A drillstem test (DST) can be run prior to completion of a well to determine if a formation is capable of producing oil or gas at commercial flow rates. In the eastern United States, drillstem tests are seldom used because a typical well will not produce at flow rates high enough to measure during a DST, especially in low permeability reservoirs where mud filtrate invasion prior to the drillstem test suppresses the gas flow rates. A closed chamber drillstem test can be used to measure these low flow rates but interpretation is often difficult. This paper examines the effects of mud filtrate invasion upon the results obtained from drillstem tests.

A two-phase, two-dimensional model was used to simulate a drillstem test following a period of mud filtrate invasion. Using the results of these drillstem test simulations, qualitative relationships between absolute permeability, gas relative permeability, filter cake permeability and thickness, and wellbore pressures during the filtrate imbibition period and flow period have been established.

All of the parameters investigated, except for the filter cake thickness, did influence the results from the drillstem test and the ability to forecast flow behavior based upon the drillstem test results. In particular, this study has shown that the parameters that affect the cleanup behavior of a formation can be ranked in order of importance as follows: absolute permeability, gas relative permeability, filter cake permeability, pressure drawdown and, finally, overbalance pressure during drilling.

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