Special Oil & Gas Reservoirs ›› 2025, Vol. 32 ›› Issue (1): 98-105.DOI: 10.3969/j.issn.1006-6535.2025.01.011

• Reservoir Engineering • Previous Articles     Next Articles

Mechanism of CO2 gravity miscible displacement and oil displacement experiment in buried hill reservoirs

GUO Fajun1, LIANG Fei2, CHEN Hong1, QI Huan3,4, ZENG Qingqiao1, WANG Ruisi1, LI Yiqiang3,4, WU Yongen3,4   

  1. 1. PetroChina Huabei Oilfield Company,Cangzhou,Hebei 061000, China;
    2. PetroChina Liaohe Oilfield Company,Panjin,Liaoning 124000, China;
    3. State Key Laboratory of Oil and Gas Resources and Engineering,China University of Petroleum (Beijing),Beijing 102249, China;
    4. College of Petroleum Engineering,China University of Petroleum (Beijing),Beijing 102249, China
  • Received:2023-12-06 Revised:2024-11-11 Online:2025-02-25 Published:2025-05-13

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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

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