ABSTRACT
This type of decline may take place in reservoirs where gravity drainage controls the production. Gravity drainage exists in tilted reservoirs where oil production is affected by drainage of oil from upstructure to downstructure which causes segregation of gas and oil in the reservoir. The cumulative production can be obtained by integrating Equation (13.25) with respect to time:
But from Equation (13.26):
Substitute in Equation (13.26)
(13.30)
(13.31)
A graphic harmonic decline analysis can be obtained by writing Equation (13.25) as:
1/qj(1 + ajt) = 1/q (13.32)
Plotting 1/ q versus t on Cartesian coordinates should result in a straight line, with a/qj being the slope and 1/qj the intercept with 1/q-axis. From
the slope aj can be known. Also Equation (13.31) can be rewritten in a different form:
In qj + (a/qj)Np = In q (13.33) A plot of q versus Np on a semilog paper will result in a straight line
with slope being aj / qj and intercept qi' This straight line can be extrapolated into the economic limit to calculate the reserve.
