Preinjection of high-molecular-weight poly-acrylamide polymers in advance of a miscible slug is proposed as a means of improving reservoir sweep efficiency by reducing the degree of "permeability" contrast between wells in a moderately heterogeneous system. The use of polymers in a preinjected mode is radically different from their " conventional" use to provide a favorable mobility condition between in-place and injected fluids and so maximize areal sweep even in a flood or a "homogeneous" sand.
Theory and model flow tests were employed in an investigation of polymer preinjection in advance of a miscible flood. Flood tests in physicalmodels of heterogeneous porous media showed that preinjection of polymers could result in better flooding efficiency because of increased volumetric sweep. This was concluded from tracer performance data and oil recovery response. Studies or theinteractions between the preinjected polymer and a subsequent micellar flood indicated that prior polymer flow had no adverse effects on oil displacement efficiency by a micellar fluid and appeared to decrease surfactant loss to the rock. Additional mobility control requirement in micellarfloods as a result of decreased mobility of fluids was moderate.
Because laboratory models cannot be scaled to reproduce the complex heterogeneities encountered in reservoirs, extensive field testing of polymer preinjection in a real environment and under carefullycontrolled test conditions is suggested. This is highly desirable since the timing of development is an important factor if the full value of having available sweep improvement methods is to be realized.
While considerable attention has been given to the chemistry of miscible flooding, little published work is available which discusses effects of variations in formation rock texture on reservoir flooding efficiency by these improved recoverymethods. Reservoir rock heterogeneity can result in an injected fluid bypassing significant portions of a reservoir under flood. Circulation of large volumes of water to achieve higher ultimate sweep in a waterflood is tolerable because of the low cost of injected water. In miscible flooding, however, the reservoir is treated with only a small size slug, usually less than 10 percent pore volume, of an expensive oil recovery fluid such as the micellar fluid. Here, more uniform contacting of the reservoir by the injected miscible fluid is critical, and even moderate variations in permeability can cause significant reduction in expected oil recovery. Mobility control behind a miscible slug may have limited effect on causing the slug to avoid flowing through the high permeability, watered-out channels in reservoirs, thus bypassing much of the in-place oil that could be recovered. It is the opinion of the authors that development and 1mplemeneation of various sweep improvement methods to maximize reservoir flooding efficiency is critical at a time when the petroleum industry is launching into an ambitious program of enhanced oil recovery.
Preinjection of polymers in advance of a miscible flood is proposed as a means of improving reservoir sweep efficiency by reducing the degree of interwell permeability contrast in a moderatelyheterogeneous system.