Study On Mechanical Properties And Fracture Mechanism Of Shale In Fuling Area

Shale gas is abundant in Wufeng and Longmaxi formations in Sichuan and Chongqing,China.In the past 7 years,the rapid development of the shale gas has been achieved,and the industrial production of shale gas has been basically realized(such as in Weiyuan,Changning,Fuling shale gas field).But compared with the geological characteristics of shale reservoirs in North America,the shale gas development in China is characterized by deep buried reservoirs and significant geological structure extrusion.Some shale reservoirs have the characteristics of high fracturing pressure and difficult migration of proppant during hydraulic fracturing treatment.In this paper,several problems such as description of mechanical characteristics of laminated shale,fracturing pressure prediction of shale reservoir,near wellbore hydraulic fracture geometry and effect of beddings on vertical propagation of hydraulic fractures are studied in order to provide references for efficient development of shale gas.The main research works is as below:(1)In view of the discontinuity and mechanical anisotropy of the shale,a discrete fracture model based on continuous damage is established.The rock mechanics experiments are carried out on shale cores,and the influence of the bedding dip angle and confining pressure on the mechanical properties of shale rock is studied.The hyperbolic Drucker-Prager model is selected to predict the strength of the rock under the conditions of tensile and compression.In order to simulate the deformation,failure and strength characteristics of shale,damage variable is introduced,which is important parameter to control the evolution of stiffness and strength of rock under stress loading conditions.According to the structural characteristics of the bedding shales,discrete fracture modelling method is adopted to describe shale beddings and matrices.The initial damage variable is used as an important parameter to distinguish the mechanical and physical heterogeneity of the material.Based on the above method,the deformation and strength anisotropy of the whole shale are simulated.(2)Aiming at the simulation of hydraulic fracture morphology and the prediction of reservoir fracture initiation pressure,a site scale strata-wellbore-perforation finite element model was established.The model uses the multi step simulation technique to simulate the drilling,cementing and perforation completion process,so as to obtain the reasonable results of the effective stress distribution around the wellbore.The model coupled with flow and damage calculation can simulate the initial damage fractures around wellbore and predict fracture pressure by observing the main hydraulic fracture initiation,which can overcome the limitations of traditional maximum tensile strength criteria.The results of field data and physical simulation test verify the effectiveness of the simulation method.Based on this model,the influence of different wellbore orientation and inclination on the hydraulic fracture morphology near wellbore,such as initiated perforation numeber,local fracture deflection and fracture connectivity between perforation tunnels,is investigated.Therefore,in addition to the prediction of reservoir fracture initiation pressure,this model can also provide guidance for the optimum design of perforation parameters.(3)To study the effect of tectonic extrusion and shale bedding structure on the vertical propagation of hydraulic fractures,a numerical model for vertical propagation of hydraulic fractures based on flow-stress-displacement-damage coupling calculation is established.The model can effectively simulate the propagation of hydraulic fractures in the rock mass and the complex fracture geometry formed by the interaction of hydraulic fractures and natural fractures.The physical simulation experiment results verify the validity of the numerical method.Combined with numerical and physical simulation methods,the effects of in-situ stress,fracturing fluid viscosity,injection rate,bedding strength and bedding dip angle on vertical propagtion of hydraulic fractures were investigated comprehensively.The simulation results show that:(1)When low injection rate(10ml/min)is used to inject low viscosity fracturing fluid(3m Pa,s),hydraulic fractures prone to offset,divert or stop propagation at beddings,resulting in the formation of complex fracture morphology,which will seriously affect the transportation and placement of proppant.(2)Under the conditions of high tectonic stress,high bedding dip angle(which is more than 30 degrees),and low viscosity fracturing fluid(3m Pa,s),even if the high injection rate(60ml/min)is used,hydraulic fractures may still shift or divert at beddings,which will eventually affect the fracturing performance.