1. Abnormally Pressured Gas Reservoirs
A New p/z Approach
for the Analysis
Abnormally Pressured Gas Reservoirs
Ronald G. Gan
Thomas A. Blasingame
Texas A&M University

2. Objective To present a new technique to calculate
gas-in-place for abnormally pressured
gas reservoirs using only average reser-
voir pressure and cumulative production
data.

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

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

5. Abnormally-Pressured Gas Reservoir
Introduction
Abnormally-Pressured Gas Reservoir
p/z Gp
GIP

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 a Knowledge of System Compressibility (continued)
Fetkovich, Reese, and Whitson
- Derived General Material Balance Eq.
- Define cumulative effective compressibilty,
- ce represents the cumulative change in
hydrocarbon PV caused by compressi-
bility effects and encroaching water.

11. Existing Methods (Continued)
Methods Which Do Not Require a Prior
Knowledge of System Compressibility
Roach
- Very sensitive to Initial Pressure.
- Method sometimes exhibits a negative
intercept, which is not possible.
Bernard
- Uses Least Squares approach.
- Very sensitive to data scatter.
Ambastha: Type Curve Approach
- Non-uniqueness problems

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

13. New Method Calculate the ce(pi-p) function for
each p/z versus Gp trend:
ce(pi-p) = ???
p/z
ce(pi-p) = ???
GIP Gp
GIPapp

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

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

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

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

18. Input Data Sheet

19. Calculation Sheet

20. Data Analysis Sheet

22. Example 1: GIP is too low

23. Example 1: GIP is too high

24. Example 1: Correct GIP Case

25. Example 2: GOM reservoir

26. Conclusions A type-curve matching procedure has
been developed to analyze production
data from abnormally pressured gas
reservoirs.
The proposed method determines the
gas-in-place without using system
compressibility data. Only pressure, Gp,
and fluid property data are required.

27. Conclusions (continued)
It may be possible to calculate rock
compressibility or acquifer size
(i.e., the M value) using pressure
and Gp data.