| Shale gas is an important unconventional natural gas resource after coalbed gas and tight sandstone gas.At present,shale gas has been an important development direction of clean energy,due to its characteristics of long mining life,long production cycle,short hydrocarbon migration distance,and large gas-bearing area.Hydraulic fracturing is one of the core technologies in shale gas exploitation.Its purpose is to form a series of complex hydraulic fracture networks in the shale reservoir and make the gas migration efficient.This requires indepth and systematic investigations on the mechanism of complex hydraulic fractures,including the study on lithology,physical properties,mechanical properties,brittleness and permeability of shale,and the extension mechanism of hydraulic fractures.In this paper,laboratory tests,theoretical analysis,and discrete element numerical simulation were conducted on shale specimens,to study its anisotropic mechanical behavior,brittleness characteristics,permeability,and hydraulic fracture propagation mechanism.The main research contents and conclusions are as follows:(1)The shale specimens in this paper were taken from the Changning area,southern Sichuan Basin.The lithology and physical properties of shale specimens were analyzed first.Conventional triaxial compression tests,Brazilian tests,triaxial cyclic loading and unloading tests,and confining pressure unloading tests were conducted on shale specimens under different bedding inclinations.Its anisotropic characteristics of strength,deformation,and failure mechanism were analyzed and revealed.Furthermore,a new failure criterion for layered rock was proposed to predict the failure behavior under the Brazilian test.(2)Different brittleness indices were used to evaluate the brittle characteristics of shale specimen.Two new brittleness indices,based on post-peak characteristics of stress-strain curves and energy balance,respectively,were proposed to demonstrate improved effectiveness and reflect the effect of confining pressure on brittleness significantly well.Furthermore,the qualitative relationship between shale brittleness and its failure pattern was revealed.(3)The permeability of intact shale specimens under different bedding inclinations and one shale specimen containing fracture were tested.The permeability of the two groups of shale specimens decreases exponentially with the increases of effective stress.The main controlling factors on permeability were revealed,besides,the flowing characteristic of fluid inside the rock containing bedding planes or sandwich structures were described.Furthermore,a model was proposed to describe the anisotropic permeability of the rock.As for shale specimen containing fractures,the function between its equivalent permeability and fracture permeability was derived,as well as the function between equivalent permeability and effective stress,and the function between fracture permeability and effective stress.(4)Based on the laboratory test results,the PFC2 D micro-parameters were analyzed and calibrated and the numerical model of shale specimen was established.Via simulation study on the shale numerical model,the strength,deformation,and failure mechanism of shale specimens under different stress loading paths were revealed on the microscopic level.Furthermore,based on the improved fluid-mechanical coupling algorithm in PFC2 D,the numerical model of shale specimen for hydraulic fracturing is established.The mechanism of hydraulic fracture extension and multi-staged fracturing were studied.The results revealed the effects of bedding inclination,bedding tensile strength,and in-situ stress on the extension characteristics of hydraulic fracture.The hydraulic fracture network distribution in vertical section and horizontal section were studied,and a method helps to design optimal perforation spacing was proposed.There are 165 figures,34 tables,and 381 references in this dissertation. |
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