Research On The Propagation Mechanism Of Hydraulic Fractures In Heterogeneous Shale And The Optimization Method Of Multi-Fracture Balanced Propagation

Shale oil and gas resources were important unconventional energy sources that required hydraulic fracturing technology to achieve commercial production.Unlike conventional reservoirs,shale was a layered sedimentary rock with significant heterogeneity.The conventional hydraulic fracture propagation law was not fully applicable to the development of shale reservoir.For different shale reservoir areas,the development degree of shale bedding was different,and the propagation law of hydraulic fractures was quite different.It was found in a test production well developed by a shale in Daqing that the hydraulic fracture could not form the expected effect after volume fracturing of horizontal well,and the well production was low and the decline was fast.Therefore,it was necessary to deepen the understanding of the hydraulic fracture propagation mechanism of heterogeneous shale reservoir and the optimization method of multi-fracture balanced propagation,which was of great significance for the efficient development of shale reservoir.This paper studies the hydraulic fracture propagation mechanism based on the heterogeneous mechanical characteristics of shale reservoir,and the main objectives of this study are as follows::By introducing the heterogeneous effective stress,anisotropic elasticity and permeability parameter matrix into the stress-seepage coupling equation of rock hydraulic fracturing,a heterogeneous numerical model of multi-fracture propagation model of horizontal shale Wells was constructed.Through the secondary development of finite element software,a numerical simulation method for simulating multiple fracture growth models of shale horizontal Wells was established.Based on the similarity theory,shale specimens with different similar material ratios were prepared,uniaxial compression tests and Brazilian disk tests were carried out,and similar shale preparation schemes with anisotropic characteristics were given.Through true triaxial hydraulic fracturing tests,the fracture height propagation law of shale reservoirs was obtained.The experimental results show that the larger the anisotropy value is,the stronger the heterogeneity is,and the more difficult the hydraulic fracture is to expand along the height direction.Strong heterogeneity is the main reason for the limited fracture height propagation in a shale block in Daqing.The hydraulic fracture propagation model of shale single cluster fractures was established,and the fracture propagation mechanism of shale reservoir was studied under different anisotropy,flow rate,viscosity and stress difference.The results show that the higher the anisotropy value of shale reservoir,the stronger the heterogeneity,the more easy to form short,long and wide fractures.The induced stress imbalance along the high,long and wide directions caused by heterogeneity is the main reason for the differentiated propagation of fractures along the three directions.At the same time,for strong heterogeneous shale reservoirs,the fracture height will increase under the conditions of high flow,high viscosity and high stress difference.However,when the flow,viscosity and stress difference increase to a certain extent,the fracture height growth trend will gradually slow down,and the increase effect was gradually not obvious.A 3D multi-fracture propagation model for shale horizontal Wells was established,and a control method for multi-fracture propagation in shale horizontal Wells was proposed by exploring the action law between perforation friction and induced stress.Based on response surface optimization method,a prediction model of fracture volume and perforation friction under the influence of multiple factors was constructed for a shale reservoir in Daqing.The optimization method within the range of global response parameters of shale reservoir in Daqing block was formed.The results show that the balanced propagation of multiple fractures could be achieved by adjusting the interference effect of the stress induced by the perforation friction balance.Based on the value of F,the main and secondary influences on fracture volume were as follows: the flow rate was greater than cluster spacing than section length was greater than stress difference.When the perforating friction was taken as the evaluation index,the main and secondary effects on the perforating friction were as follows:cluster spacing greater than section length greater than stress difference greater than flow rate based on F value.Taking the largest fracture volume as the highest weight optimization goal and the minimum perforation friction as the second weight optimization goal,the global response optimization of the four parameters was carried out,and the optimal numerical solutions of segment length,cluster spacing,flow rate and stress difference were obtained.According to the potential monitoring results of horizontal shale Wells in Daqing block,it was found that the balanced propagation of multiple fractures can be promoted by reducing the number of perforations and improving the perforation friction when multiple clusters of fractures expand.By combining theoretical analysis,experimental research,numerical simulation and field monitoring,the propagation mechanism of shale reservoir fractures and the balanced propagation control method of multiple fractures are revealed.The research of this subject provided a certain theoretical basis and reference for the efficient development of shale oil reservoir in Daqing.