Seeking Synergy Between Drawdown and Buildup Analyses

Abstract

Summary Drawdown tests are rarely interpreted unless accompanied by instantaneous surface or downhole rate measurements. Rate variations prevent a constant-rate analysis in most situations. Unsteady wellbore dynamics precipitated by progressive formation cleanup, multiphase fluid flow, and wellbore thermal effects influencing compressible fluids, to name a few, may prevent one from obtaining a stable surface rate. This reality compels an analyst to fall back upon buildup interpretation. Thus, one typically presupposes that the rate was held constant before shut-in. Drillstem tests and long-duration exploratory well tests are cases in point. This conventional practice raises an important question: Can we calculate a variable-rate history such that the drawdown analysis will be consistent with the buildup? This paper presents a method to interpret drawdown data whenever variable rates are encountered, either inherent in a test or induced by design, such as in a gas-lift well test. The method entails a stepwise procedure. First, buildup data are interpreted assuming a constant rate during drawdown. Second, the estimated parameters are used to compute instantaneous rate corresponding to each pressure data point by superposing constant-pressure solution with an appropriate pD model. The reconstructed rate history is then used to perform the variable-rate (convolution) analysis of drawdown data. Third, the new rate history is used to reinterpret buildup data. Parameters obtained from the two analyses provide the desired consistency. The proposed procedure is entirely general, in that any appropriate reservoir pD model may be used to compute the instantaneous rate values. However, best results are obtained when the late-time data are free from outer-boundary effects. We present field examples from various scenarios to show the usefulness of this approach. These examples include a short-duration onshore drillstem test (DST), a gas-lift well test involving a gas-vent period, a long-duration production test with suspected pressure-sensitive rock properties, and a slug/closed chamber test. In all cases, we have shown the synergy between the results of the two tests.