Improving recovery efficiency by CO2 injection at late stage of steam assisted gravity drainage

The high recovery performance of steam-assisted gravity drainage (SAGD) makes it a popular option for heavy oil resources. Currently, most of the heavy oil reservoirs developed by SAGD in China are in the late development phase, with high energy consumption due to reduced thermal efficiency. The use of SAGD wind-down processes involving CO2 in combination with steam for heavy oil recovery is considered as a viable alternative to limit energy consumption, and also reduce the amount of greenhouse gas emissions by leaving CO2 behind in the reservoir. Study reveals that the dissolution and demulsification of CO2 in crude oil can reduce the viscosity of emulsified heavy oil by more than 50%. When the steam chamber temperature reaches 200 degrees C, the amount of solid phase deposition induced by CO2 extraction is only 0.016 kg/m(3), the rock wettability changes from lipophilic to hydrophilic, and the higher the reservoir temperature, the stronger the hydrophilicity is, which reduces the adhesion power of the oil phase and facilitates the stripping of crude oil from the rock surface. Numerical simulation studies have been carried out utilizing STARS to obtain energy efficient utilization and improved steam chamber characteristics. Heat loss from SAGD baseline is 1.77 times that with CO2 injection process, but the recovery factor is only 2.48% higher. At the initial stage with CO2 injection, the steam chamber continues its lateral expanding, which increases the recovery factor at the initial stage of CO2 injection by about 6%. One year after CO2 injection, gas channeling results in lower recovery than traditional SAGD process, and 38.4% of the injected CO2 is stored in the reservoir from this study.