5.2 Ideal Pressure Buildup Test

In an ideal situation, we assume that the test is conducted in an infinite-acting reservoir in which no boundary effects are felt during the entire flow and later shut-in period. The reservoir is homogeneous and containing in a slightly compressible, single-phase fluid with uniform properties so that the Ei function and its logarithmic approximation apply. Horner's approximation is applicable. Wellbore damage and stimulation is concentrated in a skin of zero thickness at the wellbore. Flow into the wellbore ceases immediately at shut-in. If a well is shut-in after it has produced at rate q for time t _p and the bottom-hole pressure p _ws is recorded at time ?t, then a plot p _ws versus log (t _p + ?t) ?t will give a straight line, which is represented by the following equation:

where the slope m is 162.6 q ₀ ? ₀ ? ₀/ kh and p _i (initial reservoir pressure) is the intercept at (t _p + ?t)/ ?t = 1.0. The absolute value of the slope m is used in analyzing the test result. The formation permeability k can be calculated from the slope and given by