CO2 THICKENERS FOR IMPROVED ENHANCD OIL RECOVERY OR HYDRAULIC FRACTURING

摘要

Despite its longstanding success as an EOR technique, CO2 flooding does not recover all of the oil in the formation regardless of whether the reservoir has been previously waterflooded. Typically, primary recovery results in the production of ～5%–15% of the original oil in place (OOIP), while secondary recovery is responsible for an additional 20%–40% OOIP. Although CO2 is capable of displacing nearly all of the oil from the portion of the porous media through which it flows, miscible CO2 floods typically recover 10%– 20% of the OOIP via the injection of a volume of dense CO2 equivalent to ～80% of the hydrocarbon (oil) pore volume (HCPV). Not surprisingly, immiscible CO2 floods recover only 5%–10% OOIP because of the interfacial tension between the CO2 and viscous oil. As a result, 35%–65% of the OOIP remains unrecovered after CO2 flooding. Further, the average domestic CO2 utilization ratio is 10,700 scf of CO2/barrel of oil; a much larger than desired ratio of solvent to oil. The fundamental causes of this disappointingly low oil recovery can be traced to the density and viscosity of dense CO2. First, the low density of high-pressure CO2 relative to oil promotes gravity override of the CO2, reducing oil recovery in the lower portions of the formation. Second, the low viscosity of dense liquid or supercritical carbon dioxide at typical CO2 flooding conditions is ～0.05–0.10 cP, a value so much lower than typical oil and brine viscosity values that it results in an unfavorable mobility ratio. This leads to viscous fingering, which in turn leads to early CO2 breakthrough, high CO2 utilization ratios, delayed CO2 production, depressed oil production rates, and low percent OOIP recovery. These problems can be exacerbated when the injection well is completed in two or more producing zones; the low viscosity of CO2 promotes its flow into the more permeable layers that have been effectively waterflooded, while disappointingly small amounts of CO2 enter the lower permeability zones that contain more recoverable oil.