Numerical Simulation Study Of Hydraulic Fracture Propagation In Deep/ultra-deep Oil And Gas Reservoirs

The oil and gas resources in deep and ultra-deep reservoirs in China is abundant and have great potential for exploitation,which is the main source for increasing the oil and gas storage.Hydraulic fracturing is a key technology for successful development of deep formations.However,the production of deep formations faces the problem of “three highs”(high temperature,high pressure,and high in-situ stress),resulting in the transformation of rock mechanical properties from elasticity to plasticity.The hydraulic fracturing process in deep and ultra-deep oil and gas reservoirs involves complex multi-physical effects,and the rock mass may exhibit extensive plastic deformation.The prevailing fracture propagation models based on the linear elastic fracture mechanics are no longer applicable to predict the evolution process of hydraulic fractures in deep and ultra-deep formations,so it is necessary to establish a set of hydraulic fracture propagation theory and method for such reservoirs.In this dissertation,the main research contents and methods are listed as follows.First of all,considering rock plasticity and thermal-hydro-mechanical coupling,the mathematical models of hydraulic fracture propagation in deep and ultra-deep reservoirs are established based on elasto-plastic theory,Biot’s theory,local non-thermal equilibrium theory and cohesive zone model.Secondly,a numerical model for simulating elasto-plastic hydraulic fracture propagation in deep and ultra-deep reservoirs on the structured grid is constructed by combining the extended finite element method and the embedded discrete fracture model.Based on this model,numerical cases of elasto-plastic hydraulic fractures propagation are carried out to analyze effects of rock plasticity and matrix permeability on fracture geometry,propagation path and extension pressure.Results show that the plastic deformation around hydraulic fractures reduces the propagation velocity and enhances the fracture opening,extension pressure and stress interference between fractures.Thirdly,considering effect of natural fractures,a numerical model for simulating fracture network propagation in deep and ultra-deep reservoirs is constructed based on global embedded cohesive zone model.The proposed model is utilized to understand the mechanism of complex fracture network.A sensitivity analysis of the stimulated fracture area with respect to rock properties,natural fracture distribution,fracturing fluid viscosity,injection rate,cluster spacing and cluster number,is carried out.Results show that plastic deformation greatly reduces the effectiveness of staged volume fracturing in horizontal wells.By appropriately optimizing the cluster spacing,not only the optimal reservoir reconstruction can be performed,but also the adverse effect of rock plasticity on fracturing treatment can be weakened by using the stress interference effect.Finally,based on fixed stress split method,sequential iteration and Picard iteration,a thermal-hydro-mechanical coupling numerical algorithm for simulating hydraulic fracture propagation is established to investigate the propagation mechanism of hydraulic fractures under thermo-poro-elasto-plastic coupling.Results show thermal stress generated by the temperature difference between fracturing fluid and rock can reduce fracture extension pressure,and cooling effect can reduce the adverse effect of rock plasticity on hydraulic fracturing.In addition,the increase in fluid temperature during the fracturing process leads to a decrease in viscosity,which can also reduce fracture extension pressure.Summing up,a set of numerical simulation theory and method for hydraulic fracture propagation in deep and ultra-deep formations is developed in this dissertation,which provides a theoretical basis and technical support for the efficient development of the oil and gas resources in deep and ultra-deep reservoirs.