Effect of Pretest Pressures and Temperature on DST Interpretation

Drillstem tests (DST's) in porous media typically consist of a sequence of production and shut-in periods. Interpretations of DST's are often based on application of the Horner method to the shut-in periods. Such an analysis assumes that the wellbore rate is constant during the production period and that the pressure response during the shut-in is affected only by that production period. For many DST's, however, these assumptions are no longer valid and other interpretation methods are required. For example, prior production and shut-in periods and/or pretest pressures can significantly affect the pressure response of subsequent periods and cause erroneous parameter estimates. In this paper, we present a new DST semianalytical solution that can account for the effect of pretest pressures, prior test periods, wellbore temperature variations, and other factors such as wellbore storage and skin on the pressure response. The solution is an extension of the solution presented by Correa and Ramey¹ who derived a DST solution by writing a single time-dependent wellbore boundary condition. Application of the new solution to example DST data shows that estimates of formation pressure from the Horner analysis can deviate from the true formation pressure by several hundred psia, whereas permeability estimates can differ by a factor of two or three.