THE EFFECT OF SHEAR FAILURE ON THE CYCLIC STEAM PROCESS AND NEW PSEUDO FUNCTIONS FOR RESERVOIR SIMULATION

The steam stimulation process is a proven, commercially viable recover y process in oil sand reservoirs. However, an understanding of the rec overy mechanism is far from complete. Recent studies of the geotechnic al aspects of tar sands point to the importance of shear failure creat ed by pore pressure increase on fluid flow in the reservoir during ste am injection. The changes in mass and heat transport properties in the heterogeneously deformed failure zone have been shown to be important factors for the recovery mechanism. This paper describes the theoreti cal and practical aspects of representing the shear failure zone in re servoir simulation, by the means of pseudo relative permeability funct ions. Relative permeability hysteresis is commonly used in the simulat ion of the steam stimulation process, but its mechanism is not clear. The paper reviews the existing methods for treating hysteresis and giv es the theory for the pseudo relative permeabilities caused by heterog eneously failed media. The new functions are derived by considering th e shear failure phenomenon. A new set of stress-dependent pseudo relat ive permeabilities is proposed and a practical method for the use of t he pseudo functions (including hysteresis) in a reservoir simulator is given. The method applies to injection as well as production cycles. Its effect on simulating the cyclic steam process is given in examples and shows that the hysteresis should be stress-dependent rather than saturation-dependent.