Hydraulic Fracture Simulation Method In Complex Reservoir By Using Virtual Internal Bond

Currently,the unconventional resource is gradually taking over the conventional resource.The hydraulic fracturing(HF)technology,as the main stimulation technique,has been successfully applying for more than 70 years.But there is still a big gap between the expected and the real.HF is a hydro-mechanical coupled process.In the complex reservoir with a large number of natural fractures,it is a tough problem to simulate the HF.To explore more efficient method for HF simulation,the VIB approach is developed in this thesis.The discretized virtual internal bond(DVIB)is adopted to simulate the water flow in fractured porous medium.The intact porous medium is permeable because it contains numerous micro cracks and pores.These micro discontinuities construct a fluid channel network.The representative volume of this fluid channel network is modeled as a lattice bond cell with finite number of bonds in statistical sense.Each bond serves as a fluid channel.In fractured porous medium,many bond cells are cut by macro fractures.The equivalent permeability and volumetric storage coefficient of a micro bond are calibrated based on the ideal bond cell conception,which makes it unnecessary to consider the detailed geometry of a specific element.Such parameter calibration method is flexible and applicable to any type of element.The accuracy check results suggest this method has a satisfying accuracy in both the steady and transient flow simulation.To simulate the massive fractures in rockmass,the bond cells intersected by fracture are assigned aperture values,which are assumed random numbers following a certain distribution law.By this method,any number of fractures can be implicitly incorporated into the background mesh,avoiding the setup of fracture element and mesh modification.The fracture aperture heterogeneity is well represented by this means.The simulation examples suggest that the present method is a feasible,simple and efficient approach to the numerical simulation of water flow in fractured porous mediumThrough DVIB,both the 3D continuous seepage and mechanical process are reduced to the 1D discrete bond problem.The micro bond functions as both the fluid channel and the mechanical connection between particles.With the calibrated micro flow and mechanical parameters,the DVIB can simulate the seepage and mechanical process accurately based on a common set of bond cells.To model the fractured medium,a fracture is allowed to run through a bond cell,which makes it possible to embed massive fracture into the background meshing scheme.The special mechanical and seepage equations for the cracked cell are derived.To consider the hydraulic and mechanical coupling process,the coupling equations for the cracked cell are built up,with which the HM coupling process can be well simulated.This method unifies the HM together on the level of micro bonds,providing a micro-macro hydraulic fracture simulation method for the complex reservoir.The simulation results suggest that the present method has very high precision in simulating the coupling process.This method can capture the basic friction mechanism between fracture faces.It can simulate the hydraulic fracture propagation and interaction with natural fractures.It is efficient in dealing with the complex fractured reservoir.The improved element partition method(IEPM)is extended to simulate the hydraulic fracture.By means of virtual nodes,the seepage equation of a cracked element is derived.To eliminate the extra degrees of freedom,the virtual nodal pressure is associated with its adjacent real nodes through the Least square interpolation method.It allows a fracture to run across an element without introducing any extra degree of freedom and remeshing.The fully coupled hydraulic-mechanical equation is derived based on the IEPM.By this method,the hydraulic fracturing process in field size can be well simulated.It provides an efficient approach to hydraulic fracture simulation in the field size.In unconventional petroleum and geothermal reservoirs,a complex fracture network is mostly desired in order to enhance the reservoir permeability.As extension of research,the mechanism of hydraulic fracture branching and generation of a cluster of fractures under pulse pressure is investigated via numerical simulation by using discretized virtual internal bond method.An algorithm is developed to identify the hydraulic fracture trajectory.A sensitivity study is conducted with different pressurization and depressurization rates,fluid pressures,in-situ stresses and orientations of perforations.It is found that the law of crack propagation in large scale and small scale is consistent.A high fluid pressure and a high pumping rate are the necessary and sufficient conditions,respectively for creating fracture branching and clusters.The pressurization rate plays a critical role in branching fracture,but the role it plays is subjected to its duration as fracture propagates.The fluid pressure,as a controlling factor that integratively accounts for the effects of pressurization rate and duration,determines the degree of fracture branching.The location of fracture network is controllable by adjusting the magnitude of fluid pressure.When keeping the pressurization rate fixed,the lower depressurization rate can generate a much more complicated fracture network and a much larger fracturing zone.The impact of the depressurization rate on fracture network generation is significant.The direction of propagation and distribution of fractures are determined by the in-situ stress contrast.As the contrast in the in-situ stresses increases,the branched fractures increasingly tend to propagate along the maximum in-situ stress direction.As expected,the effect of perforation direction on fracture growth is local;with hydraulic fracture advancing forward,it gradually turns to and propagates along the maximum in-situ direction.The degree of contrast between the in-situ stresses determines how fast the hydraulic fracture turns to the maximum in-situ stress direction.Therefore,when evaluating and designing the pulse fracturing operation,the above factors should be considered comprehensively.The simulation study helps to understand the mechanism and dynamics of complex hydraulic fracture generation in unconventional reservoirs.