Research On Mechanical Properties And Fracture Damage Of Shale Reservoir Under Fluid-solid Coupling

Shale gas is a kind of clean and efficient unconventional natural gas with large reserves and wide distribution.It is considered as an ideal substitute for conventional oil and gas energy.The mechanical properties and fracture initiation mechanism of shale under fluid-solid coupling are the key scientific issues related to hydraulic fracturing in the process of shale gas exploitation.Taking the Cambrian Niutitang Formation shale in northern Guizhou as the research object,the mineral composition determination(XRD),scanning electron microscopy(SEM),Brazilian splitting test,uniaxial compression test,permeability test and numerical simulation test were carried out.The mechanical properties and fracture damage process of shale reservoir under fluid-solid coupling are studied.The main conclusions are as follows:(1)It is clarified that the organic carbon content of Cambrian Niutitang Formation shale in northern Guizhou is high,and the whole study area is in the high over mature stage.(2)The weak bedding has a significant influence on the mechanical properties and failure modes of shale,and the compressive strength of shale shows a significant bedding effect with the increase of bedding angle.When the osmotic pressure is 4MPa,shale shows five failure modes;When the osmotic pressure is 8 MPa,shale shows four failure modes;When the osmotic pressure is 12 MPa,shale shows four failure modes.(3)Microfracture and osmotic pressure have significant effects on the mechanical properties of shale.The compressive strength and residual strength of shale decrease with the increase of osmotic pressure.Under the action of low osmotic pressure(4MPa and 8MPa),micro fracture is the dominant factor of shale fracture damage.Under the action of high osmotic pressure(12MPa),the osmotic pressure is the dominant factor of shale fracture damage.(4)Osmotic pressure has a significant effect on energy evolution during shale failure.Under the action of low osmotic pressure(4MPa and 8MPa),the energy released in the process of shale fracture is relatively gentle.Under the action of high osmotic pressure(12MPa),the internal structure of shale is softened,the microcracks are penetrated,and the release energy is larger.In actual production,special attention should be paid to the energy release of shale fracture under high permeability pressure.