PETR 464 Material Balance: Dry Gas

1. PETR 464Material Balance: Dry Gas

2. Basic Definition • Application of the Law of Conservation of Mass to oil and gas reservoirs (Original hydrocarbon mass)-(produced hydrocarbon mass)=(remaining hydrocarbon mass)

3. Material Balance Model Gp GBgi (G-Gp)Bg Initial Conditions P=Pi Later Conditions P<Pi

4. Basic Uses • Understand reservoir performance • Identify drive mechanism • Predict future performance • Estimate OGIP • Predict productions rates/pressure decline • Estimate ultimate recovery

5. Basic Concepts Measured pressure Cumulative gas Production @ P Gas property, f(T,P,g) GRM-Engler-09

6. Steps • Collect data: • Initial reservoir pressure • Various reservoir pressures throughout the life of the well • Associated cumulative production at each pressure • Determine z-factor for each pressure • Calculate P/z • Plot P/z versus GP • Draw a straight line through the data points. • Extrapolate the straight line to P/z = 0 • Obtain estimate of OGIP @ P/z=0: Gp must equal G • Estimate ultimate recovery @ abandonment pressure

7. Example 1 Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 8 Step 7

8. Variations • Water drive reservoir vs. gas expansion • Abnormally pressured reservoirs (formation compressibility) • Pressure measurements/calculations • Low permeability • Retrograde gas reservoir

9. Comprehensive Gas Material Balance Geopressured component Gas in solution Water drive component Gas injection GRM-Engler-09

10. Water Drive Reservoirs Cumulative water influx, rcf Cumulative water production, stb RF (water drive) < RF (depletion) 45 to 75% >75% GRM-Engler-09

11. Gas expansion + Formation compaction + Water expansion (p/z)i p/z Gas expansion Overestimate of G Gp Abnormally Pressured Reservoirs Where, GRM-Engler-09