Research On The Mechanical Properties And Permeability Characteristics Of Reservoir Shale Under Stress Conditions

“Clean and low-carbon” natural gas,as a high-quality clean energy,meets the development requirement of the international community.In recent years,China's natural gas production has been in a steady growth trend,but the growth rate is far less than the natural gas consumption.In 2018,natural gas import reached 125.4 billion cubic meters,up to 31.7% year on year,while external dependence climbed to 45.3%.It can be seen that increasing natural gas production comprehensively is an important basis for optimizing China's energy structure and building a natural gas security system under open conditions.However,China is rich of shale gas reservoir,which has large potential for exploitation.Carrying out independent exploration and development of shale gas is an important measure to solve the current shortage of natural gas.Based on the tracking of shale gas development technology in North America and the development of demonstration bases in China,the output increases year by year,but there is a big difference compared with that in North America.Therefore,it's key to form development technology in line with China's shale gas reservoir.And reservoir permeability,as one of the important factors affecting shale productivity,is a key issue to be solved urgently.Hence,it is of great significance to study the mechanical properties and permeability of shale in the environment.Taking the outcropping shale of Long maxi formation collected on site as the research object,the mechanical properties and permeability tests of shale under triaxial stress were conducted.This paper analyzes the changes of mechanical parameters,failure characteristics and permeability change rules of original shale,hydrated shale and acidified shale,and theoretically analyzes the correlation between shale strain,damage variables and permeability under load,so as to reveal the evolution mechanism of permeability in the process of shale damage.Combined with experimental analysis,theoretical research and numerical simulation,the paper obtained the following research results:(1)The mechanical properties of shale with different angle of bedding were tested,and the mechanical characteristics of shale under load were analyzed.The results show that the anisotropy of mechanical properties of various shale under stress is remarkable.When shale stratification Angle increases from 0° to 90°,the elastic modulus increases with it,and the peak strength decreases first and then increases.In the process of loading,the cracks firstly appear in the bedding plane,and the vertical cracks form along the axial loading direction,and gradually penetrate to form the failure surface,which is mostly manifested as brittle failure along the bedding plane.(2)The mechanical properties of shale with different immersion fluids were tested,and the mechanical properties of original shale,hydrated shale and acidified shale were compared and analyzed.The results show that the elastic modulus,internal friction angle,cohesion and peak strength of the shale after acidification and hydration are sharply lower than those of the original shale,and the mechanical parameters of the acidified shale decreased more.(3)The permeability test of shale under stress was further carried out,and the variation law of shale permeability under load was analyzed.The results show that under a specific load,the permeability presents a change trend of gentle,slow increase,sharp increase and rapid decrease in the initial stage,and the change is closely related to the four stages of shale gradual fracture failure under stress.On this basis,based on the damage-seepage mechanics theory,the permeability evolution model that expressed by damage variables was established to reveal the permeability mechanism of load-induced shale damage.(4)The results of simulating shale mechanics and seepage characteristics with COMSOL Mutiphysics show that the test results of shale mechanics and seepage characteristics under stress are highly consistent with the simulation,and the numerical simulation can well reproduce the permeability of shale load induced damage evolution process,and the research results are of great significance to solve the practical engineering problems of reservoir shale.