Numerical Simulation Of Hydraulic Fracturing And The Pattern Of Fracture Propagation In Coal Reservoirs

The development history and present situation of hydraulic fracturing technique are reviewed in this thesis. The three development stages of Numerical simulation of hydraulic fracturing models, from the two dimensional, the proposed three dimensional and the three dimensional model and their scope of use, assumptions and advantages and disadvantages are introduced respectively. The south part of Qinshui basin in Shanxi province is the target area. The pattern of hydraulic fracture propagation in coal reservoirs under different geological conditions was studied, and the control action to hydraulic fracture extension by factors such as mechanical properties of coal, stress field, fracturing fluid viscosity and injection pressure was investigated through a series of finite element numerical simulation. A preliminary fracture propagation model which suits the geological conditions of coal reservoirs in south Qinshui basin was set up.The basic theories of porous media and fracture mechanics are generally summed up. Some basic concepts such as porosity, saturation, effective stress, stress intensity factor, energy release rate and J integral are introduced. Several criteria of typeâ… fracture and mixed fracture propagation are described, including stress intensity factor criteria and energy release rate criteria for typeâ… fracture, the maximum tensile stress criteria and energy release rate for mixed fracture.Hydraulic fracture propagation in coal reservoir was carried out in use of finite element software ANSYS on the basis of several assumptions. The geological model was set up using Cohesive Zone Model (CZM) material which comes with ANSYS. The various parameters in the geological model come from mechanical testing of coal samples and literature research. The hydraulic pressure expression on the fracture face along the fracture length was deduced based on Newtonian fluid flowing between two-dimensional parallel plates model. A command file was written using ANSYS Parametric Design Language (APDL), making the simulation process carry out automatically. CZM, an interface model made up of contact elements, has three kinds of interface separation modes: (1) separation normal to the interface, (2) slip tangent to the interface, (3) mixed mode involving both the normal separation and tangent slip. The software provides two kinds of fracturing criteria, the critical separation displacement normal to the interface and maximum normal contact stress.Some satisfactory conclusions are drawn out through the simulation. The fracture initiation pressure is 13.9MPa when other parameters are constant. The tensile stress in front of the perforate weakens with the increasing distance from the perforate tip and eventually disappears after certain distance. The maximum fracture length changes positively with the injection pressure of fracturing fluid, while negatively with the Young's modulus in X direction and Y direction, fracturing fluid viscosity and minimum horizontal stress, and independently with the maximum horizontal stress. The maximum fracture width changes positively with the injection pressure and fracturing fluid viscosity, negatively with the Young's modulus in X direction and Y direction and minimum horizontal stress, and independently with the minimum horizontal stress as well. The fracture initiation pressure keeps independent with fracturing fluid viscosity and maximum horizontal stress, while increases with the minimum horizontal stress. The fracture becomes wide and short when the fracturing fluid viscosity increases, while becomes long and narrow when it decreases. The research results is hopeful to provide basis for the fracturing design optimization for the CBM wells in south Qinshui basin.