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Abstract
A closed chamber test (CCT) is characterized by a variable wellbore storage coefficient, whereas, in a slug test, the wellbore storage coefficient is assumed to be constant. It is shown that for ranges of parameters often encountered in practical cases, a large span of the CCT pressure data can be converted to equivalent slug test pressure data which can then be analyzed by several available slug test analysis techniques. In addition, we derive a new convolution method for analyzing buildup data obtained in a closed chamber drillstem test. We also present a method to analyze slug test data influenced by a step change in the wellbore storage coefficient due to a change in the wellstring diameter.
Introduction
Closed chamber tests (CCT) are currently used in the oil industry as closed chamber drillstem tests (DST), underbalanced perforation and backsurge perforation cleaning. Although there are differences in operation, underbalanced perforation, backsurge perforation cleaning and the flowing period of a closed chamber DST are the same from the transient well testing point of view. In this work, therefore, the term "close chamber" is generally used to refer to all of them. Pressure responses in these tests are governed by a changing wellbore storage coefficient which results in a nonlinear mathematical problem with no known solution. Available methods to analyze data from such a test include history matching through the use of numerical simulators, convolution methods utilizing flow rate directly measured or calculated from the measurement of the chamber gas pressure, and techniques developed from the short producing time concept. In this work, we suggest new procedures for analyzing CCT pressure data. Drillstem testing is well established as a primary technique for evaluating the commercial viability of newly discovered (virgin) reservoirs. A successful drillstem test (DST) gives a measurement of flow rates, an estimate of static reservoir pressure and pressure-time data corresponding to a short pressure transient test. Successful analysis of DST pressure data yields an estimate of the flow capacity, kh, and the skin factor, s. The flowing period of a DST corresponds to a slug test. Although slug tests were first considered in the field of groundwater hydrology, the industry-standard interpretation methods are based on the type curves of Ramey et al. Recently, Peres et al. showed that pressure data from slug tests can be conveniently converted to pressure data that would be obtained for the equivalent constant surface rate wellbore storage and skin problem. This general relationship enabled them to develop new convolution, deconvolution and type-curve analysis procedures to analyze slug tests. In particular, the converted slug test data can be analyzed using existing type curves for the analogous wellbore storage and skin problem; therefore, slug test type curves are no longer needed. Closed chamber drillstem tests were first introduced to petroleum industry by Alexander as an improvement to conventional drillstem tests; the objective was to reduce operational and safety problems. The only major difference between a conventional DST and a closed chamber DST is that in a closed chamber DST, the well is closed in at the surface. Erdle et al. conducted an extensive review of closed chamber drillstem testing and concluded that the advantages of closed chamber drillstem testing over conventional drillstem testing are improved safety, reduced test time and better test engineering at the wellsite. Reid discussed the advantages of a closed chamber DST over a conventional DST for low permeability gas reservoirs. Unfortunately, the mathematical problem for a closed chamber DST is more complicated than for a conventional DST because the former has a nonlinear wellbore boundary condition. Thus, researchers have been forced to resort to numerical solution techniques; see Saldana-C. and Ramey, the work of Grader (formerly Sageev) and coworkers and Petak et al.
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