Discussion on the limit recovery factor of carbon dioxide flooding in a permanent sequestration scenario

Based on practices of CO2 flooding tests in China and abroad, the recovery factor of carbon dioxide capture, utilization in displacing oil and storage (CCUS-EOR) in permanent sequestration scenario has been investigated in this work. Under the background of carbon neutrality, carbon dioxide injection into geological bodies should pursue the goal of permanent sequestration for effective carbon emission reduction. Hence, CCUS-EOR is an ultimate development method for oil reservoirs to maximize oil recovery. The limit recovery factor of CCUS-EOR development mode is put forward, the connotation differences between it and ultimate recovery factor and economically reasonable recovery factor are clarified. It is concluded that limit recovery factor is achievable with mature supporting technical base for the whole process of CCUS-EOR. Based on statistics of practical data of CO2 flooding projects in China and abroad such as North H79 block CO2 flooding pilot test at small well spacing in Jilin Oilfield etc., the empirical relationship between the oil recovery factor of miscible CO2 flooding and cumulative CO2 volume injected is obtained by regression. Combined with the concept of “oil production rate multiplier of gas flooding, a reservoir engineering method calculating CO2 flooding recovery factor under any miscible degree is established by derivation. It is found that when the cumulative CO2 volume injected is 1.5 times the hydrocarbon pore volume (HCPV), the relative deviation and the absolute difference between the recovery percentage and the limit recovery factor are less than 5% and less than 2.0 percentage points respectively. The limit recovery factor of CCUS-EOR can only be approached by large pore volume (PV) injection based on the technology of expanding swept volume. It needs to be realized from three aspects: large PV injection scheme design, enhancing miscibility degree and continuously expanding swept volume of injected CO2.