Effect of Wellbore Storage and Wellbore Damage at the Active Well on Interference Test Analysis
Authors: Jargon J.R. 
Published: December 2007

This paper shows that wellbore storage and wellbore damage reduce the pressure response at an observation well, especially at early times, and pressure response at an observation well, especially at early times, and can cause great error in estimating transmissivity and storativity if not account for properly. New type curves are presented that include the effects of wellbore storage and damage.

Introduction

Interference testing is used to estimate the mobilitythickness product and the reservoir thickness -porositytotal compressibility product. These two terms are referred respectively to as transmissivity and storativity. In the past, wellbore storage and wellbore damage at the active well have been assumed to cause no detrimental effect on interference test analysis. Only one type curve has been used to analyze interference tests, namely the exponential-integral-solution type curve.

Interference testing has been used to estimate transmissivity and storativity in the petroleum and ground-water industries for many years. The mathematical basis for interference testing was first presented by Theis. The most practical and easiest way to analyze interference tests is by type-curve matching. The method of type-curve matching is adequately described by Ramey and Earlougher and Kersch. The type curve is a plot of log PD vs log tD/rD. Thus, for the classical exponential-integral solution, only one theoretical type curve is required to analyze all interference tests. A recent paper presented type curves for a vertically fractured presented type curves for a vertically fractured reservoir with drawdown measured at the observation well.

In single-well pressure transient tests, wellbore storage and wellbore damage have been recognized as adversely affecting the tests, and errors can be made in analyzing these tests if the period dominated by wellbore storage is not properly identified. In multiwell tests, wellbore storage and wellbore damage at the active well have always been ignored and assumed to have essentially no effect on the observation-well pressure response. Recently, Prats and Scotts presented the pressure response. Recently, Prats and Scotts presented the effects of wellbore storage at the responding well on pulse-test pressure response. pulse-test pressure response.This paper shows that wellbore storage and wellbore damage at the active well can, in certain cases, cause large error in determining transmissivity and storativity from an interference test or pulse test.

The Model

The mathematical model used to generate the following results is presented in the Appendix. This model solves the diffusivity equation by finite-difference techniques. Reservoir properties, flow rates. wellbore storage, and wellbore damage are variable in this model. However, the results presented in this paper assume an isotropic. homogeneous reservoir containing a slightly compressible fluid, the reservoir is infinite-acting for the time period under consideration, the surface flow rate at the period under consideration, the surface flow rate at the active well is constant; and the wellbore storage coefficient and wellbore damage are constant during the testing period.

The Need To Account for Wellbore Storage

Fig. 1 shows the increase in flowing bottom-hole pressure above initial pressure vs time in an injection well pressure above initial pressure vs time in an injection well for cases with dimensionless wellbore storage coefficient, CD, equal to zero and 10', where

(1)

JPT

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