Effect of leak-off on behavior of chromium(III)-PHPA gel in fractured media.

摘要

Nature's own play, or at time, the engineered production of oil renders all the hydrocarbon reservoirs fractured to some extent. During waterflooding, water may channel through such fracture or network of fractures. As a consequence, the sweeping of oil out of the reservoir becomes inefficient. One remedial practice is injection of a mixture of polymer and crosslinker that reacts to form an immobile gel in the fracture. Such placement of gel is associated with leak-off from the fracture face into the adjoining matrix. Design of a gel treatment needs understanding of the physico-chemical processes that occur as the gel flows (with leak-off), matures and is subsequently subjected to water pressure. This investigation addresses the physicochemical processes for a Cr(III)-PHPA (partially hydrolyzed polyacrylamide) gel system through experiments and mathematical modeling. Flow experiments on physical models of fractured rock had provisions for controlled leak-off into the adjoining matrix.; The displacing fluid leaked off from the fracture into the matrix as a front, resulting in a decreasing velocity (and pressure gradient) along the fracture. When the displacing fluid was more viscous than the displaced one, and the pressure in the fracture was held constant with time, the leak-off rate decreased as the viscous front progressed into the matrix. The drop in leakoff rate was rapid during the initial phase of displacement. A Cr(III)-PHPA solution (referred as gelant) flowed like a viscous solution through the fracture model and formed gel after the shut-in period.; When the gelant was placed in the fracture without leak-off, an immediate loss of chromium was observed, and the formation of gel structure was not evident. Diffusion of chromium into the brine in adjoining matrix and in stagnant pockets in the fracture explained the immediate loss of chromium. Conversely leak-off during placement drove brine from the vicinity of the fracture, ensuring availability of chromium for gelation in the fracture. Displacements in unfractured core at constant injection pressure elucidated the extent and characteristics of leak-off. Before entering the core, gelants underwent post-mixing delays (shorter than their gel time). On continued displacement, flow resistance developed, and the resistance reduced further flow. More delay after mixing of gelant, hastened the build up of resistance to flow and the resistance was contained nearer to the inlet face. The extent of leakoff in these experiments was found to be more than that required for the formation of gel in the fracture.; After gel formed inside the fracture, brine was injected to impose a pressure gradient across the gel structure. As the injection pressure was increased, a rupture occurred in the gel structure at some point. The rupture process was further studied from similar experiments in tubes. On maintenance of injection pressure after rupture, brine flowed through the ruptured gel, and eroded the gel layer to open the fracture gradually.