The thermal recovery of bitumen reservoirs by steam assisted gravity drainage (SAGD) is often designed to maximize the operating pressure while maintaining a safe and economic operation. In general, higher operating pressure can reduce thermal efficiency due to heat losses to over/underburden formation, but the other benefits usually compensate. To name a few, higher steam temperatures can maximize the reduction of oil viscosity, enhance permeability associated with lower effective stress and shear dilation, and give a larger pressure window to allow flexible control of the producer. This is especially important for shallow reservoirs where the pressure window for injection and production is smaller. The limitation of the maximum operating pressure is then based on maintaining caprock integrity. Thus, shear and tensile failure mechanisms should be quantified and managed.
This paper presents a methodology to perform a geomechanical analysis of caprock integrity for SAGD operation and illustrates the available approaches. Both analytical and numerical approaches are compared demonstrating their usefulness. Main factors in the analysis are the knowledge of the initial stress state and proper representation of the complexity of the geomaterials. A typical initial stress state for a northern Alberta SAGD property, Suncor's MacKay River project, is presented showing the potential for low initial minimum total stress and elevated initial shear stress levels. The stress-strain behavior for the MacKay River sand and caprock materials is discussed focusing on the potential for shear dilation in the sand and shear strength behavior in the caprock. An elasto-plastic constitutive model is used to represent the sand and caprock materials. The increase in pressure and temperature alter the stress state and disturb the soil matrix. This disturbance results in shear dilation of the sand matrix creating regions of enhanced permeability and porosity. Also, the transfer of stress and strain to the caprock causes dynamic stress changes and, therefore, dynamic behavior of shear and tensile failure conditions. Calculations are presented showing the stress paths associated with SAGD operations, suggesting better design of lab testing programs and the implications for shear dilation in the sand and shear failure in the caprock. Finally, the results are used to demonstrate locations that are most likely at risk for potential tensile and shear failure. Stress ratios are used to summarize the analysis and quantify and monitor the failure mechanisms.
The above methodology has been developed and applied in several studies of other SAGD projects and aided the operators in the optimization and permitting the operating conditions.