In this paper we study the effect of wellbore storage and skin on the pressure buildup behavior. The closed square was used as the basic unit for analysis because (1) this geometry is quite common, (2) it is useful as a building unit for other practical geometries, and (3) it allows a study of the boundary effects.
The principle of desuperposition, as discussed by Gringarten, et al.7 was used to generate the pressure functions. The data used were already tabulated in the literature. This same principle can be extended to broader ranges of variables than reported here.
In general the Horner graph was found to be superior to the Miller-Dyes-Hutchinson graph for analyzing these data, so only Horner graphs are shown. Even for the Horner graph we found that the correct straight line slope could usually not be found; however, for most cases an approximately correct slope was found that was in error by only 5–10%. A simple empirical equation was developed which predicts the time for the start of that straight line.
It is well known that skin damage has no effect on the buildup when there is no wellbore storage. Skin does have an effect for the cases studied here. This effect lasts for a somewhat longer time than it does for drawdown. The producing time and the boundary are also important. When producing time is too short, storage effects may dominate the entire buildup curve, and when they are too long, the boundary may mask the proper straight line slope.