Pressure and rate rebalancing to improve recovery in a miscible CO 2 EOR project

Abstract

In a CO2 EOR project the injected CO2 seldom sweeps all portions of each injection pattern uniformly. This is compounded by the fact that injection patterns are not always symmetrical and the injector itself may not be centered within the pattern. Areal sweep within a pattern is further influenced by pressure gradients which may not be uniform in all directions. At the SACROC unit the approach to maximizing the areal sweep involves detailed monitoring of individual producing well pump intake pressures and producing GOR's as well as the calculated streamlines to identify opportunities for changing the direction of CO2migration in the reservoir. It is frequently observed that response to CO2 injection occurs in only one well or on only one side of a pattern with little to no effect in the other directions. The remedy presented in this paper involves converting to flow or slowing down the pump in the responding wells (and thus raising the pump intake pressure) when their GOR exceeds a certain threshold and upsizing or speeding up the pumps in the wells on the opposite side of the pattern. The effect of these changes is to alter the pressure distribution within the pattern and change the portion of the reservoir being processed by CO2. We have documented cases where the pattern recovery has more than doubled due to altering the streamlines in this manner. This technique of monitoring and altering the streamlines is time and data intensive but in the patterns where it is applicable significant increases in the ultimate EOR recovery can be obtained for essentially zero capital cost. This paper discusses the pattern selection criteria, data requirements, and the timing and method of adjusting the withdrawal rates to alter the streamlines and increase areal sweep efficiency. Copyright 2014, Society of Petroleum Engineers.