Numerical Simulation Of Complex Hydraulic Fracture Network And Assessment Of Fracturing Effect

Shale gas is an unconventional natural gas resource with broad development prospects,which is of strategic significance for optimizing our country’s energy structure and ensuring national energy independence.Hydraulic fracturing is a widely applied technique for enhancing the oil/gas recovery of unconventional hydrocarbon reservoirs,which practically stimulates ultra-low permeability formations by generating fracture system with higher conductivity to communicate high-yield "sweet spots".Natural fractures are often observed to abound in the reservoirs,which may play a major role in affecting the propagation behavior and fracture status of hydraulic fractures,and most reservoirs matrices exhibit inherent orthotropy that may also change the hydraulic fracture trajectories.Accurately evaluating the stimulation effect of complex fracture networks to maximize the stimulated reservoir volume of horizontal wells still remains a complicated and challenging issue.In this work,a two-dimensional numerical fracturing model is presented on the basis of the extended finite element method.A complete fracturing simulation program is independently developed to analyze the intersection criterion between hydraulic fractures and natural fractures,the propagation mechanism of complex fracture networks,and the influence of reservoir anisotropy on fracture path.And an equivalent method is presented combining the Peaceman well model to evaluate the stimulation effect of fracture network on the reservoirs.The main conclusion are as follows:(1)A numerical model of random complex hydraulic fracture network is established.The interaction between natural fracture and hydraulic fracture may cause the occurrence of five patterns,namely penetration,arrest,opening,offset,and crossshape.Results show that the propagation trajectory of hydraulic fracture is related to the intersection angle,in-situ stress difference,stress shadow effect of fracture branching,net pressure of hydrau-fracture and rock fracture toughness.Smaller in-situ stress difference makes it casier for hydraulic fracture to open natural fractures,and it is easier for hydraulic fracture to directly penetrate natural fractures under the condition of larger intersection angle.The more diverse intersection patterns appear when hydraulic fracture is surrounded by random natural fractures,which results in a higher probability to generate complex fracture network.And under large in-situ stress difference,the fracture network can penetrate deeper shale layer so that the formations further away from the horizontal wellbore can be stimulated.(2)A fracturing numerical model suitable for orthotropic reservoirs is proposed,and the effect of rock anisotropy parameters on fracture structure is studied.Results show that the material orthotropy has a huge impact on the propagation behavior of the hydraulic fracture,and the propagation direction of hydraulic fracture will experience a tremendous variation in preliminary period.The hydraulic fracture will deflect by a larger angle from its initial propagation direction as the material angle increases.The larger Young’s modulus ratio can enhance the influence of rock orthotropy on the hydraulic fracture trajectory.The in-situ stresses will weaken the influence of orthotropy on fracture propagation and reduce the fracture offset.(3)An equivalent method for the production performance is presented based on the extended finite element method(XFEM)and the Peaceman well model.Results show that fracture network can enhance the reservoir production capacity.And the enhancing effect will be better if the fracture network is more broadly distributed.Fracture aperture is one of the significant influencing factors for the stimulation effect.And the branching aperture gradually decreases away from the injection point,which result in a lower stimulation effect.The apertures in crack tips are so small that the ends of branches have little stimulation effect on the production capacity.The findings can provide in-depth understandings of the propagation mechanism of complex hydraulic fracture network and the hydraulic fracturing process in orthotropic formations,which is of benefit to develop high-efficiency fracturing technology.