Experimental study on effects of CO2 and improving oil recovery for CO2 assisted SAGD in super-heavy-oil reservoirs

When developing heavy-oil reservoirs using steam-assisted gravity drainage (SAGD) technology, CO2 is often used to reduce the viscosity of heavy oil and the heat loss of the steam chamber, thereby enhancing oil recovery. The limitation of experimental apparatuses, results in a maximum temperature of the reaction kettle of no more than 150 °C. Thus, the solubility of CO2 in super-heavy oil and its impact on decreasing viscosity beyond 150 °C requires further discussion. Considering these issues, a high-temperature-resistant heavy-oil mixing apparatus was designed to measure the solubility of CO2 in super-heavy oil and the relative density and viscosity of super-heavy oil under high temperatures and pressures. Core flooding experiments under 200 °C were conducted to explore the effects of CO2 on oil displacement efficiency. The results indicated that solubility of CO2 became significantly lower at 200 °C, when the viscosity and relative density of super-heavy oil decreased as pressure increased. The viscosity of super-heavy oil saturated with CO2 decreased by 71.2%, and relative density decreased by 10.1% at 200 °C compared to those of degassed crude. At temperatures of 120 °C, 150 °C and 200 °C, the ultimate displacement efficiency of super-heavy oil increased by 6.6%, 19.2% and 21.4%, respectively, compared to a temperature of 60 °C. At pressures of 1.0 MPa, 2.0 MPa and 4.0 MPa, the ultimate displacement efficiency (46.7%) of super-heavy oil increased by 5.2%, 4.0% and 5.9%, respectively, compared to a pressure of 0.5 MPa. The experiments showed that CO2 injection can effectively reduce the relative density and viscosity of super-heavy oil and improve oil recovery under high temperatures and pressures. CO2 has good potential to improve SAGD efficiency in developing super-heavy-oil reservoirs.