Analyzing Gas Well Buildup Tests With Changing Wellbore Storage in Tight Gas

Pre-fracture pressure buildup tests in tight gas formations can be extremely difficult to analyze correctly, particularly when large pressure drawdowns precede the shut-in period. A major source of difficulty is the wellbore storage coefficient, which changes continuously throughout the test and which can change by one to two orders of magnitude from beginning to end of the test. This change causes particular problems in type-curve analysis; in turn, this can cause serious difficulties in recognizing when, if ever, the semi-log straight line with slope related to formation permeability appears. As a result, buildup test analysis results can be the subject of great controversy and uncertainty.This paper summarizes research which shows conditions under which changing storage coefficient causes interpretation problems and which also shows how the use of Agarwal's pseudo-time function can resolve the difficulties. Simulated buildup test analyses are used to show the problem and its solution. Several actual tests from tight gas formations also illustrate the difficult-to-analyze shapes suggested by theory, and the correct analysis of these tests is presented.