Reducing the Miscibility Pressure in Gas Injection Oil Recovery Processes

Abstract

AbstractIn gas flooding EOR applications the injection pressure plays a significant role. It has to be higher than the minimum miscibility pressure (MMP) for a fully miscible flood to obtain the highest oil recovery, since the swelling, and consequently the efficiency of a flood, strongly depends on the miscibility of the residual oil and the recovery fluid. Poor miscibility leads to poor recovery. On the other hand the injection pressure must not be higher than the reservoir fracture pressure to avoid the creation of high thief zones in the formation. The addition of proper additives to the injection gas can reduce the MMP significantly.Using a pressure resistant sapphire cell firstly the solubility of different additives in the gas at common reservoir conditions was determined. The compatibility of the additives with CO2 is needed to be ascertained first in order to identify compounds that enable an easy co-injection of the additive within the flow of the CO2. Then the miscibility behavior of the gas and a model oil was studied and the change of the miscibility pressure in the presence of chosen additives was investigated. Phase behavior studies at typical reservoir pressures and temperatures show the miscibility behavior of the fluids and the effect of the different additives on the mutual miscibility.It was proven that the minimum miscibility pressure can be lowered significantly by the choice of an appropriate additive. Through determination of phase diagrams it is shown how the miscibility gap between crude oil and CO2 can be decreased, thereby going from immiscible or multi-contact miscible to fully miscible conditions. Adjusting the MMP broadens the pressure range of high sweep efficiency that is limited by the formation fracture pressure as the upper and the MMP as the lower limit. Thus, conditions can be created at which the reservoir pressure leads to fully miscible floods.Using an appropriate additive can lead to an improvement of the miscibility behavior at given reservoir conditions and make it more favorable for a CO2 EOR process. A former near-miscible or even immiscible application can become a miscible flood. By this the sweep efficiency is maximized and the recovery rate is highest. The parameters that define an economic reasonable gas flooding application now comprise a larger number of reservoirs due to the improved miscibility of crude oil and injection gas.