Optimizing the efficiency of the waterflood displacement process in heavy oils is critical to reaching the oil recovery goals. However, in the process of finding an economic and stable throughput for the process, in some cases significant sand production and generation of wormholes have resulted in premature water breakthrough and channelling destroying volumetric efficiency. In order to understand such events, a simulation study using a coupled reservoir and geomechanical simulator was used to determine the physics controlling the initiation and propagation of dilated zones resulting from sand production giving the premature breakthrough. An attempt was made to identify the importance of well configuration and what operating constraints can be altered to reduce the risk of these breakthrough events.
The complex physics of sand production during oil recovery requires it to be modeled as a coupled process: multiphase fluid flow causing transient pressure gradients and geomechanics to calculate the resultant stress variation, permeability enhancement and shear/tensile failure around the induced dilated zone and finally coupling failure criterion for the dilated zone propagation combining pressure gradient and effective stress. A force balance criterion calculates the threshold fluid pressure gradient for sand mobilization based on the effective confining stresses in each numerical element. The stress variation across the loosely supported sand body (damage zone) at the edge of a dilated zone is captured by an elasto-plastic constitutive model using a Mohr-Coulomb shear failure surface combined with softening of the Young's modulus.
The application of the coupled simulator in modeling waterflooding reveals critical insights regarding the significance of different factors contributing to the sand production problem. Multiphase flow, over vs. under-injection, and inter-well pressure gradient effects are critical to controlling the sand production initiation and evolution. Gas liberation below bubble point pressure conditions causes excessive pressure gradient and increases the possibility of sand production. The oil/water relative permeability impact emerges if the mixture mobility at a certain fraction is lower than the end points. As dilated zone geometry appears to follow the weakest zones often associated with high permeable layers; it highlights the significance of the reservoir heterogeneity in contributing to the sand production problem. The results of the current study add understanding to the significance of different mechanisms contributing to sand production and may be used to help mitigate the premature breakthrough problem observed in many waterflooding operations.