Experimental investigation on CO2 injection in the Daqing extra/ultra-low permeability reservoir

This paper aims to study the effect of four kinds of CO2 injection schemes, namely water-alternating-gas (CO2/water), immiscible flooding, miscible flooding, and cyclic CO2 injection, in Daqing peripheral extra/ultra-low permeability reservoirs. Physical simulation experiments and nuclear magnetic resonance (NMR) measurements were conducted to investigate how crude oil residing in different sized pores are recovered by different gas injection schemes in extra/ultra-low permeability cores and to quantify the recovery factors and residual oil distribution in different sized pores. Experimental results indicate that the residual oil is mainly found in the <10 ms T2 spectrum (clay micro pore), followed by 10-100 ms (medium pore) and >100 ms T2 spectrum (large pore); therefore, it is important to enhance recovery in the pores and throats smaller than 10 ms. Water flooding injection gives the worst oil recovery, but converting to CO2 injection can greatly improve the recovery efficiency depending on the injection mode. For extra-low permeability (1-10 mD) cores, cyclic CO2 injection displacement of crude oil in the micro pores has the highest efficiency, followed by CO2 miscible flooding, WAG flooding and immiscible CO2 flooding in that order. For ultra-low permeability (<1 mD) cores, WAG injection shows the best performance among the four injection schemes. The distribution of remaining oil from NMR shows that both CO2 miscible and immiscible injection mainly recover the oil in large-medium sized pores, whereas early gas breakthroughs cause the crude oil in clay micro pores to be difficult to displace. However, although the overall recovery factors of WAG and cyclic CO2 injection are very close, their mechanisms of recovering oil from clay micro pores are different. WAG greatly enhances the fluid mobility ratio and delays the breakthrough time due to the water blocking effect. In the cyclic CO2 injection process, pressure equilibration during the soak time causes the crude oil in the micro pore to flow into the large-medium pore, thus greatly increasing the displacement efficiency of crude oil in micro pores.