New Methods For Analysis Of Constant Pressure Interference Testing
Authors: Chakrabarty C. , Yeung K. 
Published: March 2010

Abstract
The existing body of theoretical studies on interference testing usually applies for constant-rate conditions 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 analysing 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 analysed 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 &gt; 20 and tDr2D &gt; 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.

Introduction
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 wells(l).
In the groundwater industry, constant pressure tests are fairly common in tight formations where a constant rate test is dominated by wellbore storage effects for a significantly long time. 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 rate(2). Constant pressure situations are also a useful means of controlling off-site migration of contaminated groundwater(3).
Uraiet and Raghavan(4,5), Ehlig-Economides and Ramey(6) 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 Grader(8). In a very recent study, Hiller and Levy(3) demonstrated the use of an approximate method to analyse 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 Guyonnet(9) 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.