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

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.

Outline Introduction Overview of Existing Methods New Method Field Examples Conclusions

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

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

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

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

Methods Based on a Knowledge of System Compressibility (continued) Fetkovich, Reese, and Whitson - 1991 - 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.

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

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

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

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

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

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

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

Input Data Sheet

Calculation Sheet

Data Analysis Sheet

Example 1: GIP is too low

Example 1: GIP is too high

Example 1: Correct GIP Case

Example 2: GOM reservoir

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.

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