Improved Sweep Efficiency by Alcohol-Induced Salt Precipitation

Abstract

Abstract A selective profile modification process was studied to improve volumetric sweep efficiency. This method involved adding alcohol to brine to reduce solubility of salt, which caused the salt to precipitate. For this process, a concentrated brine preflush was injected. This was followed by the injection of a water-soluble alcohol, such as ethanol. The salt precipitation method has several potential advantages over other profile modification methods such as polymer gels. Profile modification by salt precipitation is not affected by adsorption, and other reservoir conditions, such as, fluid pH and temperature. These are conditions that may affect the gelation process and stability of polymer gels. For applications with CO2 floods, the solid salt is stable to subsequent CO2 flooding. Carbon dioxide flooding can lower the pH of reservoir fluids, a condition which may adversely affect the stability of some polymer gels. Laboratory experiments indicated that the permeabilities of cores treated by salt precipitation were resistant to subsequent waterfloods with low-salinity brines. Core permeabilities recovered only 5 to 10% of the original permeability after injecting several pore volume of low-salinity brine. Also, corefloods demonstrated the feasibility of the new process to plug high-permeability regions. In parallel coreflooding experiments, the injection of alcohol and NaCl-saturated brine selectively reduced the permeability of highly watered-out cores. Using about 7% PV of reagent grade ethanol for salt precipitation, oil recovery with parallel corefloods of high and low permeabilities was increased by about 20% of the OOIP or about 60% above that of conventional waterflooding. For CO2 coreflooding experiments, the injection of ethanol and NaCl-saturated brine slugs resulted in 12% additional oil recovery of the OOIP or 45% higher oil recovery compared to that of CO2 floods without salt precipitation.