1. A Semianalytical p/z Technique Abnormally Pressured Gas Reservoirs
SPE 71514
A Semianalytical p/z Technique
for the Analysis of
Abnormally Pressured Gas Reservoirs
Ronald Gunawan Gan,
VICO Indonesia
and
T. A. Blasingame,
Texas A&M University

2. Objective To present a new technique that can be used to :
Calculate gas-in-place for an abnor-
mally pressured gas reservoir using
only average reservoir pressure and
cumulative production data.
Calculate pore volume compressibi-
lity as a function of reservoir pressure.

3. Presentation Outline Introduction Overview of Existing Methods
New Method
Field Examples
Conclusions

4. p/z schematic for a normally-pressured
Introduction
p/z schematic for a normally-pressured
volumetric gas reservoir
p/z Gp G

5. p/z schematic for an abnormally-pressured
Introduction
p/z schematic for an abnormally-pressured
gas reservoir
p/z Gp
Gapp G

6. Introduction Reasons for the non-linear p/z behavior:
Rock and water compressibility effects —
"rock collapse theory" (Hawkins, 1969)
Shale water influx (Bourgoyne, 1989)

7. Existing Methods Methods based on presumed knowledge of
system compressibility:
Hammerlindl (Constant Compressibility), 1971
Ramagost & Farshad (Constant Comp.), 1981
Yale et al. (Variable Compressibility), 1993

8. Methods based on presumed knowledge of
system compressibility (continued)
Fetkovich, Reese, and Whitson
- Derived General Material Balance Eq.
- Define cumulative effective compressibility,
- ce represents the cumulative change in
hydrocarbon PV caused by compressi-
bility effects (and water influx).

9. Methods which do not require a prior
knowledge of system compressibility
Roach
- very sensitive to initial pressure.
- method sometimes doesn’t exhibit
a negative intercept (which is not possible).
Bernard
- using Least Squares approach.
- very sensitive to data scatter.
Ambastha : Type Curve Approach
- non-uniqueness problems.

10. New Method Satisfies both "rock collapse" and
"shale water influx" theories
Develops 2 new plotting functions: 1. 2.
Requires production data only (p and Gp)

11. New Method Uses general material balance equation
(proposed by Fetkovich, et al.)
Rearranging, we obtain

12. New Method Calculate the ce(pi-p) function for
each p/z versus Gp trend
ce(pi-p) = ??? Gp
p/z G
Gapp
ce(pi-p) = ???

13. New Method For early time data (1st straight line) :
For late time data (2nd straight line) :
where: A is the inflection point

14. New Method Plot of log ce(pi-p) versus (p/z)/(pi/zi): log ce(pi-p)
inflection point
G/Gapp=0.6
G/Gapp=0.7
G/Gapp=0.8

15. New Method Plot of log ce(pi-p) versus (p/z)/(pi/zi) : log ce(pi-p)
inflection point

16. New Method (p/z)/(pi/zi) Gp/G 1 Infl. Point: GpA/G, (p/z)A /( pi /zi )1
Gp/G

17. New Method (p/z)/(pi/zi) Gp/G 1 G/Gapp=0.6 Inflection point h1
Gp/G

18. New Method 1
G/Gapp=0.8
Inflection point h
(p/z)/(pi/zi) 1
Gp/G

19. New Method Dynamic Type Curve Matching.
Automatic Matching using SOLVER m(Excel function for non-linear regression).

20. New Method Data required for analysis: Fluid property data
Initial Reservoir p and T
p and Gp data

21. Easy to use - especially for analysis
New Method
Computer program:
Visual Basic Application in MS Excel
Only requires MS Excel
Easy to use - especially for analysis

22. Data Analysis Sheet

23. Example 1: G is too low

24. Example 1: G is too high

25. Example 1: Correct G

26. Example 2: Long transition period

27. Example 3: Early time data

28. Example 4: Synthetic Dry Gas Case

29. Example 4: Backcalculated cf
Procedure to calculate cf vs. p from
production data:
1. Get
from type curve matching
2. Use the following equation to calculate :
3. Calculate cf (p):

30. Example 4: Backcalculated cf

31. Conclusions We have developed a straightforward
approach for analyzing p/z versus Gp
behavior for abnormally pressured gas
reservoirs — the approach considers
that two straight-lines must be ob-
served on the p/z plot.
The proposed method determines
gas-in-place without using system
compressibility data. Only p, Gp, and
fluid property data are required.

32. Conclusions (continued)
Our approach of using ce(pi-p) versus
(p/z)/(pi /zi) and (p/z)/(pi /zi) versus Gp/G
as dynamic type curve matching func-
tions has been shown to work extreme-
ly well.
Using our new method, it is possible to
calculate rock compressibility as a func-
tion of pressure from p and Gp data

33. Conclusions (continued)
The "dynamic type curve matching technique" used for calculating gas-in-place from production data is more representative (and more stable) than the non-linear optimization method provided by SOLVER.

34. A Semianalytical p/z Technique Abnormally Pressured Gas Reservoirs
SPE 71514
A Semianalytical p/z Technique
for the Analysis of
Abnormally Pressured Gas Reservoirs
Ronald Gunawan Gan,
VICO Indonesia
and
T. A. Blasingame,
Texas A&M University