Pressure Falloff Analysis in Reservoirs With Fluid Banks
Authors: Merrill L.S. , Kazemi Hossein , Gogarty W. Barney 
Published: December 2007

A radial reservoir simulator has been used to generate a wide range of conceptual pressure falloff curves in systems having fluid banks. The experience gained should improve the analysis of field pressure falloff curves. Also, a procedure has been developed to estimate the distance to the front nearest the injection well and the saturations on both sides of the front in liquid-filled waterflood reservoirs.

Introduction
A detailed background for falloff testing in composite reservoir situations was given by Kazemi et al. Briefly, falloff tests are the counterpart of pressure buildup tests in production wells and are used for many of the same purposes. The fluid injected before a falloff test, however, is frequently different from the reservoir fluid and is used to displace the reservoir fluid. This creates what might be called saturation discontinuities between the region where the injected fluid predominates and the region or regions where the original reservoir fluids predominate. Examples of such systems arereservoirs being waterflooded,reservoirs undergoing in-situ combustion, andreservoirs undergoing gas injection for various purposes, such as gas storage, pressure maintenance, and purposes, such as gas storage, pressure maintenance, and miscible displacement.
Falloff testing has occasionally been used in such systems to determine the front radius and the reservoir properties ahead of and behind the front. As Kazemi et al. have demonstrated, such analyses are subject to many pitfalls, and must be made with considerable caution. The slope of the first straight-line segment that develops on a standard plot of pressure vs logarithm of time is commonly plot of pressure vs logarithm of time is commonly used to determine the mobility of the first zone. The slope of this segment, however, can be affected by afterflow. The slopes of the straight-line segments beyond the first one have also been used to estimate the properties of other zones. These straight-line segments, however, are functions of both the mobility and the specific storage of zones that develop and, in general, one cannot use the slopes of the lines for direct calculation of mobilities.Deviation from the first straight-line segment of a plot of pressure vs the logarithm of time for falloff tests in reservoirs with fluid banks can be due to sensing of the first front. The time of such deviation for a given system is indicative of the distance from the wellbore to the front. The dimensionless time of deviation, delta tDf1 *, for many reservoir situations is frequently assumed to be constant. The equation (1) is then used to estimate the distance to the front. The value for delta tDf1*, however, is actually a function of mobility ratio and specific storage ratio. The assumption that it is constant can frequently lead to large errors.The work reported here was undertaken to see if correlations could be developed to more accurately analyze field pressure falloff curves when fluid banks are present.

Model of Systems Investigated
Fig. 1 is an idealization of the type of system under consideration. In this figure, Zone 1 is the area dominated by the injected fluid, with rf1 being the distance from the injection well to the nearest front.
JPT
P. 809