Maintaining an effective caprock seal is of prime importance in any enhanced oil recovery (EOR) stimulation involving subsurface injection of fluid. Hydraulic or mechanical breaching of the caprock may entail leakage of injected fluids and/or hydrocarbons into shallower formations, or even to the surface, with the potential for adverse environmental impact. Determining a safe optimal injection pressure which minimizes the likelihood of such an occurrence is a challenging problem. For example, assessing caprock integrity with the static stress measurement alone at the virgin state of reservoir without considering dynamic stress changes is likely to be both unreliable and inadequate.
In this paper, we present a case study of hydraulic and mechanical integrity of Wabiskaw caprock at multiple injection scenarios under dynamic conditions. In this study a multi-disciplinary approach was used which integrates geology, petrophysics, reservoir engineering and geomechanics followed by a finite element coupled reservoir-geomechanics simulation. A 3-D mechanical earth model (MEM) was constructed utilizing advanced azimuthal shear anisotropic sonic log and core based mechanical properties. All the available LOT mini-frac as well as MDT micro-frac tests data were assessed and used to calibrate the anisotropic stresses at initialization step in addition to assessing the lateral variability of the vertical stress. Several tight streaks expected to provide additional abutment to the primary caprock were honored in the model. The 3-D MEM was extended to the surface and embedded with the sideburden and underburden layers incorporating adjacent injectors and producers to minimize the boundary effects.
This study evaluated tensile and shear failures within the reservoir and the caprock at several injection scenarios for a period of 30yrs with injection bottomhole pressures of 116 and 155bar at the rate of 150 m3/day. No shear failure or tensile failures were predicted by the coupled geomechanical simulator in the caprock for all the injection scenarios. This model also predicted the vertical displacement within the reservoir, caprock and at the surface. The amount of heave at the ground surface was negligible.