Experimental Study On Coal-Sandstone-Shale Erosion Failure Characteristics Under Ultrahigh Pressure Water Jet

As a high-efficiency oil and gas resource extraction method,high-pressure water jet has been widely used in the field of unconventional natural gas extraction.The structural characteristics and physical and chemical properties of several typical unconventional reservoir rocks(coal,sandstone,shale)vary greatly.Among them,coal-heterogeneous rock mass,sandstone-homogeneous rock mass,shale-transverse view Isotropic rock masses result in different action mechanisms of high-pressure water jets during rock breaking,which ultimately affects the setting of jet parameters(target distance,pressure,angle,etc.)of high-pressure water jets in practical applications.At present,domestic and foreign scholars have done more research on the destruction of rock masses by lowpressure water jets,but the lack of damage to ultrahigh-pressure water jets(greater than200 MP)and the consideration of rock mass structures(homogeneous,heterogeneous and isotropic)There are few studies on the destruction mechanism.This paper uses a combination of laboratory experiments and theory,using CT scanning reconstruction,SEM and NMR to reveal the failure mode and damage mechanism of ultrahigh pressure water jets impacting different rock masses with different pressures from both macro and micro aspects..The main research results are as follows:(1)The failure modes of three typical structural rock bodies(coal,sandstone,shale)impacted by ultra-high pressure water jet at different pressures are revealed.Coal damage is mainly splitting damage under the action of water wedges;sandstone dominates with jet erosion;and shale has two stages of failure after reaching a jet pressure of 200 MPa,and the shallow part is a transverse ring under the action of stress waves The crack is the main failure mode,and the bottom is dominated by longitudinal tensile failure under the combined action of stress wave and water wedge effect.(2)The damage distribution and pore evolution law of rock mass in three-dimensional space under the impact of ultra-high pressure water jet are obtained,which quantitatively characterizes the damage degree and pore variation of rock mass.The study found that the damage degree of each section of the coal body is inversely proportional to the pressure of the water jet.At the same time,some mesopores in the coal body are converted into large pores under the erosion of high-pressure water,and the total porosity is increased by about 15% to 40%;sandstone damage The amount is extremely small compared with the other two rock masses,but the micropores of sandstone after erosion are converted into mesopores and macropores under the erosion of the jet,which causes the negative growth of micropore pores and the growth of total pores 14.34 ～ 24.77%;Shale under 250 MPa water jet pressure showed overall damage and peak damage degree of each section.After erosion,the large pores and fissures of shale increased sharply under the action of stress wave.(3)Revealed the fracture mechanism of ultra-high pressure water jet eroding three different structural rock masses.The research shows that the crystal fractures at the coal fractures are staggered,and the overall performance is brittle tensile fractures;while the crystal fractures at the sandstone fractures are neat and mainly shear failure;the traces at the shale fractures are similar to the coal fractures.Also manifested as tensile failure.The research results of this paper are of guiding significance for improving the application efficiency of ultra-high pressure water jet technology in practical engineering and optimizing the hydraulic parameters.