Numerical Simulation Of Complex Fracture Network In Shale Gas Reservoir

Hydraulic fracturing has been widely used in conventional and unconventional oil and gas development as an important measure for reservoir stimulation.Due to the characteristics of tight and low-permeability of shale reservoirs,it's difficult to achieve economical exploitation by conventional single fracturing mode.Compared with conventional fracturing,hydraulic fracturing in shale gas reservoir can form fracture network to achieve volume fracturing.Under effects of in-situ stress and induced stress by adjacent fractures,hydraulic fractures continuously communicate natural fractures and weak interface to form a complex fracture network system.The degree of fracture network development and reservoir stimulation is an important factor affecting shale gas production.How to effectively obtain a larger area of fracture networks is a primary consideration for hydraulic fracturing in shale gas reservoir.Therefore,fracturing design is needed before fracturing operation to evaluate the fracturing effect.The core of fracturing design is the simulation of fracturing effects.The optimal design scheme is selected by simulating the extension of cracks under reservoir conditions and different fracturing operations.In this paper,a complex fracture network model is established for volume fracturing technology in shale gas reservoir.The intersections between hydraulic fracturing and natural fractures as well as fault are studied.The effect of different fracturing construction techniques is investigated.Based on Extended Finite Element Method(XFEM)and Matlab language,a hydraulic fracturing numerical simulation software was created.With the self-developed software,a series of numerical simulations were performed.The main results and innovations of this paper is as follows:1.For the fracture intersection problem,the intersection algorithm between hydraulic fractures and natural fractures as well as weak planes is presented respectively.By simulating brick wall structure in shale reservoirs,the effects of different rock material parameters,fracture intersection angles,in-situ stress difference,and surface friction on the intersecting morphology were studied.Meanwhile,different intersection criteria were compared in the generation of complex fracture network.The results show that whether hydraulic fractures penetrate or open natural cracks when intersect with nature fractures are mainly affected by intersecting angles and pressure inside fractures.2.The influence of stress shadows between fractures was investigated.The process of sequential fracturing,alternating fracturing and zipper fracturing was simulated based on stress interference among fractures.The results show that the stress effects of sequential fracturing and alternating fracturing are mainly concentrated near the crack tip,and the direction of in-situ stress changes greatly.The interference between zipper fracturing is more complicated,and the direction of in-situ stress is little changed.The natural fractures can be communicated to a greater degree and the effect of fracturing is improved.3.A complex fracture network model was established and a volumetric fracturing effect evaluation method was put forward.According to the simulation results,it is found that the initial orientation of natural fractures has a great influence on fracture network.Hydraulic fractures in reservoirs of which the natural fractures with the same azimuth are more uniform.The trend of hydraulic fractures can be changed more easily when the orientation of natural fractures distributes randomly,That can form a more complex fracture network and get better reservoir reconstruction results.4.A two-dimensional XFEM hydraulic fracturing software FRAC2D-XFEM was developed based on Matlab language.The software can simulate the generation of complex fracture network during actual fracturing.The intersection of hydraulic fractures and different types of non-continuum has been considered.The influence of the internal flow on rock deformation also has been taken into account.After comparing with the classic examples in the literature,it verifies the correctness of the software.The software has a high numerical accuracy and certain guiding significance for the development of high-efficiency complex fracturing technology.5.Based on DLA model,the phenomenon of viscous fingering in cracks was studied.Through fingering experiments in real cores and numerical simulation results,it was found that the injected CO2 preferentially breaks through from high-permeability layer and CO2 flooding has a low recovery efficiency in formations with very high permeability contrast.In the fractured formations,CO2 will first move along fracture channels and little CO2 in matrix can escape.The occurrence of fingering in reservoirs will seriously affect the recovery efficiency of crude oil,which should be avoided in practical engineering.The complex fracture network model established in this paper has the advantages of high numerical accuracy,simple modeling,and fast calculation speed.The 2D fracturing software developed independently has good pre-processing and post-processing module and core analysis module,which can provide reference for hydraulic fracturing design during actual shale gas reservoir development.