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 20feet 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.
Analysis of well tests from gas condensate reservoirs is a significant challenge for engineers. If pressure drops below the dew point near the wellbore during the test, a condensate ring will accumulate immediately around the well. This can cause a significant loss in well productivity. The formation of this ring is documented by McCain and Alexander1. In this paper, we report an investigation of the effect of the condensate ring on pressure transient analysis and document the distinctive behavior of the pressure derivative caused by the ring.
A drill stem test (DST) can include a series of production and shut-in periods, and thus can produce particularly "interesting" pressure derivative curves in gas condensate reservoirs. The test that we analyzed and discuss in this paper was from a multi-flow period, multi-shutin period DST.
Although many papers discuss fluid flow in gas condensate reservoirs, we found none that propose adequate methodology to determine formation properties from analysis of well test data from gas condensate reservoirs. Bourbiaux2 investigated depletion behavior in gas condensate wells using a parametric modeling study. Carlson and Myer3 studied the effect of condensate drop out on the performance of fractured wells and presented some information on well test analysis of fractured gas condensate reservoirs. Afidick, et al.4 presented a case study of a gas condensate reservoir. Jones, et al.5 presented a two-phase analog that can be used for build up analysis from wells producing below the dew point pressure.
