DOES GAUGE PLACEMENT MATTER IN DOWNHOLE TRANSIENT-DATA ACQUISITION

Transient pressure data are often collected at various points in the w ellbore, including the wellhead, for test interpretation. When data ga thering takes place at a point other than the midpoint of producing (M PP) interval, either inadvertently or by design, serious interpretatio n problems may arise because of wellbore thermal effects. Many field t ests conducted on gas, oil, and water injection wells show the severit y of this wellbore-induced problem. This paper explores the consequenc es of thermal effects upon test analysis in some cases and presents ne w insights into both data collection and their interpretation. Existin g methods do not capture the borehole physics to allow us to study thi s problem. We present interesting synthetic and field cases. One field example shows that pressure actually can decrease during buildup test s 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 becaus e of mechanical restrictions downhole. We reproduced this observed beh avior 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 t he MPP. For the same reason, pressure increases during a drawdown test . Equally important, results of the computation show that even if data were collected at the MPP, the gauge's ability to resolve small chang es in pressure would have been tested severely.