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

The steam assisted gravity drainage (SAGD) process requires high energy input to maintain the continuous expansion of the steam chamber for achieving high oil recovery. In the late stage of SAGD operation where the oil rate is low and the heat loss is high from a mature steam chamber, maintaining steam chamber pressure with a lower steam injection is the key to maintaining the economic oil-to-steam ratio (OSR). Both laboratory studies and field tests have demonstrated the effectiveness of adding a non-condensable gas (NCG) to the SAGD steam chamber for improving the overall thermal efficiency. In this study, a multi-well reservoir model was built based on the detailed geological description from an operating SAGD project area, which contains thick pay and top water. Grounded with the history matching of more than 20 years of production using CSS (cyclic steam stimulation) and SAGD as follow-up process, the model was applied to optimize the operating strategies for the late stage of SAGD production. The results from this study demonstrated that the co-injection of steam with CO₂ or the injection of CO₂ only has potential to improve the OSR and reduce emissions by more than 50% through the improvement in steam-saving and the storage of CO₂. The results from reservoir modeling indicate that, with the current volume of a steam chamber and an operating pressure of 4.0 MPa, about 55 sm³ of CO₂ could be sequestrated and utilized for producing 1.0 m³ of oil from this reservoir through the replacement of a steam injection with CO₂ in the late stage of SAGD operation.