Quantitative Evaluation Of Critical Leakage Conditions Of Fault Rock When Gas Driving Development In Xin48 And Yong66 Blocks Of Bohai Bay Basin

After 50 or 60 years of exploration and development,Xin48 and Yong66 Blocks in Bohai Bay Basin have entered an extremely high water-cut period.The degree of water flooding in layers varies greatly,the water channeling is extremely serious,the remaining hydrocarbon is scattered,and the development benefits are sharply reduced.The gas driving technology is an important means to maintain formation pressure,keep high productivity of production wells and improve hydrocarbon recovery.However,under the effects of high gas injection volume and high formation pressure,hydrocarbon and other fluids may leak through fault rock.Therefore,quantitative evaluation of critical leakage conditions of fault rock is of great significance to formulate a safe and reasonable gas driving development plan.On the basis of the internal structural of the fault zone and the characteristics of its leakage conduit,combined with the mechanism of capillary sealing and deformation of brittle rock,this paper systematically summarizes the mechanism and influencing factors of fault rock leakage through pore,tension fracture and shear fracture.Based on the experimental results of entry pressure and clay content of typical rock samples in the study area,a quantitative evaluation method of critical leakage conditions through the pore of fault rock when gas driving is established by defining the variation law of diagenesis degree of fault rock and surrounding rocks with diagenesis time and diagenesis pressure.Based on the triaxial compression experimental results of typical rock samples,the tensile fracture pressure of surrounding rocks was determined by using the Huang Model with pressure correction,a quantitative evaluation method of critical leakage conditions through the tension fracture of fault rock when gas driving is established by multi-factor correlation analysis.By fitting the relationship between the clay content of fault rock and the friction coefficient of fault surface,combing with the three-dimensional stress analysis results of fault,a quantitative evaluation method of critical leakage conditions through the shear fracture of fault rock when gas driving is established based on the Coulomb rupture criterion.According to the above quantitative evaluation model,the conditions required to prevent oil drived by gas leakage through fault rock in Xin48 and Yong66 Blocks of Bohai Bay Basin are quantitatively studied.The results indicated that:(1)Influenced by the burial depth and residual hydrocarbon of fault related trap,the critical residual pressure for pore leakage in fault rock produced by gas injected during development phase of Xin48 Block(3.688~10.255 MPa)is significantly higher than that of Yong66 Block(0.144~0.783MPa),which can withstand greater gas injection volume.(2)Influenced by the burial depth of fault,the critical pressure increment for tensile fracture leakage in fault rock of Xin48 Block(21.234~39.852 MPa)is obviously greater than that of Yong66 Block(6.534~10.500MPa),which can withstand greater gas injection pressure.(3)Influenced by the clay content,burial depth and the variation of principal stress,the distribution range of the critical pressure increment for shear fracture leakage in fault rock of Yong66 Block(0.802~21.961MPa)is wider than that of Xin48 Block(6.207~18.802MPa).From the perspective of fault trap,the maximum injection pressure during gas driving of Xin48 Block is also significantly higher than that of Yong66 Block.(4)For Xin48 Block with relatively deep burial depth,the critical pressure increment for tensile fracture leakage in fault rock is relatively large,with the increasing injection pressure when gas driving,the fault rock preferentially leakage through shear fracture.While for Yong66 Block,which is relatively shallowly buried due to the uplift and erosion,except for the f4 and f5 faults,the critical pressure increment for shear fracture leakage in fault rock is relatively small,the rest fault rocks are preferentially leakage through tension fracture.This is consistent with the fact that brittle rocks are prone to shear fracture leakage under deep burial and tension fracture leakage under shallow burial.