Reservoir Simulation of Steam Fracturing in Early Cycle Cyclic Steam Stimulation

Abstract

In Cyclic Steam Stimulation (CSS), steam is injected above the fracture pressure into the oil sands reservoir. In early cycles, the injected steam fractures the reservoir creating a relatively thin dilated zone which allows rapid distribution of heat within the reservoir without excessive displacement of oil from the neighborhood of the wellbore. Numerical reservoir simulation models of CSS that deal with the fracturing process have difficulty simultaneously capturing flowing bottom hole pressure behaviour and steam injection rate. In this research, coupled reservoir simulation (flow and heat transfer) and geomechanics models are investigated to model dynamic fracturing during the first cycle of CSS in an oil sands reservoir. In Alberta, Canada, in terms of volumetric production rate, CSS is the largest thermal recovery technology for bitumen production with production rates equal to about 1.3 million bbl/day in 2008. The average recovery factor from CSS is between 25 and 28% at the economic end of the process. This implies that the majority of bitumen remains in the ground. Since the mobility of the bitumen depends strongly on temperature, the performance of CSS is intimately linked to steam conformance in the reservoir which is largely established during steam fracturing of the reservoir in the early cycles of the process. Thus, a fundamental understanding of the flow and geomechanical aspects of early cycle CSS is critical. A detailed, thermal reservoir simulation model, including dilation and dynamic fracturing, was developed to understand their effects on bottom hole pressure and injection rate. The results demonstrate that geomechanics must be included to accurately model CSS.