Observations of Thermal and Pressure Transients in Carbon Dioxide Wells

Abstract

Abstract Carbon dioxide wells are different to oil, gas and water wells because large density changes due to transient thermal effects can decouple surface pressure from downhole pressure. This means, for instance, that wellhead pressure can decline while reservoir pressure is building after shut-in of a production reservoir. Similarly the opposite can occur. This presents challenges for the interpretation of surface measurements. An ability to forecast wellhead pressure is desirable for the optimum design of surface facilities for both carbon dioxide production and injection. The interpretation of surface measurements is also useful for monitoring carbon dioxide reservoirs as flow rates are more easily measured at the surface, and surface gauges are more easily replaced if a gauge fails. Here we report on detailed pressure and temperature observations involving a mixture of 77 mole % carbon dioxide, 20 mole % methane and 3 mole % other gas components. The measurements are from two wells, one production and one injection, involving both surface and downhole gauges. These two wells when connected can act as a strong siphon due to thermal effects. Thermal transients are observed to last up to two months before the surface response matches the downhole response. The use of two downhole gauges in the injection well allows measurement noise to be separated from real pressure events. This in turn allows the performance of the downhole gauges to be evaluated. The results are useful for input into the engineering design of future carbon dioxide facilities and the use of surface pressure for interpreting reservoir behavior.