Tight gas reservoirs are characterized by very low matrix permeability. In these types of reservoir systems long shut in periods are required to establish pseudo radial flow making conventional well testing methods impractical. This is especially true once the well has been hydraulically fractured due to the added flow complexity and the resultant additional time required to reach pseudo radial flow.
Injection fall off test (IFOT) is a pre frac injection technique that can be used to estimate reservoir pressure and get a qualitative estimate of formation permeability-thickness (k*h) in tight gas reservoirs. This is somewhat equivalent to a cleanup flow on a DST (Drill Stem Test); the idea is to establish flow to the reservoir, get the well-bore fluid to a known density, restrict the flow duration to minimize the reservoir volume affected by the flow, and shut-in long enough that the Horner/Superposition plot extrapolation is as small as possible.
BP has applied this technique to infill Cotton Valley (CV) wells in the East Texas Basin in order to get an updated value of reservoir pressure for use in material balance depletion analysis, amongst other applications. After successful implementation of this technique in vertical and horizontal tight gas sandstone wells, the testing method was extended to three Cotton Valley Lime wells and also to the first BP Haynesville Shale vertical well. This document summarizes field examples that illustrate the analysis and application of this IFOT technique in both vertical and horizontal wells including the test results on the Hayneville shale well. In the latter, the application of the test results on basic reservoir simulation is also discussed.