Use of Pressure Transient Analysis for the Detection of Gas/Water Contact Movement in a Gas Reservoir

Abstract

Abstract A significant challenge in the development of gas reservoirs connected to strong aquifers is the prediction of aquifer movement and the timing of water breakthrough. Generally reservoir simulation techniques are used to solve this problem but we cannot calibrate the prediction until water production at the well is seen – this may be too late. The use of permanent downhole pressure gauges to record pressure transients throughout the life of a well allows us to identify the influence of nearby water contacts and to track their movement. The pressure derivative provides a dynamic signature of the well and reservoir response and, in the case of a mobile aquifer, indicates a boundary effect appearing to move closer to the well over time. By monitoring the change in distance, the time for it to reach the well can be predicted and thus gives us an estimate of the time to water breakthrough. The method leads to a recovery estimate that is independent of traditional reservoir simulation and material balance techniques. One of the benefits of this methodology is to assist in the calibration of reservoir simulation forecasts and therefore business planning. Another is to evaluate whether the gas water contacts are moving under the current production strategy and hence to optimize reservoir management and maximize recovery. It can also be used to estimate the location of a potential down-dip gas-water contact not penetrated by any of the wells and therefore help in the quantification of reservoir volumes. It is also a potential alternative/additional methodology for prediction and forecasting of reservoir behaviour. This novel method was applied to a gas field in the offshore Trinidad where it was estimated that the current reservoir simulation model was under-predicting the recovery of gas prior to water breakthrough. While the reservoir model predicted that water breakthrough should have happened more than six months ago, no sign of formation water production has been detected at the time of writting which is in line with the findings from our method.