Isolated mechanism study on in situ CO2 EOR

•IFT reduction between crude oil and hydrolyzed/unhydrolyzed urea solution was measured.•The wettability alteration of in situ CO2 EOR was directly observed by contact angle measurement.•The effectiveness of the proposed in situ CO2 EOR system was demonstrated in carbonate and sandstone reservoirs.•The mechanism and the factors that affect the effectiveness of the in situ CO2 EOR system were quantified.•The screening criteria for in situ CO2 EOR candidates are proposed.