Study On Hydraulic Fracture Initiation-propagation And Nonlinear Flow Behavior Of Shalestone Under Bedding Effect

Broad-scale hydraulic fracturing of shale gas has disturbed the formation for a long time and on a large scale,facing risks of inducing fault activation,destroying the integrity of the caprock,and fracturing fluid seepage’s polluting groundwater.That is to say,its effects of safety and environmental have attracted increasing public attention.Among them,the fracture initiation and propagation and fracture seepage involved in fracturing are tightly associated with the above problems.Thus,it is necessary and urgent to conduct research on the impact of artificial disturbance of shale gas hydraulic fracturing to ensure safe development of shale gas and dispel public doubts and worry.Different from conventional reservoirs,shale reservoirs have developed bedding structure,which makes mechanical behavior,and laws of hydraulic fracture initiation and propagation and fracture seepage more complicated.Therefore,this paper closely focuses on hydraulic fracture initiation and nonlinear seepage of bedding shale,adopting means including laboratory experiments,theoretical analysis,and numerical simulation to systematically study the anisotropic mechanical properties of shale.Besides,the influence of bedding and critical initiation characteristic length on the initiation pressure of shale hydraulic fracturing fractures is discussed,and the influence of bedding on the morphology of hydraulic fracturing fractures and the nonlinear seepage characteristics of shale fractures are explored.The main results and conclusions obtained are as follows:(1)The anisotropic characteristics and evolution laws of shale tension,uniaxial mechanics and breaking energy evolution are expounded.Experimental tests on tensile strength,uniaxial compressive strength and acoustic emission have been carried out for shale.Research shows that the tensile and uniaxial strength in the vertical bedding direction are greater than that in the parallel bedding direction,and the direction of angle changes from the vertical bedding to the parallel bedding demonstrates a trend of first decreasing and then increasing;the local failure mode is quite complicated.The Brazilian split is mainly divided into three types: straight,curved and crescent,and the uniaxial failure mainly consists of split tension failure and shear-tensile failure;the acoustic emission wave signal indicates that shear failure plays a dominant hole in the shale generally and as the bedding angle increases,the proportion of shear failure increases first and then decreases.The research results provide parameters and basis for the following experimental analysis and numerical simulation modeling.(2)The influencing rules and mechanisms on the initiation pressure of shale hydraulic fracturing between boreholes and bedding angle are revealed,and a shale hydraulic fracturing initiation pressure prediction model considering the critical initiation characteristic length is established.A shale hydraulic fracturing experiment with different bedding angles is carried out.It is found that the initiation pressure of shale shows a trend of first decreasing and then increasing varying by the bedding angle.Macroscopically,they all appear as single fracture with tensile failure;through numerical simulation and experiments,it illustrates that there are more macropores in the matrix around the wellbore,better connectivity of the pores,and obvious plastic damage.Differences in damage thresholds of shale matrix in different bedding directions are the potential cause of the change in initiation pressure;based on the elastoplastic theory,an analytical model of stress distribution of the rock with borehole under axisymmetric condition is established.Combined with the experimental data of hydraulic fracturing,the critical initiation characteristic length of Longmaxi shale in the confining pressure range of 15 MPa to 25 MPa is between 0.28 mm and 0.56 mm,and the plasticity enhances with the increase of confining pressure.The prediction model of hydraulic fracturing initiation pressure considering the critical fracture initiation characteristic length is established,revealing the influencing rules of critical fracture initiation length and borehole radius on hydraulic fracturing initiation pressure.(3)The influencing rules of shale bedding on hydraulic fracturing fracture propagation morphology are clarified.First of all,the validity of the hydraulic fracturing numerical model in terms of hydraulic fracturing fracture propagation and multi-fracture fluid-solid coupling response is verified;at the same time,based on the shale hydraulic fracturing experiment,the numerical simulation analyzes the interaction of the hydraulic fracturing fracture and bedding;through CT scanning and image digitization technology,the average bedding spacing of the shale in the area is 2.929 cm,and the bedding dip is approximately horizontal.Furthermore,through the three-dimensional numerical simulation system,the influencing rules of geological factors and optimizing fracturing parameter on the vertical penetration of hydraulic fracturing fractures are analyzed.It is found that the bedding plane is easy to open under the conditions of low stress difference,low bedding cohesion,low pump rate and low fluid viscosity.It is also revealed that the vertical propagation height of hydraulic fractures is insufficient and that the tensile strength of the bedding has a relatively small influence on the vertical propagation of the fractures,which indicates that the bedding is mainly opened by the shear mode.Additionally,the increase of bedding angle is beneficial to the opening of bedding plane.(4)The nonlinear seepage laws of shale fractures are discovered.Considering the evolution of microstructure of fractures,a critical Reynolds number model and a nonlinear seepage model are established.As studies have manifested,fluids show strong nonlinearity in shale fractures,and the friction effect on the fluids is significant.Furthermore,as the effective stress increases,nonlinearity appears earlier;the gas in shale fractures is mainly transitional flow,and laminar flow occurs rarely.In the effective stress range of 1MPa ～30 MPa,the critical Reynolds number of shale fractures is between1.80～102.88 and goes down with the increase of effective stress;the channel effect of fluid flow in shale fractures is obvious,and the flow path is relatively tortuous.The evolution law of shale fracture mesostructure(contact area,void space,contact fractal,aperture,etc.)with s effective tress are obtained by numerical simulation.It is observed that the fluid flow in the fracture has obvious channel effect,the flow path is relatively curved.Besides,the change of fracture aperture is greatly affected by the contact area and surface roughness;Simultaneously,the anisotropic seepage characteristics of vertical bedding fractures and parallel bedding fractures are analyzed by numerical simulation and experiments.The ratio of the two is between 0.463～1.735,which means that anisotropy is not significant.Furthermore,considering the evolution of microstructure of fractures,a critical Reynolds number model and a nonlinear seepage model are established.