Crack Model Study And Hydraulic Fracture Propagation Simulation Based On CDEM Theory

Numerical simulation of crack initiation and propagation has been the research focus inmechanics and engineering field. The essence of this process is continuum converting intodiscontinuum, so how to deal with the conversion is the crucial problem. Furthermore, theeffective stress and conductivity of rock can be changed by hydraulic pressure, which meansthe complex coupling of fracture–stress–seepage should be considered. In this paper, thecombined finite–element method is adopted to simulate solid crack growth. After exploringon the compatible flow between fracture and rock mass, the mechanism of hydraulic fractureis studied by indirect coupling analysis. Focus on crack growth, the following work has beenconducted:1. Based on the continuum-based discrete element method (CDEM), a crack model isestablished. The planar half-spring method is applied to detect and calculate contacts betweenelements. The choice of parameters is studied which emphasizes on the fictitious density andtime step. It is pointed out that the key for fictitious density accelerating convergence issetting densities corresponding to elements size to ensure they have same convergence timestep.2. A two-dimensional program based on crack model mentioned above is coded tosimulate crack propagation with various primary conditions under different loading. Thesimulation results are verified through agreement well with the experiment results. Thefindings can be concluded as following:(1) various crack pattern due to different failuremechanics;(2) shear cracks is more complicated;(3) composite failure mode is not the simplesum of two single-mode, but results from mutually influence of tension and shear;(4) enechelon cracks with90°rock bridge loaded unidirectional displacements is linked by wingcracks.3. By studying on the compatible flow in fracture and rock mass and couplingmechanism, fracture–stress–seepage coupling model is established. Then, correspondingprogram is coded to simulate hydraulic fracturing. The results show that usingtwo–dimensional finite volume method to simulate pore seepage, applying one–dimensional finite element method to simulate flow in fracture, and assuming that fracture pressure worksas boundary condition of porous flow and the flow rate of porous media affects the calculationof fracture pressure, the compatible flow in fracture and rock mass can be simulatedeffectively. Through simulation of hydraulic fracturing, there are some findings listed asfollowing:(1) the initial pressure in the well bore is greater than the expansion pressure;(2)the curve of pressure head at well bore is discontinuously fluctuant;(3) the width of crack tipincreases at beginning, then don’t occur apparent change;(4) the fracture aperture is inverseproportional to its distance to crack tip;(5) crack propagates along the minimum principlestress.