Transient Analysis of Acoustically Derived Pressure and Rate Data
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SummaryA pressure-buildup test conducted on a sucker-rod pumping well is often distorted by long-duration wellbore storage. In fact, this distortion could be so severe that even a weeks shut-in period may not allow a semilog analysis. A longer shut-in period becomes economically discouraging because of lost production.Low energy and low transmissivity in the reservoir, coupled with increased fluid compressibility, contribute to this long-duration storage phenomenon. One way of reducing the storage effect clearly lies in the simultaneous analysis of downhole pressure and flow rate, estimated from casinghead pressure and rising annular liquid-level measurement made by acoustic well sounding (AWS). Ascertaining the quality of the indirectly measured pressure and rate data constitutes one of the objectives of this study.Several methods exist to translate the AWS measurement to downhole pressure and rate data for the subsequent transient analysis. We show that even an empirical hydrodynamic correlation provides satisfactory transient-pressure/flow-rate data for convolution and deconvolution analyses for moderate pumping-liquid columns. When long annular liquid columns are encountered, translating the AWS measurement with a mechanistically based hydrodynamic model appears to be a prudent approach.Interpretation of several transient tests show that automated convolved-type-curve or history matching of field data is a powerful tool for reservoir-parameter (total mobility, skin, fracture half-length, and storage coefficient) estimation. Use of downhole rate for the convolved type-curve matching reduces the variable storage coefficient by several orders of magnitude, thereby enhancing the quality of the estimates. Deconvolution of downhole pressure and rate data and the pressure-derivative approach significantly aided in well/reservoir flow-model identification.A simple algorithm for computing the Laplace transform of the wellbore pressure for an infinite-conductivity vertically fractured well in an infinite reservoir is developed in this work for a rapid, iterative-type computation used in automated convolved-type-curve analysis.
