Carbon dioxide miscible flooding is a proven EOR method. It faces two significant challenges: gravity override and early CO2 break through. Many researchers have investigated different methods to control CO2 mobility and improve its sweep efficiency. This paper, for the first time, investigates the use of a new and unique CO2-foam (emulsion) to control CO2 mobility and also as a conformance control technique in heterogeneous reservoirs with high permeability contrast within carbonate rocks.
The new CO2-emulsion system consists of 50-70 vol% supercritical CO2, 50-30 vol% of water-based polysaccharide linear polymer and a foaming agent (surfactant). Several experiments were conducted using HPHT visual cell to assess the CO2-emulsion stability as a function of temperature, pressure and time. In addition, the rheological properties of the CO2-emulsions were investigated at different shear rates, different pressures and mixing ratios, and at an operating temperature of 220°F. Special dual core flooding experiments were conducted using live oil at reservoir conditions to investigate the effectiveness of CO2-emulsion system in enhancing oil recovery. Several experiments were conducted to explore the effect of injection rate, injection mode and slug volume on incremental oil recovery. These experiments were performed using dual core holders with different permeable carbonate composite stacks, and permeability contrasts up to 35.
Results based on this study have shown that the CO2-emulsion system is stable at 210°F for extended periods of time without any emulsion breakage or phase separation. The effective viscosity of CO2 was increased by 3-4 orders of magnitude and approached 100 cP at reservoir conditions. Results also show the emulsion's ability to severely reduce permeability of the higher permeable cores resulting in significant incremental oil recovery from the lower permeable cores. This new and unique emulsion system has the ability to be created in-situ to provide better mobility control of the injected CO2. Additionally, this paper provides optimal design parameters of the new emulsion system to behave as a conformance control agent, and also to enhance the recovery of oil following water and CO2 floods.