CO2 utilization and sequestration in Reservoir: Effects and mechanisms of CO2 electrochemical reduction

Owing to the harmful consequences of the greenhouse effect, mankind has reached a consensus to reduce CO2 emissions. CO2 utilization and sequestration is considered the most feasible solution for realizing an effective and economical sequestration of CO2. Herein, CO2-enhanced oil recovery (EOR) is a representative strategy owing to its tremendous sequestration capacity and reasonable economic viability. However, the low sequestration stability and the accompanying secondary disasters have hindered its wider application. Considering this, a novel CO2 utilization and sequestration technology, i.e., CO2 electrochemical reduction (CO2RR)-EOR technology was proposed and its effects and mechanisms were studied. First, a SnO2 catalyst that can be used in the coreflooding experiments was synthesized and its efficiency on CO2RR was evaluated. Then, coreflooding experiments using the SnO2 catalyst were conducted to study the influence of CO2RR on the CO2 sequestration and oil recovery. Next, contact angle and interfacial tension were measured to analyze the effect of the CO2RR on the CO2-crude oil-brine-rock interaction. Finally, the CO2RR-EOR mechanism was proposed. Multiple experiments revealed that the low-cost and easy-synthesis SnO2 catalyst, which was characterized by the 3-dimensioned hierarchical and porous structure, was in the high efficiency and strong stability. Coreflooding results illustrated that the CO2RR remarkably enhanced CO2 sequestration and oil recovery. At the optimal cell potential of 3.75 V, the CO2 sequestration efficiency and oil recovery were as high as 75.93% and 43.8%, respectively, which are much higher than those (50.56% and 37.9%, respectively) achieved in conventional CO2 flooding. Contact angle and interfacial tension results indicated that the CO2RR has a significant impact on the CO2-crude oil-brine-rock interaction. At the optimal cell potential, the crude oil-contact angle and the interfacial tension were 36.2° and 8.9 mN/m, respectively, which are much lower than those (64.6° and 16.4 mN/m, respectively) achieved in conventional CO2 flooding; the CO2-contact angle was 52.2°, which too is much larger than that (31.7°) achieved in conventional CO2 flooding. In this technology, the CO2RR converts the injected CO2 into formate to sequestrate itself, and meanwhile alters the CO2-crude oil-brine-rock interactions, resulting in enhanced CO2 wettability and consequently increases CO2 trapping and retention; causing the weakened oil wettability and the enhanced oil-brine interaction, and finally enhances oil recovery. CO2RR-EOR can be a novel, efficient, and economically-viable CO2 utilization and sequestration technology, which provide a feasible option for the future industrial CO2 emission reduction.