Error Analysis and Design Considerations for Backpressure Testing of Gas Wells

Abstract The theoretical validity of backpressure tests has been analyzed for wells in radial flow and for wells intersected by vertical fractures. It is demonstrated that stabilized flow-after-flow and isochronal tests are valid tests. Unstabilized flow-after-flow and modified isochronal tests result in error in predicted deliverability caused by continued influence of prior flow and shut-in periods (superposition error). It is shown how the superposition error can be minimized by proper selection and design of the backpressure test. Isochronal and modified isochronal tests are unsteady-state methods and provide the transient deliverability of a well. An extended flow period is generally used to determine the stabilized deliverability. In many reservoirs extended flow periods are unreasonably expensive and impractical. Analytical solutions for predicting stabilized deliverability are required. The calculations used in the prediction of stabilized gas deliverability are based on a radial flow geometry. When these calculations are applied to other flow geometries, errorneous conclusions result. In this study a fully-implicit, two-dimensional gas simulator was used to demonstrate the extension of radial flow theory to vertically fractured wells. The effect of fracture length and conductivity was investigated. It was observed that the performance coefficient (Ct) was inversly related to pressure. A proof is presented for the Darcy flow condition. An understanding of how Ct changes with time will allow a more precise estimate of stabilized deliverability.