Gas Material Balance

1. Gas Material Balance

2. Outline: • Volumetric depletion reservoir. • Water drive mechanism. • Burns et al method. • Gas material balance exercise.

3. Volumetric depletion reservoir • The term volumetric depletion reservoir applies to the performance of a gas reservoir in which water influx due to pressure decline is insignificant. • Volume of the hydrocarbon remains constant and can be calculated by the following equation. • As the pressure decline from the initial reservoir pressure for a given volume of production Gp material balance equations can be written. Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

4. Volumetric depletion reservoir Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

5. Volumetric depletion reservoir • The assumption that hydrocarbon pore volume is constant is problematic. • Connate water saturation expansion. • Grain pressure increases due to fluid pressure reduction. Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

6. Volumetric depletion reservoir Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

7. Volumetric depletion reservoir Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

8. Water drive reservoir • If the reduction in reservoir pressure leads to water influx into the reservoir material balance equation is modified. Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

9. Water drive reservoir • is the fraction of the hydrocarbon which is flooded by the water. • The effect of water influx is to maintain the pressure. • Material balance equation for water drive mechanism in gas reservoir is a non linear equation. • A mathematical model needs to be defined to interpret history matching and prediction. • Aquifer fitting. • If the aquifer is the same size of reservoir then a simple mathematical model can be applied. Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

10. Water drive reservoir • If the production history of the reservoir is available Burns et al proposed the following method. • Step1: Depletion material balance ( apparent gas in place). • Step 2: corrected value of the gas in place can be calculated by the following formula: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

11. Water drive reservoir Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

12. Exercise: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

13. Exercise: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

14. Solution: • GIIP: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

15. Solution: • Pressure at GWC: • Temperature at GWC: • Z factor: 0.888 • Gas formation factor at GWC: • Gas pressure gradient: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

16. Solution: • Gas pressure at centroid: • Temperature at centroid: • GIIP: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

17. Solution: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

18. Solution: • Step1: calculate cumulative gas production until the pressure reduced to 1200 psi. When P=1200 psi, Z=0.832. • Step2: cumulative gas production in the build up period: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

19. Solution: • Step3: Cumulative gas production in plateau period: • Step4:Time in which reservoir can produce at the rate of 100 MMscf/d: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

20. Solution: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development

21. Solution: Fundamentals of Reservoir Engineering, L.P. Dake , Shell Learning and Development