ALLOW FOR COMPLEX LITHOLOGY RECOMPUTE VP
NOTE HIGH Kfr/G RATIO!

26. SOME EXPECTATIONS FOR SEDIMENTARY ROCKS
NOTE: Plagioclase feldspars are non-radioactive, and have a N-D response nearly identical to quartz
“DRY” FRAME POISSON’S RATIO
NOTE: for rocks with >65% quartz, PRfr values should be < ~0.2 1 .9 .8 .7 .6 .5 .4 .3 .2 .1
FRACTION OF QUARTZ

27. UNDERSTANDING COMPLEX LITHOLOGY
KEY POINT: VARIATIONS IN PRfr MAY BE INDICATIVE OF VARIABLE COMPOSITION IN THE ZONE OF INTEREST

28. OTHER USES Kdry G POROSITY MODELING PREDICTION OF BIOT COEFFICIENT+
NOTE: IT IS NOT SUFFICIENT TO SIMPLY ADJUST POROSITY IN GASSMANN’S EQUATION

29. STRESS ANISOTROPY (Xu, 2002) SANDS AT LOW NET EFFECTIVE STRESS
FAILURE MODES
CARBONATES
STRESS ANISOTROPY (Xu, 2002)
SANDS AT LOW NET EFFECTIVE STRESS
EFFECT OF GRAIN CONTACTS?

30. Kfr (GPa) G (GPa) PRfr = 0.125 PRfr = 0.29 PRfr = 0.08
Increasing Effective Stress
Kfr (GPa)
G (GPa)

31. CONTACT MODELS Winkler Hertz-Mindlin (PRfr = 0.008) Walton
- for pure quartz
Walton
(PRfr = 0.250)

32. OBSERVATIONS
Kfr/G ratios are sensitive to the composition of a rock, and may vary from ~1 to 3
It is not appropriate to simply assign a constant dry rock PR (or Kfr/G) ratio when applying Gassmann.
Will vary with composition
Rocks with high PRfr values are not uncommon
Utilize the explicit calculation of Kfr
May provide insight into the composition of the rock
High PRfr values may not be wrong

33. OBSERVATIONS
Use model-based Kfr/G ratios as a quality check against the explicit calculation for Kfr
Frame property models can be used to help solve difficult problems.
fluid substitutions in shaley sands
velocity problems in low porosity gas sands
p-wave invasion corrections
porosity modeling
Biot Coefficient