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A Semianalytical p / z Technique for the Analysis of 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-

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A Semianalytical p / z Technique for the Analysis of Abnormally Pressured Gas Reservoirs

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

  5. p/z Gp Introduction • p/z schematic for an abnormally-pressured • gas reservoir 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 - 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).

  9. 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.

  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, etal.) • Rearranging, we obtain

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

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

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

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

  16. New Method 1 Infl. Point:GpA/G, (p/z)A /( pi /zi ) h (p/z)/(pi/zi) 1 0 Gp/G

  17. G/Gapp=0.6 Inflection point G/Gapp= 0.8 G/Gapp=1 New Method 1 h (p/z)/(pi/zi) 1 0 Gp/G

  18. Inflection point G/Gapp=0.8 New Method 1 h (p/z)/(pi/zi) 1 0 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. 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. 1. Get from type curve matching 2. Use the following equation to calculate : 3. Calculate cf (p): Example 4: Backcalculated cf • Procedure to calculate cf vs. p from production data:

  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. 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

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