Reservoir evaluation of a gas condensate reservoir using pressure transient analysis
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Annual Technical Meeting 1997, ATM 1997. All rights reserved. This paper presents a case history of characterization of a gas condensate reservoir using pressure transient analysis. Pressure transient tests from wells in this field led to test data plots with complex shapes. Specifically, the pressure derivative in a typical test flattened at intermediate shut-in times (after wellbore storage effects diminished) and then trended downward. This curve shape indicates lower mobility near the wellbore and increased mobility some distance away. Using conventional interpretation techniques, this pressure derivative response may be interpreted (erroneously) as a composite reservoir with low transmissibility in a region with radius of almost 500 feet near the well, surrounded by a region of higher transmissibility, and a positive skin factor. In this study, we modeled well tests in this field with a fully compositional reservoir simulator. We demonstrated that we can reproduce the observed test behavior in a homogenous reservoir. The decrease in pressure derivative is caused by reservoir fluid property changes with pressure, and the apparent positive skin factor is a result of liquid condensing in the formation near the wellbore. The region with reduced transmissibility (high liquid saturation) was on the order of only 20 feet in radius. Our study included sensitivity analysis to determine the effect of selected variables on pressure transient test response. Production time prior to shut-in proved to be particularly important. Longer production periods prior to shut-in can modify the shape of the derivative curve plot but do not change the possible erroneous interpretations resulting from essentially perfect fits of test data with composite reservoir models.
