Interpreting short-term buildup tests from low-productivity gas wells using deconvolution
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This paper illustrates the application of deconvolution to analysis of wellbore storage distorted pressure buildup test data from low productivity gas wells. These wells generally require long times to reach the correct semilog straight line, rendering conventional Horner analysis impractical. The deconvolution process removes the effects of wellbore storage and allows for the use of semilog analysis. Also, times required for buildup tests can be reduced by more than an order of magnitude with no loss in accuracy in results. In fact, analysis is sometimes even more definitive with the short-term, corrected test data than with much longer-term but uncorrected data because some important characteristics of a formation (such as the effects of layering or dual porosity) appear only in the first few hours of shut-in and are usually masked by wellbore storage distortion. The problem with deconvolution is that it requires accurate sandface rates after shut-in. These rates are difficult and expensive to measure, so we present a method to calculate the sandface rates from the observed pressure buildup data. This paper presents verification of the rate calculation and the use of the calculated rates in deconvolution with simulated data. We also present several field cases from low permeability gas wells. The success of the deconvolution analysis implies that shorter tests may be run on low productivity gas wells, significantly reducing the cost of the tests and increasing opportunities to identify important reservoir heterogeneities.
