# PETR 464 Material Balance: Dry Gas

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PETR 464 Material Balance: Dry Gas

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

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

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

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

Steps Collect data: Determine z-factor for each pressure Calculate P/z
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 P/z=0: Gp must equal G Estimate ultimate abandonment pressure

Example 1 Step 1 Step 2 Step 3 Step 4 Step 6 Step 5 Step 8 Step 7
P (psia) Gp (MMscf) z P/z 4000 0.8 5000 3500 2.46 0.73 4795 3000 4.92 0.66 4545 2500 7.88 0.6 4167 2000 11.2 0.55 3636 Step 1 Step 2 Step 3 Step 4 Step 6 Step 5 Step 8 Step 7

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

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

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

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