Low-permeability heavy oil reservoirs are characterized by poor flowability, generally mandating hydraulic fracturing to commence production. CO_(2) huff-n-puff in fractured reservoirs is an effective enhanced oil rec...Low-permeability heavy oil reservoirs are characterized by poor flowability, generally mandating hydraulic fracturing to commence production. CO_(2) huff-n-puff in fractured reservoirs is an effective enhanced oil recovery method. This paper uses nuclear magnetic resonance imaging to elucidate the role of propped and unpropped fractures on CO_(2) huff-n-puff in cores under different confining pressures. In presence of fractures, significant improvement in the rate of early stage oil recovery is observed, up to 0.255 mL/min. Fractures enlarge the contact area between CO_(2) and the heavy oil, hence improve CO_(2) dissolution and oil flowability. Fractures improve oil recovery from micropores, small pores, and mesopores, as well as reduce CO_(2) consumption ratio. The oil recovery factor in propped fractures is significantly higher than that in unpropped fractures, and with higher oil recovery from small pores and mesopores. The oil recovery in fractured cores noticeably decreases with increasing confining pressure. The extent of fracture closure increases and the matrix pore throats compress under pressure leading to lower apparent permeability. The decrease in oil recovery factor is more pronounced in unpropped fractured cores. A relationship between the apparent permeability of the fracture aperture is derived based on the modified cubic law of percolation to quantitatively characterize the fracture. Additionally, both the reduction in heavy oil viscosity and the increase in experimental temperature and pressure can improve the CO_(2) huff-n-puff oil recovery factor in fractured cores.展开更多
Huge numbers of induced unpropped(IU)fractures are generated near propped fractures during hydraulic fracturing in shale gas reservoirs.But it is still unclear how their fracture space and conductivity evolve under in...Huge numbers of induced unpropped(IU)fractures are generated near propped fractures during hydraulic fracturing in shale gas reservoirs.But it is still unclear how their fracture space and conductivity evolve under in-situ conditions.This paper prepares three types of samples,namely,manually split vertical/parallel to beddings(MSV,MSP)and parallel natural fractures(NFP),to represent the varied IU fractures as well as their surface morphology.Laser scan and reconstruction demonstrate that the initial fracture spaces of MSVs and MSPs are limited as the asperities of newly created surfaces are wellmatched,and the NFPs have bigger space due to inhomogeneous geological corrosion.Surface slippage and consequent asperity mismatch increase the fracture width by several times,and the increase is proportional to surface roughness.Under stressful conditions,the slipped MSVs retain the smallest residual space and conductivity due to the newly sharp asperities.Controlled by the bedding structures and clay mineral hydrations,the conductivity of MSPs decreases most after treated with a fracturing fluid.The NFPs remain the highest conductivity,benefitting from their dispersive,gentle,and strong asperities.The results reveal the diverse evolution trends of IU fractures and can provide reliable parameters for fracturing design,post-fracturing evaluation,and productivity forecasting.展开更多
基金supported by the Open Research Fund of State Key Laboratory of Deep Oil and Gas (No. SKLDOG2024-KFYB-14)the support received from the National Nature Science Foundation of China under Grant No. U20B6003China Scholarship Council (CSC) for financial support
文摘Low-permeability heavy oil reservoirs are characterized by poor flowability, generally mandating hydraulic fracturing to commence production. CO_(2) huff-n-puff in fractured reservoirs is an effective enhanced oil recovery method. This paper uses nuclear magnetic resonance imaging to elucidate the role of propped and unpropped fractures on CO_(2) huff-n-puff in cores under different confining pressures. In presence of fractures, significant improvement in the rate of early stage oil recovery is observed, up to 0.255 mL/min. Fractures enlarge the contact area between CO_(2) and the heavy oil, hence improve CO_(2) dissolution and oil flowability. Fractures improve oil recovery from micropores, small pores, and mesopores, as well as reduce CO_(2) consumption ratio. The oil recovery factor in propped fractures is significantly higher than that in unpropped fractures, and with higher oil recovery from small pores and mesopores. The oil recovery in fractured cores noticeably decreases with increasing confining pressure. The extent of fracture closure increases and the matrix pore throats compress under pressure leading to lower apparent permeability. The decrease in oil recovery factor is more pronounced in unpropped fractured cores. A relationship between the apparent permeability of the fracture aperture is derived based on the modified cubic law of percolation to quantitatively characterize the fracture. Additionally, both the reduction in heavy oil viscosity and the increase in experimental temperature and pressure can improve the CO_(2) huff-n-puff oil recovery factor in fractured cores.
基金supported by the National Natural Science Youth Foundation of China(No.52104003)the Open Fund of Engineering Research Center of Development and Management for Low to Ultra-Low Permeability Oil&Gas Reservoirs in West China,Ministry of Education(No.KFJJ-XB-2020-5)+2 种基金the Science and Technology Planning Project of Sichuan Province(No.22NSFSC4005)the National Natural Science Foundation of China(No.52274031 and No.52374005)the Natural Science Youth Foundation of Sichuan Province(No.2023NSFSC0930)。
文摘Huge numbers of induced unpropped(IU)fractures are generated near propped fractures during hydraulic fracturing in shale gas reservoirs.But it is still unclear how their fracture space and conductivity evolve under in-situ conditions.This paper prepares three types of samples,namely,manually split vertical/parallel to beddings(MSV,MSP)and parallel natural fractures(NFP),to represent the varied IU fractures as well as their surface morphology.Laser scan and reconstruction demonstrate that the initial fracture spaces of MSVs and MSPs are limited as the asperities of newly created surfaces are wellmatched,and the NFPs have bigger space due to inhomogeneous geological corrosion.Surface slippage and consequent asperity mismatch increase the fracture width by several times,and the increase is proportional to surface roughness.Under stressful conditions,the slipped MSVs retain the smallest residual space and conductivity due to the newly sharp asperities.Controlled by the bedding structures and clay mineral hydrations,the conductivity of MSPs decreases most after treated with a fracturing fluid.The NFPs remain the highest conductivity,benefitting from their dispersive,gentle,and strong asperities.The results reveal the diverse evolution trends of IU fractures and can provide reliable parameters for fracturing design,post-fracturing evaluation,and productivity forecasting.
