Variable-Skin and Cleanup Effects in Well-Test Data

Methods to analyze well-test data influenced by variable-skin and cleanup effects are introduced, with emphasis on verifications of consistency in drawdown and buildup data. It is shown that a stable cleanup trend can be modeled by a simple hyperbolic skin factor in solutions for wells with constant skin. It is also shown that if the skin effect stabilizes at constant values within the general cleanup trend, then segments of data fitting a model with constant skin should be detectable. In some cases, repetitions of such segments can be used to estimate the formation flow capacity and the well cleanup rate. We also discuss how a gradual reduction in the skin effect in drawdown data can produce a false semilog straight line and lead to gross overestimation of flow capacity and skin. Well cleanup can produce constant or nearly constant bottomhole pressures (BHP's) during periods of constant-rate production, thus giving a false impression of pressure support in the reservoir. Methods presented will help to recognize whether a well is strongly influenced by either gradual or abrupt cleanup effects.