This paper will present experimental results that demonstrate a novel plugging mechanism for improving the areal sweep efficiency for EOR solvents such as carbon dioxide or light hydrocarbons, or mixtures of the two. Early gas breakthrough due to thief zones or gas override is the single most significant factor affecting the sweep efficiency of miscible gas processes. WAG is routinely used and foams are being investigated for diverting gas to upswept zones. Both approaches require injecting an aqueous phase which leads to water blocking and slows gas injection, thus delaying oil recovery. Additionally, foams have a short life, are limited to near-wellbore applications due to surfactant during every cycle of surfactant injection.

The optimal solution calls for a deep penetrating plugging agent carried in the solvent phase for accurate and economical delivery to thief or override zones. Additionally, the process should be simple, safe in the wellbore region, robust, and easily activated. It has already been demonstrated that cosolvents like toluene will successfully solubilize polydimethylsiloxane-type polymers in CO2, and that polymer solubility decreases with increasing temperature and decreasing pressure. Consequently, it is feasible to define a polymer + cosolvent combination that is homogeneous at the lower temperature of the injector, but will destabilize as the injected slug moves through the formation and is subjected to higher temperatures and lower pressures.

Figure 1 represents the solubility phase boundary for a 600,000 cst polydimethylsiloxane polymer in CO2- toluene mixtures as a function of temperature, pressure, and two cosolvent concentrations. For either cosolvent concentration, a single phase condition exists for temperature and pressure combinations that would place the system above the phase boundary line, with polymer dropping out of solution once the system conditions drop below the line. Consider the specific case of a CO2 flood in a reservoir at 180°F. and 4,000 psia, where the injector is at 130 ° F., and early gas breakthrough has taken place. The working of this process would require injecting a CO2 phase enriched with 7.1 vol.% toluene and 600,000 cst polymer as a one phase system at the injector. As this mixture moves through the reservoir and increases in temperature, the stability decreases, and once the temperature at fixed pressure crosses the phase boundary line, the polymer phase separates out to plug the zone.

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