Problems in Interpretation of Pressure Fall-Off Tests in Reservoirs With And Without Fluid Banks

Abstract

Secondary- and tertiary-recovery injection projects may cause fluid banks to form in the reservoir. Pressure fall-off testing is often used in hopes of determining the parameters for each side of the fronts that delineate such banks. A mathematical model was used to investigate the practicality of such an approach. Introduction Pressure fall-off testing consists of shutting in an injection well and measuring the surface pressure or preferably the bottom-hole pressure vs time. This test is the counterpart of a pressure buildup test in a production well. The analyses of the pressure-time data in both tests, therefore, are remarkably similar. Nonetheless, because of the,.jnjection mode of operation, pressure fall-off testing has some unique characteristics. In secondary- and tertiary-recovery projects three major fall-off situations may arise: liquid injection, gas injection, and alternate gas and liquid injection. The first two cases can be subdivided as follows: Liquid Injection Case (1) Liquid injection before fillup, (2) Gas injection in aquifers for storage, Gas Injection Case (1) Air injection as in in-situ combustion, (2) Gas injection in aquifers for storage, (3) Gas injection for pressure maintenance, (4) Gas injection in miscible displacement, (5) Gas injection in gas cycling. From this classification we learn that the mathematics of pressure fall-off testing must account for multiphase flow in many complex situations. Fortunately, numerical methods and computers have made the task of simulating such situations fairly simple and economical. However, to interpret actual fall-off test data from the field is still very difficult. The main problem is the nonuniqueness of the situations that give rise to nearly identical pressure fall-off behavior. The analysis of pressure-time data from a pressure fall-off test yields information about the interwell transmissibilities, interwell average pressure, and skin factor. Fall-off testing has also been used to track the movement of the burning front in in-situ combustion, to locate the position of the outer radius of a water bank, and to help in calculating formation damage depth. In general, "front tracking," or locating discontinuities by pressure fall-off is very intricate and must be used with extreme caution. Often, the front or discontinuity cannot be detected, and what appears to be a front or a transmissibility change (damage or improved zone) on the fall-off curve is actually a reflection of some other phenomenon. Considerable care must also be taken in determining transmissibilities from the slopes of the straight-line segments. As we will show, there are several factors besides the transmissibility that can influence these slopes. Afterflow (wellbore storage) invariably occurs in fall-off testing and in many cases can cause misinterpretation of data. Recent advances in this area have laid the foundation for understanding such problems.