Effect of Pre-Test Pressures and Temperature on DST Interpretation
Authors: Johns R.T. , Ma L. 
Published: December 2007

Drill-stem tests (DST's) in porous media typically consist of a sequence of production and shut-in periods. Interpretation of DST's is 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 For many DST's, however, these assumptions are no longer valid and other interpretation methods are required.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 pre-test pressures can significantly affect the pressure response of subsequent periods and cause erroneous parameter estimates.

In this paper, we present a new DST analytical solution that can account for the effect of pre-test 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 Ramey 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 hundreds of psia, whereas permeability estimates can differ by a factor of two or three. The errors in the estimated formation parameters increase as the permeability is reduced, but can be significant even for large permeability reservoirs. These errors are more pronounced for longer pre-test pressure periods and when the pre-test pressures are much greater or smaller than the true formation pressure. The effect of wellbore temperature variations are also shown to be important for DST interpretation in certain cases when the permeability-thickness product is small.

Introduction

The interpretation of drill-stem tests (DST's) provides important reservoir information for determining the profitability of a particular reservoir or well. The DST can provide good estimates of formation permeability and initial pressure as well as samples of the reservoir fluid. Because DST's are typically composed of a sequence of very short test periods, however, accurate interpretation of DST' s can be challenging.

Methods to interpret drill-stem tests have been extensively studied in the literature. Interpretation of DST's is often based on application of the Horner method. 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.

In many cases, however, these assumptions are not satisfied and interpretation of DST's by classical methods may be very inaccurate. For example, prior production and shut-in periods and/or pre-test pressures can significantly affect the pressure response of subsequent periods. The focus of this work is to examine the effect of pre-test pressures, such as those encountered during drilling, logging, or coring, on the interpretation of drill-stem tests (DST). A secondary focus is to examine the effect of borehole temperature variations on the pressure response, which can also impact the DST pressure for certain low-permeability reservoirs.

Neuzil and Pickens et al. describe processes and mechanisms associated with well tests in relatively low-permeability formations. The measured formation pressure response in those formations is primarily affected by the following:formation flow parameters and model;wellbore storage, skin and tool compliance;variable pretest pressures;wellbore temperature changes; andcomplex test-sequence design.

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