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

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.

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

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

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

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 presumed knowledge of system compressibility (continued) Fetkovich, Reese, and Whitson - 1991 - 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).

Methods which do not require a prior knowledge of system compressibility Roach - 1981 - very sensitive to initial pressure. - method sometimes doesn’t exhibit a negative intercept (which is not possible). Bernard - 1985 - using Least Squares approach. - very sensitive to data scatter. Ambastha - 1991: Type Curve Approach - non-uniqueness problems.

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)

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) = ??? Gp p/z G Gapp ce(pi-p) = ???

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 G/Gapp=0.6 G/Gapp=0.7 G/Gapp=0.8

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

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

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

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

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

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

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

Data Analysis Sheet

Example 1: G is too low

Example 1: G is too high

Example 1: Correct G

Example 2: Long transition period

Example 3: Early time data

Example 4: Synthetic Dry Gas Case

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):

Example 4: Backcalculated cf

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.

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

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.

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