CO2 flooding can improve crude oil recovery,and realize CO2 underground storage, which is an important means to increase the recovery rate in tight reservoirs. CO2-water alternating flooding,which is a method to combine the advantages of CO2 flooding and water flooding,has good applicability and is the most commonly used method in gas injection preventing channeling. However,due to the low viscosity of water medium under reservoir conditions, plugging ability for water to heterogeneous reservoirs is limited during CO2-water alternate flooding, and the oil displacement effect of CO2 flooding needs to be further improved. Therefore, a low interfacial tension small molecule tackifying system (DXZT) was constructed based on small molecule oligomeric organic active substances. When implementing alternative flooding with CO2, the DXZT has good injection performance and environmental adaptability in heterogeneous tight reservoirs, meeting the needs of improving the swept volume and oil washing efficiency of CO2 flooding. In this paper, the interfacial activity, viscosity increasing, emulsification, wettability, foam properties and reservoir environmental adaptability of the DXZT were systematically evaluated. The injectivity and preventing gas channeling effect of the CO2 and DXZT alternating flooding were investigated, and the preventing CO2 channeling mechanism of DXZT was analyzed. The results show that DXZT has good interfacial activity (≤0.02mN/m ), viscosity enhancing (2.0-15.4mPa·s), environmental adaptability of high salinity, high calcium ion and acidic CO2 ( salinity ≤ 80000mg/L, Ca2+ ≤ 15400mg/L, pH ≥3 ), oil-water emulsifica -tion, water wettability and foam performance(foam comprehensive index ≥ 15443mL·min). CO2 and DXZT alternate flooding has good injectivity for tight core (0.242mD). After water flooding in heterogeneous tight cores, CO2-DXZT alternative flooding recovery increased by 14.8% than comprehensive water flooding recovery, which was better than single CO2 flooding or DXZT flooding. With the increase of alternating drive rounds, the recovery rate of low permeability core and total comprehensive recovery rate are increasing. Which shows that DXZT has the ability to block the high permeability channel,divert CO2 to the low permeability channel,start the remaining oil,and prevent CO2 channeling in high permeability zone. The anti-gas channeling mechanism of DXZT includes: DXZT increases viscosity and has better mobility adjustment ability than water; is easy to inject high permeability channel or fracture,plug and seal these channelings, and form foam with CO2 that has entered the channel,strengthen the sealing ability to high permeability channel. That is beneficial to the subsequent CO2 divert, start the remaining oil in the low permeability channel and expand the swept volume of CO2 flooding. Thus, DXZT is a new method and technology to prevent gas channeling in CO2 flooding, which has broad application prospects in improving the recovery of CO2 flooding in heterogeneous tight oil reservoirs.
Keywords heterogeneous tight oil reservoirs, CO2 flooding, small molecule tackifying system, Preventing gas channeling mechanism, New technology for preventing gas channeling