Sweep efficiency for solvent injection into heavy oil reservoirs at grain-scale displacement of extremely viscous fluid



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The movement of low viscosity fluid through a porous medium containing extremely viscous fluid is emerging as an important phenomenon in several petroleum engineering applications. These include the recovery of heavy oil by solvent injection, the preferential reduction of water flow using polymer gels, and the enhancement of acid fracturing treatments. The displacement of one fluid from a porous medium by a second, immiscible fluid has been extensively studied in two cases: when capillary forces are dominant, and when viscous forces are comparable to capillary forces. This dissertation research examines a third case: when viscous forces are dominant. The viscosity of the fluid initially present in the porous medium is four or more orders of magnitude greater than the viscosity of the displacing fluid. Consequently, the displacement through an individual pore will be dictated by the hydrodynamic forces required to move the high viscosity fluid. However, very little is known about grain-scale behavior of such displacements. The research will develop a mathematical model of the viscosity-dominated displacement in a network of conduits. By neglecting pressure drop within the low viscosity fluid, the model will treat the displacement as a moving boundary problem. The high viscosity fluid will be assumed Newtonian and will move in response to the pressure gradient imposed via the low viscosity fluid. The movement can be treated as pseudo-steady state flow of the highviscosity fluid. The flow field will be updated whenever the low viscosity fluid advances into a pore previously occupied by high-viscosity fluid. Swept volume will be calculated in each run for comparison and further investigation. We will use classical methods for direct and iterative solutions of large, sparse linear systems to compute these steady states. Key practical insights to be obtained from the model are the nature of the displacement and effects of geometry and hydraulic conductivities on the sweep efficiency. The model will form the basis for examining additional physical processes, notably mass transfer between fluids, and the possibility that fingering of the low viscosity fluid occurs within individual pore throats.