| The exploration and development of shale gas is of great significance for ensuring China's oil and gas safety.The government attaches great importance to the exploitation of shale gas,and has formulated the Plan of Shale Gas Development(2016-2020)and supporting policies.The storage space of shale gas is mainly nano-scale organic pores,and it mainly exists in the adsorption state,which makes it difficult to obtain economic productivity by using the conventional hydraulic fracturing method.According to the shale gas exploitation experience of North America and China,horizontal well staged multi-cluster large-scale hydraulic fracturing technology is a necessary method to economically and efficiently develop shale gas,but the current fracture propagation theory research is not well developed to effectively guide engineering design.The shale reservoirs in China have the feature of large variations in mineral composition and rich in natural fractures,which results in strong heterogeneity and discontinuity of shale,and makes the propagation mechanism of hydraulic fractures in shale reservoirs very complicated.There exists the problem of unclear mechanism of hydraulic fracture propagating in heterogeneous shale reservoirs,unclear mechanism of hydraulic fractures in propagating discontinous(naturally fractured)shale reservoirs,and unclear mechanism of competitive propagation of fractures simultaneous propagation after multi-cluster perforating.These three problems are also the bottleneck that limits the shale gas post-fracturing performance in China,and effective theoretical research methods are in urgent need to solve these problems.Supported by National Science and Technology Major Project of petroleum,and based on the phase field method,this paper established a shale hydraulic fracture propagation model,and studied the above three problems to reveal the fracture propagation mechanism under complex shale reservoir conditions,and provides theoretical guidance for improving the hydraulic fracturing performance in shale.According to the characteristics of multi-cluster fracturing of horizontal wells in shale gas,this paper considers the elastic energy generated in shale under external force,which leads to the process of gradual damage of rock mass from intact state to complete failure to form fracture and generate dissipation potential.The fracture density function with diffusive fracture topology,the elastic energy evolution functional,the fracture dissipation potential functional and the external force functional are derived.The mathematical models of fracture damage evolution by phase field method and the stress field of shale matrix rock are established based on the principle of energy conservation,continuum mechanics,fracture mechanics,damage mechanics and variational principle.And the two models are combined to form the fracture propagation mathematical model in shale.During fracturing,the fracturing fluid entering the complex fracture after the flow distribution at the perforation,then passing through the wall pores and micro-fractures to seepage into the matrix.Considering the above process,and based on plate flow,Darcy flow and Kirchhoff's law,this paper established the mathematical model of flow system in fracturing,which is composed of fracture fluid flow mode,matrix seepage model and the source and sink terms related multi-fracture flow distribution model and fluid loss model.The phase field method based shale gas hydraulic fracturing mathematical model is established by coupling flow-stress-damage with above models.According to the complex model feature in moving boundary of matrix and fracture,mathematical model are discretized by finite difference method.Explicit method and successive over relaxation interaction of implicit method are combined to get numerical solution,and the numerical model is coded on C++ platform.The model is verified against Wu's double-fracture simultaneous propagation model based on displacement discontinuity method,which proves that the model can accurately simulate the fracture propagation process.Regarding the above-mentioned problems of propagation in heterogeneous shale,propagation in discontinuous shale and fractures simultaneous propagation,the phase field fracture propagation model proposed in this paper are used to establish the correspond numerical models to study the branching,interaction and competition propagation of fractures.The research reveals that:in a reservoir with soft heterogeneous strip,when the in-situ principal stress difference is zero and the Young's modulus ratio(Estrip/Emain)is below the critical value of 0.25,the hydraulic fracture branches after it enters the strip;at a greater principal stress difference,the no branching phenomenon occurs.In a reservoir with stiff heterogeneous strips,fracture branches when the in-situ principal stress difference is zero and the Young's modulus ratio is greater than the critical value of 2.5,and the branching occurs before the crack enters the heterogeneous strip.As the principal stress difference increases,the critical Young's modulus ratio at which the branching occurs increases;under the same in-situ principal stress difference,with the increase of Young's modulus ratio,the larger the fracture branching angle and drainage area get larger.In the case of the hydraulic fracture interacting with a single natural crack,natural fracture is initiated to some extend under the simulated conditions.The condition of the natural fracture approaching angle no less than 45°,the principal stress difference equals 0MPa,the injection rate no less than 5m3/min,and the fracturing fluid viscosity equals 10mPa·s are beneficial to increase the drainage area of the fracture and the length uniformity of the branched fracture.In the case of hydraulic fracture interacting with large number of natural fractures,the degree of control of the natural fracture orientation,pumping rate and stress difference on the fracture network distribution is determined.In the case of the weak discontinuous(without natural fracture)reservoir being fractured by three perforating clusters,as the cluster spacing increases,the side fractures' morphology gradually becomes flat from curving,and the uniformity of the three fractures gradually increases.As the pumping rate increases,the length of all fractures increases,the overall diverting angle of the side fractures grows,and the uniformity of the fractures increases at first and then decreases.Based on the fracture morphology and stress variation,the optimal cluster spacing of 40m and the pumping rate of 15m3/min under simulated conditions are obtained.In the case of three-cluster perforation fracturing propagating in strong discontinuous(with large number of natural fractures)reservoir,the degree of control of natural fracture... |
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