Experimental Study On Mechanical Properties And Permeability Of Sandstone Under ScCO₂

The development and application of fracturing technology has greatly improved the global output of coalbed methane.China is rich in coal-bed methane resources,but coal-bed methane resources are mainly distributed in the lack of water resources areas,the method of hydraulic fracturing permeability is limited.Due to the high density,low viscosity and high diffusion coefficient of supercritical CO₂,compared with hydraulic fracturing,supercritical CO₂ fracturing not only saves water resources,has no pollution and the fracturing fluid flows back completely,but also has the advantages of low fracturing pressure and complex cracks,which can be used for unconventional oil and gas resources exploitation.For deep fractured soft reservoir,direct fracturing of coal seam is not effective in increasing permeability,so roof perforation and directional fracturing can be used.When supercritical CO₂ is injected into deep formation,physical and chemical reactions will occur between supercritical CO₂ and coal and rock,which will change the composition content,microstructure,physical and mechanical properties and permeability of coal and rock.This will cause a series of engineering problems:fracturing permeability enhancement effect,fracturing pressure change and feasibility of CO₂ deep geological storage.At present,there are few studies on the mechanical properties and permeability of rock after different time of supercritical CO₂action.Therefore,through a series of laboratory experiments,this paper studies the influence of supercritical CO₂ on mechanical properties and permeability of hard sandstone at different time,which provides theoretical support for supercritical CO₂ mining of deep coalbed methane.In this paper,the roof hard sandstone of 3-5#coal seam in Tashan Coal Mine of Datong is taken as the research object.The large volume high pressure reactor is used to carry out the immersion test of supercritical CO₂ on hard sandstone(temperature is 40?and pressure is10 MPa).The mechanical properties of hard sandstone after 0,15,30 and 60 days of supercritical CO₂ immersion are studied by uniaxial compression test,Brazilian splitting test,variable angle shear test and permeability test The effect of supercritical CO₂ on physical and mechanical properties and permeability of hard sandstone is analyzed.In the process of uniaxial compression test,acoustic emission monitoring technology is used to analyze the process of sandstone fracture initiation,propagation and failure after different time.The main conclusions are as follows:(1)When the temperature is 40?and the pressure is 10 MPa,the uniaxial compressive strength and elastic modulus of sandstone decrease monotonously and the Poisson's ratio increases gradually after 0,15,30 and 60 days of supercritical CO₂ treatment.After 60 days,the uniaxial compressive strength and elastic modulus decreased to 51.1%and 78.1%of the original state respectively,and the Poisson's ratio increased to 144.1%of the original state.With the increase of action time,the uniaxial compressive strength decreases linearly,and the change range of elastic modulus and Poisson's ratio decreases gradually.(2)Under different action time,the stress-strain curve of sandstone has four stages:pore fracture compaction stage,elastic deformation stage,plastic deformation stage and failure stage.When the stress of the original sandstone reaches the peak strength,the sandstone is destroyed instantly and reduces to the residual strength.With the increase of the action time,the sandstone still has a certain bearing capacity after reaching the peak strength,which gradually develops from brittleness to ductility.(3)The acoustic emission data of uniaxial compression test show that the crack initiation(the boundary point between elastic and plastic stages)occurs at a smaller strain with the increase of action time.The crack initiation and failure of sandstone in the original state release huge energy,accompanied by loud sound.With the increase of action time,the energy released during crack initiation decreases.(4)With the increase of action time,the tensile strength of hard sandstone decreases gradually,from 8.62 MPa at 0 day to 6.05MPa at 60 day,and the tensile strength shows a negative exponential decreasing trend.(5)The cohesion and internal friction angle of sandstone are reduced by 21.8%and 13.9%respectively after 60 days of action through 45°,50°and 55°shear tests.The results show that the clay minerals in sandstone will absorb supercritical CO₂,which reduces the cohesion between mineral particles;supercritical CO₂ will dissolve feldspar and clay minerals,which reduces the internal friction angle.(6)The permeability of sandstone decreases with the increase of effective stress after different time.There is no significant difference between 0,15 and 30 days,but the permeability of 60 days is 12.0%higher than that of short time.The results show that supercritical CO₂ can not affect the porosity in a short time,but has a certain effect after 60days.