The existing body of theoretical studies on interference testing usually applies for constant-rate condition at the active well. For constant-pressure situations, which are fairly commonly encountered during injection tests in low-permeability rocks (e.g., crystalline rocks, which are often tested for waste repository projects), alternative approaches must be sought for analyzing interference tests. This paper presents a new method for analysis of interference tests with the active well at constant pressure. This new analytical technique makes use of both type curve matching and straight line fitting methods for analysis of such tests.
We show in this paper that with a proper grouping of the relevant variables, a constant-pressure interference test can be analyzed using constant-rate Cartesian (or semilog) derivative type curves for interference testing. Thus, this method utilizes existing constant-rate solutions for interpretation of constant-pressure interference tests. The solution is exact and extends a previously presented approximate solution. Moreover, for rD ≥20 and tD / r 2D ≥0.5, we can also use a semilog plot to conduct a straight line analysis of the data from a constant pressure interference test for estimation of interwell transmissivity and storativity.
In an interference test between two wells, the ‘active’ well undergoes a variety of production or injection tests, with the ‘observation’ well used only to monitor the pressure transients released into the formation by the active well. Analysis of interference tests are commonly performed using the assumption of a constant rate or of a series of constant rate steps. Such analyses of the observation well data can be performed using wellbore storage and skin at both active and observation wells1.
In the groundwater industry, constant pressure tests are fairly common in tight formations where a constant rate test is affected for a relatively long period of time by wellbore storage effects, so that assessment of theformation characteristics close to the active well are difficult by the latter method. The constant pressure situation is also well suited to injection testing, as constant rate methods need a prior knowledge of the formation permeability to select a suitable rate2. Constant pressure situations are also a useful means of controlling off-site migration of contaminated groundwater3.
Uraiet and Raghavan4,5, Ehlig-Economides and Ramey6 and Ozkan et al. 7, among others, have presented a number of tools to aid in analysis of constant pressure tests in a variety of flow situations and reservoir conditions. Analytical methods to interpret constant pressure interference tests in a double porosity situation have been detailed by Grasman and Grader8. In a very recent study, Hiller and Levy3 demonstrated the use of an approximate method to analyze the late time data at an observation well, when the active well is being produced at a constant pressure. Their method yields the formation diffusivity directly from the analysis.
Mishra and Guyonnet9 presented a simple but elegant technique to compute formation transmissivity and storativity from analysis of observation well data during constant pressure tests. Their method involves a generalization of the Theis exponential integral solution for constant pressure situations. This approximate method is adequate for usage when rD ≥300 and/or D / r 2D ≥5. However, the method of Mish