关键词: CO₂重力混相驱, CCUS, 潜山油藏, 作用机制, 驱油特征, 在线核磁

Abstract: To address the unclear mechanism of CO₂ gravity flooding implemented in Balixi fracture-type buried hill reservoir,this paper,considering the reservoir characteristics and development status,explored the interaction mechanisms between reservoir fluids and CO₂ through the gas injection expansion,core displacement,and NMR scanning experiments,and studied the effect of CO₂ gravity flooding after water injection at the bottom of the reservoir,clarifying the production patterns of crude oil within the pore and fracture reservoir spaces.The study shows that:after the injection of CO₂,the viscosity of crude oil is reduced by 57%,and its volume is expanded by a factor of 1.83,indicating that CO₂ has a significant effect on reducing the viscosity and expanding the volume of crude oil.The results of PVT (Pressure-Volume-Temperature) experiments indicate that the presence of a water phase has almost no effect on the interaction between crude oil and CO₂.During the oil-gas mass transfer process,CO₂ can extract and strip the C₂-C₁₂ components from the oil,which is conducive to the recovery of lighter fractions,while heavier fractions (C₁₀-C₃₆₊) remain in place.Implementing CO₂ gravity miscible displacement after water injection at the bottom of the reservoir can further enhance the recovery of crude oil.Both the degree of oil-gas mass transfer and the gas injection rate affect the oil displacement efficiency.The stronger the degree of oil-gas mass transfer and the lower the injection rate,the more effectively CO₂ can extract,strip,and dissolve,thereby fully producing the contiguous remaining oil and the remaining oil that has not been in direct contact with CO₂.Based on NMR experiment results,the reservoir space can be divided into pore space and fracture space.Compared with CO₂ non-miscible flooding,CO₂ gravity miscible flooding after water flooding can improve the recovery from both matrix and fractures.After miscible flooding of twice the pore volume,the recovery from the matrix is 49.3%,and that from the fractures is 99.3%.Field practice has demonstrated that top-injected CO₂ gravity miscible displacement is an effective means of succession development for enhancing oil recovery in the Balixi buried hill reservoir.

Key words: CO₂ gravity miscible displacement, CCUS, buried hill reservoir, mechanism, oil displacement characteristics, online NMR