Flow Behavior In Granite Fractures And Mechanism Of Hydraulic Fracture Propagation

The deep geothermal reservoir has the characteristics of low porosity and low permeability due to its deep burial depth.In order to realize economic exploitation,reservoir stimulation must be conducted.Scientists suggest the Enhanced Geothermal Systems(EGS),through which the geothermal reservoir with certain conductivity is formed by artificial method(hydraulic fracturing)and then the geothermal energy can be exploited efficiently and economically to generate electricity.In general,geothermal reservoir contains natural fractures and the tectonic stress is high,so the hydraulic fracture network propagation is very complex,containing both tensile and shear fractures.There is also a safety risk of induced earthquake.Therefore,it is necessary to study the propagation law of hydraulic fracture network.Since the geothermal reservoir only can be connected with the surface through drilling,the propagation of the fracture network is in a "black box",and it is difficult to observe it in a direct and effective way.Numerical modelling has the innate advantages.It is a simulation process which makes full use of physical model and historical work data,combining methods of multi-disciplinary,multi-physical quantities,multi-scale,multi-probability.The mapping is completed in a virtual space,thereby the physical process of hydraulic fracture network propagation can be comprehensively predicted and quantitatively analysed.However,the existing fracture flow models are not totally suitable for hot dry rock fractures,numerical solving method for propagation of hydraulic fracture network in the naturally fractured reservoir remains to be further developed.In this thesis,focusing on the theme of flow behavior in hot dry rock fractures and mechanism of hydraulic fracture propagation,through laboratory experiments,the fluid flow law of tensile and shear fractures and the influence from mesoscopic structure of fracture contact are revealed.A nonlinear flow model based on fracture mesoscopic structure evolution is established.Based on theoretical analysis,the hydraulic fracturing mathematical model for three dimension fractured rock mass under multi-field coupling is established,and the established fracture flow model is implanted into it.The numerical solution is realized by finite difference method,finite volume method and equivalent continuum theory.Finally,the propagation law of the hydraulic fracture network and influencing factors in geothermal reservoir are revealed by large scale field numerical simulation.The main conclusions are as follows:(1)The evolution of mesoscopic structure and its influencing mechanism on the flow characteristics of tensile and shear fractures are revealed.The results show that under the same stress,the deformation,average embedded depth and contact area of shear fracture are smaller than that of tensile fracture,while the void space is larger than that of tensile fracture.With the increase of stress,the channelling effect is gradually obvious.The element flow rate and flow channel in shear fracture are much higher than that in tensile fracture.The Reynolds number and friction factor of shear fracture are significantly higher than that of tensile fracture and transitional and turbulent flow dominates in shear fractures while laminar and transitional flow dominates in tensile fractures.The upper and lower critical Reynolds Numbers of shear fracture are generally lower than those of tensile fracture,indicating that the flow regime in shear fracture is easier to change from linearity and nonlinearity.The contact of shear fractures is more clustering so the resistance ability to deformation is stronger.There are more mesoscopic asperities in tensile fracture,when the two surfaces get to contact,tip contact and stress concentration happen,resulting in a stronger ability to deform and a weaker ability for fracture flow.When the effective stress increases from 1MPa to40 MPa,the permeability of shear fracture is 7.6 to 33 times of tensile fracture.(2)The flow model considering mesoscopic structure of fracture contact is established.Considering effective stress and fracture contact cluster,a mechanical model to describe the average embedded depth was established based on Hertz contact theory.Considering void space and fracture contact cluster,a hydraulic aperture model was established.Considering Reynolds number and fracture contact cluster,a friction factor model was established.By embedding these models into the cubic law,a flow model considering the mesoscopic structure of fracture contact is obtained.The model is applicable to both tensile and shear fractures.When the 3D topography data of fracture surfaces are obtained,the flow rate under certain stress condition and pressure gradient can be predicted.(3)The hydraulic fracturing mathematical model for three dimension fractured rock mass under multi-field coupling and the numerical solution method are established.The equilibrium,geometric and elastic constitutive equations of elastic mechanics are used to describe the mechanical behavior of rock matrix.Mass conservation and seepage equation are used to describe the seepage behavior in pores and fractures.The tensile and Coulomb strength criteria are used to describe the tensile failure,shear failure,fracture initiation and propagation behaviors of fractures.The mathematical model is solved in the form of step by step iterative coupling.By this method,the problem of mass and energy non-conservation caused by the strong coupling effect,which caused by the change of fracture width during the step by step coupling,is solved.This method significantly reduces the difficulty of solving the equation.The embedded element method is used to calculate the additional strain tensor and stress tensor caused by the discontinuity of multiple fractures,thus the discontinuous displacement field of rock mass element considering multiple fractures under multi-field coupling is calculated efficiently and accurately.By embedding the established nonlinear flow model into this mathematical model,the simulation process of hydraulic fracturing in geothermal reservoir containing natural fractures and tectonic faults can be realized.(4)Numerical investigation of the hydromechanical response of a natural fracture during fluid injection under field large scale was conducted.It is revealed that mixed shear–tension failure mechanism happens during the injection.The fracture propagation law and its influencing factors during hydraulic fracturing in geothermal reservoir with complex natural fracture network were investigated.The results show that the distribution mode(parallel,non-crossed orthogonal,crossed orthogonal,random orthogonal,etc.),angle,size of natural fracture network and three dimension stress influence the propagation of hydraulic fracture network.The distribution of hydraulic fracture network is more complex and the area is larger when the natural fracture network is random,orthogonal and crossed.In the range of 0-30°,with the increase of the angle of natural fracture network,the influence on hydraulic fracture network increases and the area also increases.Longer natural fracture network means higher degree of reservoir segmentation,it is easier for hydraulic fractures to be captured by natural fracture network,so the hydraulic fracture network is more complex and the area is always larger.The increase of the maximum horizontal principal stress increases the hydraulic fracture propagation resistance,so the hydraulic fracture network area is smaller,and it is easier to generate hydraulic fractures parallel to the maximum horizontal principal stress in the matrix.