Study On Fractal Fracture Mechanism Of Rocks Under Water&Gas Fracturing And Fractal Discrete Fracture Networks

China holds abundant unconventional natural gas reserves,and the efficient exploitation of unconventional natural gas is of great significance to the improvement of energy structure and sustainable development in China.The fracture system palys an important role in unconventional natural gas exploitation,and it is closely related to the hydraulic fracturing process of reservoir rocks and the seepage process of underground energy gas,as well as the diffusion process of underground energy gas.However,due to the irregular geometric shape and fractal effect of fractures,the understanding of fracture-making mechanism of hydraulic fracturing on rock and the seepage and diffusion process of underground energy gas is not yet comprehensive.In order to further reveal the initiation and propagation mechanism of hydraulic induced cracks in reservoir rocks by considering fractal effect of fractures,and to explore the evolution law of seepage and diffusion of fluid in fractal fracture network.In this paper,experimental study,theoretical analysis and numerical simulations are used comprehensively.The water&gas fracturing tests were conducted on different reservoir rocks.The new criteria for fractal crack initiation and propagation of rocks under water&gas fracturing were carried out.The solution method of anomalous diffusion model under generalized fractional operator is explored.The main research results are as follows:?1?From the water&N₂ fracturing laboratory tests of different reservoir rocks,it is found that the rock fracturing effect under N₂ fracturing is better than that under hydraulic fracturing.The main manifestations include that the breakdown pressures of rocks are lower under N₂ fracturing,the fractal dimensions of the macro-induced crack traces are large,and the micro-induced cracks propagate in the transgranular fracture mode mostly.In addition,the morphology of longitudinal single crack being liking wings indicates that the main failure mode of rocks under water&N₂ fracturing is the tensile failure.?2?In view of the limitations of rock crack initiation criteria in hydraulic fracturing based on the micro-fractal fracture models,a new criterion for rock crack initiation in hydraulic fracturing considering fractal effect of cracks is proposed based on the modified formula of the stress intensity factor of fractal cracks.In addition,based on the stress field and energy density field at a fractal crack tip,a new criterion for judging the rock crack propagation direction in hydraulic fracturing considering the fractal effect of crack is established,and the influence of the crack fractal dimension on the propagation direction angles is discussed.The results show that the fractal dimension of cracks mainly affects the propagation direction of hydraulic induced fractal cracks of mode?or mixed mode?-?,and the influence degrees under the plane stress condition are greater than that under the plane strain condition.?3?Considering the fluid-solid coupling effect in water&gas fracturing,the breakdown pressures of different rocks is solved numerically by secondary development on“COMSOL 3.5 with MATLAB 7.0”software platform under the new fractal crack initiation criterion in hydraulic fracturing.By comparing the numerical results and experimental results of fracture pressures of different rocks,it is found that the deviations between them are all less than 20%,which verifies the validity of the new criterion.At the same time,by further solving the breakdown pressures of different rocks in water&gas fracturing under different fractal dimensions,it is found that neglecting actual fractal dimension of cracks will lead to a large prediction of rock breakdown pressure,and the deviations exceed 50%.The results show that the fractal effect of cracks should not be ignored in the designs of hydraulic fracturing.?4?The effects of different engineering and geological factors on the rock breakdown pressures under water&gas fracturing are investigated by numerical simulations.Within the certain ranges,the larger the initial permeability of rock or the smaller the viscosity of fracturing fluids,the smaller the breakdown pressure of rock.When the initial permeability of rock is lower than a critical value or the viscosity of fracturing fluid is higher than a critical value,the deviation of rock breakdown pressure caused by the fractal dimension of cracks becomes larger.In addition,generally speaking,the increase of Biot coefficient or the decrease of loading rates of fracturing fluids will reduce the rock breakdown pressure.However,the reduction degrees are restricted by the initial permeability of rock and the viscosity of fracturing fluid.?5?It is revealed that the connection topological structure of rock fracture networks has the characteristics of local clustering,showing the typical"small world"nature of complex networks.Based on this,by considering the average clustering coefficient and fractal dimension of fracture networks,the ADFNE open source program is redeveloped to realize the construction of fractal discrete fracture network model.The permeability of fractal discrete fracture network models with different structures is evaluated by using LBM.The results show that the larger the average clustering coefficient of the fracture network is,the better its permeability is,and the larger the fractal dimension of the fracture network is,the worse the permeability of the fractal fracture network is.?6?The time fractional anomalous diffusion models of fluid were theoretically described based on a new Wiman's generalized fractional operator,and their analytical series solutions under special initial and boundary conditions were obtained by Laplace transform and Taylor series expansion.The paper has 108 figures,14 tables and 184 references.