Does gauge placement matter in downhole transient data acquisition?
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abstract
Transient pressure data are often collected at various points in the wellbore, including at the wellhead, for test interpretation. When data gathering takes place at a point other than the midpoint of producing (MPP) interval, either by design or inadvertently, serious interpretation problems may arise because of wellbore thermal effects. Many field tests conducted on gas, oil, and water injection wells show the severity of this wellbore-induced problem. This paper explores the consequences of thermal effects upon test analysis in some cases and presents new insights into both data collection and their interpretation. Existing methods do not capture the borehole physics to allow us to study this problem. We present two interesting field cases here. One example shows that pressure can actually decrease during buildup tests in a gas well, in a high-transmissivity reservoir. In this test, data were collected some 1,200 ft above the MPP in a 9,000-ft well because of mechanical restrictions downhole. We reproduced this observed behavior by using the simulator. Pressure decrease in a buildup is caused by the dominating influence of thermal diffusion over pressure diffusion in a high-transmissivity system when measurements are made away from the MPP. For the same reason, pressure increases during a drawdown test. Equally important, the results of the computation show that even if the data were collected at the MPP, the gauge's ability to resolve small changes in pressure would have been tested severely.
