Study On The Formation Mechanism Of Complex Hydraulic Fractures In Deep Shale Reservoirs

The Deep Wufeng-Longmaxi shale reservoirs with the buried depth of more than3500 m in the southeastern Sichuan Basin are rich in reserves and have great potential for exploration and exploitation.However,as the buried depth increases,the brittleness of the shale decreases,the in-situ stresses increase,and the propagation resistance of hydraulic fractures increases.Therefore,hydraulic fractures are prone to being captured when they encounter natural fractures in the near-wellbore zone,thus cannot get connected to the natural fractures far away from the well.In this case,the stimulated reservoir volume(SRV)of the hydraulic fractures is small.To solve the aforementioned problems,this thesis conducts research from two aspects: the characterization of mechanical properties and the simulation of hydraulic fracture propagation in deep shale reservoirs.The main research works are as follows:(1)The characterization of rock mechanical properties of deep shale reservoirsTriaxial compression tests were conducted on the core samples from Longmaxi shale reservoirs with different buried depths,and the rock mechanical properties of deep and shallow shale reservoirs were compared.The results show that under the influences of clay diagenesis and reservoir temperature and pressure environment,deep shale has higher compressive strength and lower brittleness index than shallow shale,while the elastic modulus and Poisson’s ratio of deep and shallow shales are almost the same.Then,in view of the serious broken sample problem during the coring process,the feasibility of using instrumented indentation test to measure the hardness and elastic modulus of shale was analyzed,and a brittleness index based on instrumented indentation test was proposed.The research results show that,the measured Martens hardness can be used to predict the proppant embedment depth and the measured indentation modulus can be used as an approximate value of Young’s modulus.Moreover,the proposed brittleness index can reflect the degree of shale fracturing during the indentation test.(2)Experimental study of hydraulic fracture propagation in deep shale reservoirsBy conducting triaxial hydraulic fracturing experiments on the outcrops of Longmaxi shale reservoir and comparing the propagation behaviors under different geological and engineering conditions,a preliminary understanding of the difficulties and main controlling factors of complex hydraulic fracture formation in deep shale reservoirs is obtained.The research results show that,in shallow reservoirs,where the confining pressure and in-situ stress difference are low,hydraulic fractures tend to divert and reinitiate at the natural fracture tips.However,in deep reservoirs,where the confining pressure and in-situ stress difference are high,hydraulic fractures tend to be captured by the natural fractures,resulting in a small SRV.Increasing the fracturing fluid displacement and viscosity within a certain range is conducive to hydraulic fractures penetrating the natural fractures near wellbore and increasing SRV.(3)Study of hydraulic fracture propagation in deep shale reservoirs using phase-field methodTo make up for the small number of experiments and insufficient controlling factors in the physical simulation study,a phase-field model for hydraulic fracture propagation under compressive stress boundary conditions,which is able to model mixed-mode fracture propagation in compressive rocks and consider the influence of fracture residual permeability,is established.The fracture propagation behaviors in the hydraulic fracturing experiment under different geological and operational conditions are simulated.The formation mechanism of complex hydraulic fractures is analyzed and some suggestions for fracturing operations are given.The numerical simulation results show that,under the deep reservoir stress condition,using high displacement(60ml/min)and moderate fracturing fluid viscosity(200m Pa·s)are beneficial for hydraulic fractures to increase net pressure while maintaining the transmission of hydraulic energy to the fracture tip.Decreasing perforation cluster number and adopting moderate cluster spacing(30mm)are conducive to decreasing fracturing fluid loss,concentrating hydraulic energy and reducing the competition between hydraulic fractures.Considering both the simulation results and field data,it is recommended to adopt high displacement,inject a moderate amount of gel at early stage,decrease cluster number and adopt moderate perforation space in the hydraulic fracturing operation in the deep Longmaxi shale reservoirs at Jiaoshiba structure.This combination of operational parameters is beneficial for hydraulic fractures to propagate sufficiently,increasing SRV and injected proppant volume.