Numerical Investigating The Hydraulic Fracturing Of Horizontal Well And The Optimization Of Stimulation Parameters

Hydraulic fracturing plays an important role in the exploition of unconventional oil and gas reservoirs. Due to the complexity of stimulation process and reservoir conditions, part of the created hydraulic fractures does not has much contribution to the production. It is significant to investigate the hydraulic fracturing to optimize stimulation parameters. Duo the limitations of measurement and experiment,numerical simulation is the most important tool to investigate hydraulic fracturing.The paper presents two dimentional hydraulic fracture models to study the important engineering problems, including high injection rate stimulation for improving fracture complexity, optimal spacing of hydraulic well in different stimulation method,multiple fractures initiation in unconventional reservoirs through injection control of horizontal well, critical conditons of creating frature network. The main works are summery as follows:(1) The fully coupled fluid and solid hydraulic fracturing model is presented based on the extened finite element method. The fracture network is characrized by the field of low in situ stress contrast. The variations of in situ stress and fracture network are investigated. The effects of injection rate on the field of fracture network for single and multiple fractures stimulation are investigated. The simulation results shown that the high injection rate stimulation improves the fracture complexity for different stimulation methods. Sensitivity studies verified that the fracture complexity is not sensitive to injection rate at low brittleness rock metrix, initial in situ stress contrast plays an important role in the creating of fracture network, the fracture width and stress perturbation depend on the minimum in situ stress, the stress purtabation is accentuated for the condition of low permeability reservoirs, which facilitates to improving fracture complexity. Due to the increasing of mechanical interaction between fractures of sequenctial fracturing as subsequent fractures created, the new created fracture network is increasing with the number of fractures. The increament of fracture network due to high injection rate stimulation for sequential fracturing is significant large than single fracturing treatment.(2) The fractures spacing of horizontal well is one of the most important controllable parameters of stimulation treatment. The fractures aspects and the variation of fracture network of sequential and simultaneous fracturing are simulated through a fully coupled fluid and solid hydraulic fracturing model. The simulation results shown that the subsequent fractures of sequential fracturing will deflecting to the same side due to the mechanical interaction. The field of fracture network is decreasing with the fracture spacing. Part of hydraulic fractures will be closed in the injection or deflection point at extremely low fracture spacing. Both the field of fracture network and fracture width are incorporated in fracture spacing optimization method. Sensitivity studies of optimal spacing for simultaneous fracturing verified that the optimal spacing is not sensitive to Young's modulus, and decreases with the in situ stress contrast.Two dimentional numerical model is presented to simulate the process of Texas Two Step method. The variations of in situ stress in the vicinity of propped fractures are calculated by the model. The distribution of stress relief fractures is predicted by the direction of in situ stress. The field of fracture network is achieved through calculating the distribution of stress releaf fractures of different fracturing stages. The simulation results demonstrated that the area of fracture network has one peak value with the varying of fracture spacing, and the corresponding spacing is optimal spcing.Sensitivity studies shown that the optimal spacing decreases with the in situ stress contrast, and increases with the fracture net pressure. The distribution of in situ stress has significant effect on optimal spacing.(3) The mechanical interaction between fractures of simultaneous fracturing causes the deflection of fractures as well as the restriction of fracture length and width.A numerical model of nonplanar fractures propagation in porous media is established based on the extended finite element method to simulate multiple fractures initiation with simultaneous injection method. The amount of fluid flowing into each fracture is dynamically calculated with a total injection volume in this model. Sensitivity studies of formation parameters on fractures geometry of simultaneous fracturing are presented. The in-situ stress contrast is found to be the main factor controlling the fractures propagation. The initiation of interior fracture will be restricted or compressed to be closed in the vicinity of the injection point. Decreasing injection rate or enlarging perforation friction facilitate the propagation of interior fracture.Strategies to optimize fractures geometry are presented through injection control of each fracture.(4) Numerical simulation fracture network is significant for the development of shale gas. Complex fracture network model (GEOS) is adopted to simulate the effect of reservoir parameters on the creation of fracture network. The simulation results verified that in situ stress and natural fracture are the controlling factors of creating fracture network. As the hydraulic fractures intersection with parallal distributed natural fractures, the hydraulic fractures propagate along the direction of natural fractures, and large field of fracture network cannot be created in this condition.Hydraulic fractures will be bifurcation resulting large field of fracture network as intersection with vertical intersecting natural fractures. The hydraulic fractures geometry depends not only on in situ stress contrast, but the absolute value. High in situ stress contrast or in situ stress will restrict the propagation, bifurcation of hydraulic fractures as well as the creating of fracture network.The numerical models, analysis method and calculation results presented in this paper can be used in hydraulic fracturing design of unconvention oil and gas development to optimize stimulation parameters.