The Effect of Wellbore Storage and Skin on Interference Test Data
Authors: Tongpenyai Y. , Raghavan Rajagopal 
Published: December 2007

Summary
A solution for analyzing interference test data influenced by wellbore storage and skin effects at the active and observation wells is presented. The results presented here can be used to analyze and/or design presented here can be used to analyze and/or design interference test data influenced by storage and skin effects at the active and/or observation wells.

Introduction
Interference tests have become popular due to the need for information about reservoir heterogeneity. The advent of sensitive and high-precision pressure gauges has hoped to make this kind of test procedure a practical tool. Estimates of the mobility-thickness product kh/ and the storativity-thickness product product kh/ and the storativity-thickness product Cth usually are obtained from interference test data. In addition, as already mentioned, the degree of heterogeneity in the reservoir can be obtained from an interference test.Most investigators in this area of pressure analysis have directed their efforts toward examining the effect of reservoir heterogeneities on the observation well response. A few investigators have examined the effect of wellbore conditions at the flowing well -i.e., the effects of wellbore storage and skin and vertical fractures. Surprisingly, the effect of wellbore conditions at the observation well has not received attention. Virtually all studies have assumed that the observation well is shut in at the sandface and that plane radial flow conditions govern the measured pressures. However, if wells are shut in at the surface (which is usually the case), the wellbore storage and skin effects at the observation well should affect the pressure response.In this work, we examine the effects of wellbore storage and skin at the observation well on the pressure response. A new analytical solution to the pressure response. A new analytical solution to the problem is presented. The solution includes the problem is presented. The solution includes the effects of wellbore storage and skin at both wells.To our knowledge, only Fenske and Prats and Scott have considered the effect of wellbore storage at the observation well. Prats and Scott briefly examined the effect of wellbore storage at the observation well on pulse test data, and Fenske considered the influence of wellbore storage and both wells on interference test data.The results given here, however, are more comprehensive and complete. Unlike the earlier studies we have considered the influence of the skin region around both wells, as well as wellbore storage, and have been able to correlate the results in terms of dimensionless groups commonly used in well test analysis. This is a unique feature of our study. The dimensionless groups reduce the number of computational runs and type curves that are needed to analyze the data. In this regard our work provides a significant improvement over Fenske's results because we have been able to combine the parameters of interest and obtain correlations of general utility. Furthermore, as mentioned earlier, this solution also can be used when the wellbore storage and skin effects exist at only one of the wells (active or observation).

Assumptions and Mathematical Model
The mathematical model considered here assumes the flow of a slightly compressible (small pressure gradients) fluid in a homogeneous, uniform, and isotropic porous medium. Gravitational forces are assumed to be negligible. The reservoir is assumed to be infinitely large - i.e., the outer boundaries have no effect on the pressure response. The initial condition assumes that the pressure Pi is constant at all points in the reservoir.
JPT
P. 151