Numerical Simulation Study On Complex Cracks In Coal Fracturing

Hydraulic fracturing is the main measures of CBM production stimulation. Theoretical exploration and practice has fully proved that the cracks produced by the fracturing of coal seam are extremely complex, not only the conventional vertical and horizontal cracks can be formed, but also the complex cracks can be formed too. The asymmetric horizontal crack we studied in this thesis is one of the most common complex cracks.In this thesis, we studied the numerical simulation technology of asymmetric horizontal fracture, since it is a new and difficult research topic. Based on the model and numerical simulation algorithm, etc, we carried out the following research work:(1) By the technical means of micro seismic fracture monitoring and fracturing pressure analysis and so on, we revealed the various types of cracks produced by the fracturing of coal seam (vertical crack, horizontal crack, "T" shaped crack, "?" shaped crack, network cracks and asymmetric cracks, etc.). From the forming mechanism of the asymmetrical horizontal cracks, we expounded that the main reason of the asymmetric horizontal crack appearing is the result of the change of ground stress of coal seam, heterogeneity of coal seam and other factors. Through researching the existed dates about in-situ stress interpretation model, we optimized an appropriate three-dimensional stress calculation method which is suitable for coal seam geological features.(2) Based on the asymmetric horizontal crack, using the micro element method and considering the force balance of crack tip, we built the pressure drop equation of the asymmetric horizontal crack. Using the knowledge of rock mechanics and elastic mechanics, we establish the equations of crack width. While, using the principle of material balance and considering the filtration of fracturing fluid, we established the equation of material balance. Combining with the initial conditions and boundary conditions, we constituted a complete mathematical model of the numerical simulation of the asymmetric horizontal crack.(3) Considering the characteristics of the mathematical model of the numerical simulation of the asymmetric horizontal crack and using the difference principle, a specific problem was numerically solved by modified Euler method and Newton iterative method.(4) We also developed corresponding software. Through calculations we could find that the calculation stability of the software was good, and the simulation results were in accordance with the results of micro seismic test.