1. PETR 464 Material 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 ModelGp
GBgi
(G-Gp)Bg
Initial Conditions P=Pi
Later Conditions P<Pi

4. Basic Uses Understand reservoir performance Predict future 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: 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

7. 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

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