| There are abundant continental shale oil resources in the Sedimentary strata of Chang 7~2Member of Yanchang Formation of Upper Triassic in Ordos Basin,which is expected to become a major replacement area for the increase of crude oil reserves and production in China.In recent years,domestic and foreign research has mainly focused on hydraulic fracturing of marine shale.The generally low production capacity of shale gas in marine shale is closely related to the development of structural plane such as bedding and fracture.Chang 7~2is dominated by continental shale,and the structural plane formed during the sedimentary diagenesis are more developed,which has an important impact on the production capacity of continental shale oil.Due to the late exploration and development of continental shale,the current research mainly focuses on reservoir characteristics,and there is less research on the mechanical properties and hydraulic fracturing characteristics of continental shale containing structural plane.However,structural plane is an essential factor for analyzing the mechanical properties of continental shale,and also has an important influence on the hydraulic fracturing characteristics.Therefore,the thesis takes the Chang 7~2as the research object,and investigates the mechanical characteristics of continental shale containing structural plane,hydraulic fracture communication structural plane by conducting uniaxial compression test,Splitting tensile load acoustic emission test,triaxial compression test,hydraulic fracture acoustic emission test and displacement-shear slipping test for continental shale with different bedding dip angles.The mechanical properties,the extension mechanism of hydraulic fractures communicating with the structural plane and the extension characteristics of penetration,the conditions and slipping modes of fracture activation and slipping in the structural plane under the two hydraulic fracture fluid injection methods of continuous injection and step injection were compared and analyzed,and the probability of fracture activation induced by hydraulic fracture to destabilize the formation under the coupling effect of multiple factors was discussed.The major research contents and conclusions are as follows:(1)By analyzing the deformation and failure characteristics of continental shale under uniaxial compression and split tension under different bedding angles,the results show that:With the increase of bedding angle,uniaxial compressive strength presents a wide“V”shape distribution,with the maximum at 0°and the minimum at 60°.Both elastic modulus,transverse and longitudinal wave velocities increase,fracture initiation stress and expansion stress and Poisson’s ratio decrease first and then increase,and the maximum is at 0°and the minimum is at 60°.The elastic modulus and Poisson’s ratio are distributed at 24GPa and 0.25,indicating that Chang 7~2continental shale is favorable for volumetric fracturing.The tensile strength shows a decreasing trend,with the maximum at 0°and the minimum at 90°.The splitting modulus decreases first and then increases,with the maximum at 0°and the minimum at 45°.(2)The triaxial compression deformation and failure characteristics of continental shale under different confining pressures and fracture angles are compared and analyzed,the results show that:When the confining pressure is the same(12MPa)and the fracture angle is different,the variation trend of triaxial compressive strength with fracture angles is 90°>0°>30°>60°>45°,with a“U”shape distribution.The variation trend of elastic modulus is:30°>90°>60°>0°>45°;the trend of Poisson’s ratio is:30°>60°>90°>0°>45°.When the fracture angle is the same but the confining pressure is different,the triaxial compression strength and elastic modulus both increase with the increase of confining pressure,and Poisson’s ratio decreases with the increase of confining pressure.(3)Comparative analysis of the failure modes of continental shale under compre-ssion and splitting load shows that:The dip angle range of shear failure along the structural plane of continental shale is between 27°and 79°under both compression and splitting load,and beyond the dip angle range,tensile failure is dominant.Among them,30°and 60°dips are accompanied by tensile failure through the structural plane,and 45°dips are accompanied by shear failure along the structural plane.(4)The hydraulic fracture tests on cylindrical specimens of continental shale were conducted to compare and analyze the behavioral characteristics of hydraulic fractures communicating with the structural plane under different vertical stress differences and different fracturing fluid injection rates.The results show that:Increasing the vertical stress difference by 3MPa can promote the transformation of the hydraulic fracture from opening bedding to penetrating bedding.When the communication angle between the hydraulic fracture and the structural plane is large(≥58°),increasing the vertical stress difference is favorable to the extension of the hydraulic fracture through the structural plane;while increasing the fracturing fluid injection rate promotes the transformation of the hydraulic fracture from penetrating behavior to slipping behavior.When the communication angle between the hydraulic fracture and the structural plane is small(≤18°),increasing the vertical stress difference tends to open the structural plane only;while increasing the injection rate of fracturing fluid facilitates the extension of the hydraulic fracture through the structural plane.(5)The hydraulic fracture penetration and extension characteristics of hydraulic fractures under different horizontal stress differences and different injection rates were compared and analyzed by acoustic emission test of concrete-encased continental shale cubic specimens.The results show that:Steeply dipping natural fracture has a good guiding effect on the hydraulic fracture extension height,and a"+"shape hydraulic fracture is formed under the joint action of steeply dipping natural fracture and horizontal bedding.During the extension of hydraulic fractures from continental shale to sandstone,the rock interface inhibits the extension height of hydraulic fractures,and hydraulic fractures can only open the rock interface to form"T"type fractures.When the horizontal stress difference and the injection rate of fracturing fluid increase to12MPa and 75 ml/min,respectively,the hydraulic fracture can penetrate the bedding to form a"step-type"fracture.(6)Comparative analysis was made on the conditions and slipping modes of activated structural plane fractures under continuous injection and step injection in two different hydraulic fracturing fluid injection modes.The results showed that:For structural plane fractures with 45°dip angle,the open of stick-slip motion depends on the critical shear stress state of the fracture and the roughness characteristics of the fracture surface.When the fracture is already in the critical shear stress state,hydraulic pressure is the main trigger to activate the fracture start stick-slip,and the non-uniform distribution of hydraulic pressure and permeability evolution are second triggers.The fracturing fluid pressurization rate controls the slipping mode of the fracture.Increasing the fracturing fluid pressurization rate will intensify the stick-slip motion of fracture,while decreasing the fracturing fluid pressurization rate cannot effectively avoid the stick-slip motion of the fracture,but can delay the open time of the stick-slip motion and reduce the slipping scale.The degree of hydraulic non-uniform distribution increases with the increase of fracturing fluid pressurization rate,and the permeability of the fracture surface decays with the continuous increase of shear displacement,but increases instantaneously with the sudden increase of shear displacement.(7)Constructing a new Coulomb stability function,solving and analyzing the probability of formation destabilization induced by pore elastic stress,pore pressure and induced shear stress,the results show that:For fracture with large dip angle(usually≥45°)and in critical shear stress state,near the fractured wells,pore pressure increase more than 3MPa will accelerate the rate of formation destabilization,and pore pressure of 10-20MPa will seriously affect the formation stability.In the far zone of fractured wells,Coulomb stress transfer caused by pore elastic stress is an important factor to induce formation instability,and the propagation distance of Coulomb stress depends on the relative growth rate of pore elastic stress and pore pressure.The induced shear stress is an influential factor that cannot be ignored to activate the high dip fracture-induced formation instability.In view of the high dip angle natural fractures developed in the Chang 7~2,especially for the≥45°dip angle fractures,it is suggested that the increase of pore pressure should not exceed 3MPa. |
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