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Injectivity of CO2 WAG in low permeability oil reservoirs based on B-L equations
MENG Fankun1,2, SU Yuliang2, HAO Yongmao2, LI Yajun2, TONG Gang3
(1.Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;2.School of Petroleum Engineering in China University of Petroleum(East China), Qingdao 266580, China;3.The First Oil Production Plant of Changqing Oilfield Company, PetroChina, Xi 'an 710200, China)
Abstract:
The loss of injectivity has become one of the key factors restricting the recovery efficiency in CO2 water-alternating-gas (WAG) projects for low permeability reservoir. In this paper, considering the solubility of CO2 in oil and water and the evaporation of oil components by CO2, a method of characteristics was used, and a retardation factor was introduced to modify the Buckley-Leverett(B-L) equations during CO2 flooding and water displacing CO2. Then, the front and rear velocity for CO2 flooding and water displacing gas can be determined. A mathematic model to calculate the injectivity of CO2 WAG was proposed based on the flowing theory of composite reservoirs and the front and rear velocity of CO2 and water flow in the WAG process. The methods of Laplace transformation and Stehfest numerical inversion were applied to solve the model and obtain an approximate solution of the bottom-hole pressure. The analytical model was verified by numerical simulation results for reservoirs with different permeability. Sensitivity analysis shows that, in comparison with partial oil-wet reservoirs, the injectivity of water is lower for partial water-wet formations. The water injectivity increases with the reduction of CO2-oil interfacial tension. When the reservoir permeability is lower than 5×10-3μm2, the injectivity of water deceases significantly.
Key words:  low permeability oil reservoir  water-alternating-CO2 flooding  injectivity  B-L equations  composite model