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Overestimation of OGIP in water-Drive gas reservoirs due to a misleading linear p/z plot

Overestimation of OGIP in water-Drive gas reservoirs due to a misleading linear p/z plot. M.H. El-Ahmady, R.A. Wattenbarger, Texas A&M University, and T.T. Pham, El-Paso Production Company June 13, 2001. Outline of Presentation. The uniqueness problem.

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Overestimation of OGIP in water-Drive gas reservoirs due to a misleading linear p/z plot

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  1. Overestimation of OGIP in water-Drive gas reservoirs due to a misleading linear p/z plot M.H. El-Ahmady, R.A. Wattenbarger, Texas A&M University, and T.T. Pham, El-Paso Production Company June 13, 2001

  2. Outline of Presentation • The uniqueness problem

  3. p/z plot for a volumetric reservoir p/z OGIP Gp

  4. Effect of water-drive on p/zplot p/z OGIP Gp

  5. What is the OGIP? p/z False straight line OGIP G’ Gp

  6. Non-Uniqueness Problem Early data may indicate - (a) large OGIP with no water influx or(b) smaller OGIP with water influx

  7. Outline of Presentation • The uniqueness problem • A Field example

  8. Field A (Gulf Of Mexico) • Early production data shows OGIP = 270 Bscf without water drive • Aquifer analysis showsOGIP = 110 Bscf with water drive

  9. Field A (Gulf Of Mexico)

  10. Zoomed view of p/z plot

  11. Field A (Gulf Of Mexico)

  12. Field A (Gulf Of Mexico) After more production: • p/z showed lower OGIP • One well “watered out” • The current estimate of OGIP is 120 Bcf , close to the early aquifer analysis (110 Bcf)

  13. Outline of Presentation • The uniqueness problem • A Field example • Simulation, const. prod. rate

  14. Identifying the problem

  15. Constant rate simulations

  16. A close-up of early time

  17. Water influx rate

  18. Outline of Presentation • The uniqueness problem • A Field example • Simulation, const. prod. rate • Simulation, false straight line

  19. What rates cause the false straight line? (simulation) • We give a value of G’ • Assume constant production rate for the first 6 months • p/z plot gives a perfect straight-line after 6 months.

  20. Data used for the Study

  21. Simulation of false straight line p/z False straight line OGIP G’ Gp

  22. Results

  23. Results

  24. Outline of Presentation • The uniqueness problem • A Field example • Simulation, const. prod. rate • Simulation, false straight line • Conclusions

  25. Conclusions • At very early times, the p/z plot extrapolates to the correct OGIP, even with water drive. (However, this is probably not practical because of data errors and lack of data at very early times).

  26. Conclusions • A false linear p/z plot may occur for a particular production rate schedule. This makes it impossible to determine OGIP with this rate schedule.

  27. Conclusions • The “non-uniqueness” production rate schedule generally increases, then peaks and decreases.

  28. Overestimation of OGIP in water-Drive gas reservoirs due to a misleading linear p/z plot M.H. El-Ahmady, R.A. Wattenbarger, Texas A&M University, and T.T. Pham, El-Paso Production Company June 13, 2001

  29. Equation for a closed reservoir

  30. Equation for a water-drive gas reservoir

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