Page 270 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Material Balance and Volumetric Calculations 239

The original pressure establishes values for B,, B and RG, and the pressure at
the given level of depletion establishes values of bo, Bg, and R,. Cumulative gas
production can be obtained from:
GP = XRsANp (5-160)

A procedure for using this method is [197]:
1. At a set value of reservoir pressure, assume the cumulative amount of gas
produced, Gp.
2. Calculate NP by Equation 5-159.
3. Calculate So using Equation 5-175.
4. Determine the relative permeability ratio based on the liquid saturation
from data such as that in Figure 5-55.
5. Calculate GP using Equations 5-160 and 5-163.
6. Compare the calculated GP with the assumed GP from Step 1.
7. If the assumed and calculated values do not agree to a satisfactory degree,
repeat the calculations from Step 1 for another value of Gp.
8. If the assumed and calculated Gp values agree, return to Step 1 and set a
new pressure.

The Tamer Method

This [250] is a trialand-error procedure based on the simultaneous solution
of the material balance equation and the instantaneous gas-oil ratio equation.
For a pressure drop from p1 to ps, the procedure involves a stepwise calculation
of cumulative oil produced (NP)* and of cumulative gas produced (Gp&. Several
variations in the procedure and equations are possible; the straightforward
procedure outlined by Timmerman [ 1801 is reproduced here.
1. During the pressure drop from p1 to ps, assume that the cumulative oil
production increases from (Np)l to (Np)*. At the bubble point pressure, NP
should be set equal to zero.
2. By means of the material-balance equation for Wp = 0, compute the
cumulative gas produced (GP)* at pressure pn:

3. Compute the fractional total liquid saturation (SJ2 at pressure pn:
(SL)* =s, +(l-S,)& [ 1-* ,,)I

Bm (5-1 62)

4. Determine the kdk, ratio corresponding to the total liquid saturation <SJ2
and compute the instantaneous gas-oil ratio at pn:

(5163)