Wellbore Effects in Injection Well Testing

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A case history is used to illustrate that wellbore storage can act differently in injection well falloff and injectivity tests. This can cause test curves to have shapes characteristic of mobility banks and thus render some falloff tests useless. Injectivity tests, however, may be interpretable. Introduction Injectivity or pressure falloff tests in injection wells are commonly used to investigate formation properties. Ideally, such tests provide information about formation permeability, skin factor, distance to fluid banks, and permeability, skin factor, distance to fluid banks, and distance to boundaries. Wellbore storage 9 and its effects on transient testing have been described in the literature. The storage of fluids in the wellbore due to compression or changing liquid level causes transient tests to act differently at short times than they would in the absence of wellbore storage effects; long-time behavior is essentially unaffected. Wellbore storage is a general term encompassing the more specific terms, "afterflow" (pressure buildup) and "unloading" (pressure falloff and drawdown); we use only the general term in this paper. The specific term commonly, applied to injectivity tests is wellbore storage.We show here that under certain circumstances wellbore storage effects, in particular changing wellbore storage, can make test interpretation for formation characteristics practically impossible. We include field data illustrating this problem, provide an explanation of the behavior of these data, make suggestions for injection well testing and test analysis, and illustrate the analysis technique. The problem of wellbore storage effects in injection well testing is much too broad and complex to be treated exhaustively in this paper, but we feel that the material presented is detailed enough to identify, illustrate, and at least partially solve the problem. partially solve the problem.We first encountered and finally recognized this problem as a result of a series of injectivity and problem as a result of a series of injectivity and falloff tests on several wells. The purpose of the testing was to locate and estimate the distance to fluid banks. Fig. 1 shows data from one of these falloff tests, from a 1,000-ft-deep water injection well. Pressure data are from a permanently installed surface-recording down-hole gauge. Fig. 1 has several bends that might be interpreted as banks, boundaries, interference from adjacent injection or production wells, commingled zones, etc. It is not difficult to pick four, five, or even six different slopes from this particular test. As a result of the multiple slope changes, there were great differences of opinion about how to interpret these data; test interpretation was never satisfactory to all involved.Fig. 2 shows injectivity test data for the same well. This figure also has several bends that might be interpreted as banks, boundaries, interference, etc. (The straight line in Fig. 2 is used later in an example calculation.) Detailed analysis of these two figures shows that they do not have the same sequence of slopes and that the slopes do not change at the same time. There are one or two places where the slopes do appear to be the same, but these slopes do not occur at equivalent times. Furthermore, we expect the general appearance of the injectivity and the falloff tests to be the same; clearly, the shapes of these two curves are quite different. JPT P. 1244