Does Gauge Placement Matter in Downhole Transient-Data Acquisition?
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Transient pressure data are often collected at various points in the wellbore, including the wellhead, for test interpretation. When data gathering takes place at a point other than the midpoint of producing (MPP) interval, either inadvertently or by design, 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 interesting synthetic and field cases. One field example shows that pressure actually can decrease during buildup tests in a gas well in a high-transmissivity reservoir. In this test, data were collected about 1,200 ft above the MPP in a 9,000-ft well because of mechanical restrictions downhole. We reproduced this observed behavior with 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 draw-down test. Equally important, results of the computation show that even if data were collected at the MPP, the gauge's ability to resolve small changes in pressure would have been tested severely.