Fracture Toughness And Damage Characteristics Of Marine Laminated Shales From The Wufeng-Longmaxi Formations Under Combustion-explosion Loading

Shale gas is a promising unconventional gas resource due to its cleanliness and abundance.China’s shale gas resources have great potential for extraction,with a wide range of reserves and distribution,but the reservoirs are generally characterized by low porosity（<5%）,low permeability(<0.001×10^-3μm²),deep burial and high stress difference.In order to achieve commercial shale gas extraction,shale reservoirs must be fractured.At present,hydraulic fracturing is the main fracturing method in China,but in China’s water-scarce areas and water-sensitive shale extraction,hydraulic fracturing has certain limitations due to large water consumption（more than 10,000 m³in a single well）and easy groundwater resource pollution.In order to explore green and efficient waterless fracturing technology,a new transformative waterless reservoir modification technology,shale gas reservoir methane in-situ combustion-explosion fracturing,has been proposed in the National Key R&D Program,which uses shale reservoir in-situ desorption of methane gas and fractures the reservoir by the impact of high-pressure gas generated by the synergistic combustion-explosion of accelerant and methane gas.However,the theoretical research related to the construction of complex fracture networks in reservoirs by combustion-explosion fracturing is not yet mature.Shale fracture toughness is an important index to evaluate the fracturing effect of shale reservoirs,and it is important to investigate the fracture toughness and damage mode of shale under hydraulic fracturing and combustion-explosion fracturing for the economic development of shale gas.Based on this thesis,a combination of theoretical,experimental and numerical simulations is used to compare and analyze the fracture toughness and damage modes of shale under different intensity of impact loads,starting from the fracture toughness and damage modes of rock planar fractures.In this thesis,the fracture toughness and damage characteristics of laminated shale under different impact strength tests were simulated,and the number of cracks and expansion direction of specimens under different loading modes were compared and analyzed to characterize the ability of building complex crack networks under different combustion-explosion loads.Firstly,the half-disc shale specimens with prefabricated seam grooves were loaded using a three-point bending experimental device,and the loading process was monitored in real time with the VIC-3D non-contact full-field strain measurement system to investigate the fracture toughness and damage characteristics of shale specimens with different laminae angles under different symmetry loading;Then,SHPB impact experiments with different loading rates were conducted on different laminated shale specimens,and the dynamic fracture characteristics of the shale specimens were studied by numerical simulation tests using PFC^2D discrete element software,and the dynamic fracture toughness and damage characteristics of the laminated shale under different ignition and explosion loading rates were analyzed;Finally,shale specimens were loaded with different burst loads using a burst test pipe to analyze the damage pattern and the change in the initial crack initiation angle of the specimens under different combustion-explosion loads.The main findings are as follows:（1）The fracture toughness and crack extension direction of laminated shale type I-II composite fracture under three-point bending test are affected by the laminae and S₂/R values（different base positions of the three-point bending test）,which exhibit significant anisotropy;The composite fracture toughness value increases with the S₂/R value,mainly showing an"uphill"distribution,with the average growth rate decreasing from 138.6%to 7.14%,with the minimum value of 1.009～1.473 MPa/m^1/2 is taken at S₂/R of 0.05,and the maximum value of 2.651～3.864 MPa/m^1/2 can be reached;The composite fracture toughness showed a positive correlation with the lamina angle and achieved a maximum value at 90°lamina(1.471～3.864 MPa/m^1/2)and a minimum value of 1.009～2.689 MPa/m^1/2.（2）The test results of different combustion-explosion loading rates show that the larger the loading angle,the greater the dynamic fracture toughness of the specimen is linearly increasing,perpendicular to the laminar C0 specimen fracture toughness is second only to 90°of 4.98 MPa/m^1/2;As the loading rate increases the dynamic fracture toughness also increases,for example,atβ=0°,the fracture toughness value of the specimen is 4.93 MPam^1/2 at a loading rate of 157.57 GPam^1/2s^-1,which is 2.9 times higher than the fracture toughness of 1.72 MPam^1/2 at a loading rate of 35.43 GPam^1/2s^-1;Due to the influence of laminae,the direction of crack extension of specimens at higher loading rates is more likely to be offset,down-layer fissures and cut layers.（3）In the combustion-explosion load test,as the combustion-explosion load increased from 20-25 MPa to 66-71 MPa,the initial crack initiation angle distribution range increased from 25°～43°to 13°～63°,a 1.78 times increase in the distribution range,26%improvement in cracking angle distribution of specimens compared to three-point bending test;when the combustion-explosion pressure reached above 50 MPa,The number of cracks and deflection patterns of the specimen are more abundant when the firing pressure reaches above 50 MPa,accompanied by the shear slip phenomenon between the rock masses,which can form canopy-like cracks of greater width and have a positive effect on the complex fracture network of shale reservoirs.The thesis contains 50 figures,8 tables and cites 115 literatures.