Study On The Stability Of Coal-rock Interface In Indirect Fracturing Coalbed Methane Mining Engineering

Coalbed methane is an important unconventional natural gas resource.Because its reservoir permeability is lower than that of conventional natural gas and the coal seam is inclined to a certain extent,fracturing technology is usually used to improve the permeability of the reservoir,which is aimed at the existence of CBM in China.Some people propose to use indirect fracturing technology to extract coalbed methane.However,the indirect fracturing technology is not yet mature,and the propagation of hydraulic fractures is not clear.If hydraulic fractures extend along the direction of natural fractures,fracturing Liquid entering the coal-rock interface will affect the stability of the coal-rock seam.Therefore,it is of great value to study the stability of the coal-rock interface in the indirect fracturing mining coal-bed methane project.In order to better understand and grasp the characteristics of coal-rock interface stability in indirect fracturing,this paper first conducts the physical and mechanical properties test of coal and sandstone;then there is no cohesive force at four inclination angles of 0°,15°,30°and 45° The free interface coal-rock combination is the research object,and the triaxial compression test is carried out in the dry state,the water-saturated state and the interface water pressure of 3MPa;then the free interface coal-rock combination with a 30° dip angle and no cohesive force is the research object.Triaxial compression tests with different interfacial water pressures;finally,using Abaqus simulation software,the meso-mechanism of coal-rock combination interface slippage was studied,and the main conclusions were as follows:(1)The mechanical strength of the unbonded interface coal-rock combination remains basically unchanged with the increase of the interface inclination,and then decreases rapidly.In the state of water saturation,the triaxial compressive strength of the coal-rock combination is 37.67 MPa and 36.91 MPa at the inclination angles of 0° and 15°,which remain basically unchanged.The failure mode is split failure inside the coal body,which is 17.18 at the inclination angle of 30°.MPa is reduced by about 20 MPa,and 7.84 MPa at an inclination angle of 45° is reduced by about 30 MPa.The shear slip failure of the coal-rock interface mainly occurs.(2)The attenuation effect of water saturation on the triaxial compressive strength of 0°and 15° dip angle coal-rock assemblies is greater than that of 30° and 45°.Compared with the water-saturated state,the triaxial compressive strength of the coal-rock combination at a 0°dip angle is reduced from 50.78 MPa to 37.67 MPa,which is about 13 MPa,and at a dip angle of 15°,it is reduced from 50.83 MPa to 36.91 MPa,which is about a decrease of about13 MPa.14MPa;when the inclination angle is 30°,it is reduced from 20.07 MPa to 17.18 MPa,which is a decrease of about 3MPa,and when the inclination angle is 45°,it is reduced from8.72 MPa to 7.84 MPa,which is a decrease of about 1MPa.(3)Low-angle coal-rock assemblages are more obviously weakened by water,and high-angle coal-rock assemblages are more prone to shear slip instability due to the weakening of water and the influence of interfacial water pressure.Under the action of dry state,water saturation state and 3MPa interfacial water pressure,the strength of 0° and 15°dip angle coal and rock assembly is reduced from 50 MPa to 37 MPa,and then to 34 MPa;the strength of 30° dip angle coal and rock assembly is reduced from 20.07 MPa to 17.18 MPa,and then reduced to 15.16 MPa,the strength of the 45° dip angle coal-rock combination is reduced from 8.72 MPa to 7.84 MPa,and then to 6.03 MPa,and shear slip instability is more likely to occur under the action of interfacial water pressure.(4)There is a negative linear relationship between the peak strength and residual strength of the 30° dip angle coal-rock combination and the interfacial water pressure.When the interfacial water pressure increases from 0 to 4 MPa,the peak strength of the coal-rock combination decreases from 18.53 MPa to 11.67 MPa,and the residual strength decreases from 13.35 MPa to 8.84 MPa.The more obvious the effective stress at the interface,the greater the coal-rock combination It is easy to produce shear slip failure along the interface.(5)The Mises stress of the 0° and 15° dip angle coal-rock combination is the largest at the interface,which mainly produces damage in the coal body;the 30° dip angle coal-rock combination stress change mainly occurs around the coal-rock interface and extends to both sides of the interface,The specimen produced shear slip instability along the interface;the stress concentration of the 45° dip angle coal-rock combination specimen was mainly at the upper left corner,the lower right corner and the interface,and the specimen was sheared along the coal-rock interface.