Carbon dioxide (CO2) flooding is an effective method to enhance oil recovery in low-permeability reservoirs. Studying key geological factors controlling oil displacement efficiency is of great significance to the CO2 injection scheme design in low-permeability reservoirs. Focusing on low-permeable H reservoir in Songliao Basin, China, this paper describes the contact and connection of sand bodies, natural fractures and high-permeability zones with core samples, log data and experiment firstly. After that, the impact of interaction of sand body connection, natural fracture and high-permeability zone on oil displacement efficiency is determined by using geological and dynamic data in CO2 injection area. Results indicate that the connection of single sand bodies between injectors and producters wells primarily controls CO2 flooding in low-permeability reservoirs. Furthermore, coupling of sand body connection, natural fractures and high-permeability zones is the key geological factor governing oil displacement efficiency of CO2 injection in low-permeability reservoirs, where well or generally-connected sand bodies can improve the efficiency significantly. Meanwhile, the dominant seepage channels in other directions have no influence on producers, which is beneficial to improve CO2 flooding efficiency.
Natural fractures are identified as high-quality storage space and seepage channels for the Triassic tight sandstone reservoirs in the Dongpu Depression, playing an important role in the tight sandstones oil production. In this paper, outcrops, cores, thin sections and imaging logs were used to describe the growth of natural fractures at different scales. The correlation between fracture and crude oil production capacity was analyzed using production data. Results show that natural fractures are primarily distributed in fine sandstones and siltstones, which are mostly shear fractures of near E-W and NE-SW strikes. The natural fractures of near E-W strike are generally parallel to the present-day maximum horizontal principal stress with the biggest apertures and the highest permeability, which are the main seepage channels, next being the fractures of NE-SW strike. The natural fractures of near E-W strike are also the most important contributors to the crude oil production in the Triassic tight sandstones of the Dongpu Depression. The intensity, permeability and direction of natural fractures govern the crude oil productivity in the per unit sandstone thickness.
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