Influences Of Shale Meso-mechanical Properties On Hydraulic Fracture Propagation And Its Quantitative Evaluation

Shale formation is usually of low porosity and low permeability,the enhancement of shale gas production needs a large-scale formation treatment technology.Hydraulic fracturing is an effective technology and has been widely applied to the formation treatment for unconventional reservoirs such as shale gas reservoir.Shale gas reservoirs have the internal beddings,the non-uniformity of mineral distribution,and the initial near-wellbore fractures.These can increase the complexity of hydraulic fracture network but are difficult in the evaluation on hydraulic fracturing effect.The mechanisms of crack initiation and propagation process have been unclear so far.The fracturing mechanism of shale gas reservoir and related engineering control measures are key scientific issues to the success of hydraulic fracturing treatments.Therefore,the investigations on the roles of mechanical properties of bedding,mineral heterogeneity,and the initial near wellbore fracture in the crack propagation process are of important scientific significances and engineering guidance to hydraulic fracturing in shale formations.This dissertation employed laboratory experiment,numerical simulation,and theoretical analysis to comprehensively study the hydraulic fracturing process of shale.The effects of bedding strength parameters,the mass fraction of low brittle mineral particles,the geometric parameters of the initial near-wellbore fractures on the microscopic crack types and macroscopic fractures networks,fracture initiation pressure and breakdown pressure are investigated.The fractal dimension is introduced to quantitatively evaluate the complexity of hydraulic fracture network.Based on these investigations,following main conclusions can be drawn:(1)A notched three-point beam bending test model and a conventional compression test model were established for laminated shale specimens.The influence of joint strength parameters on the failure strength,microscopic crack types,and macroscopic fracture network of shale were systematically analyzed under tension and compression loadings.It is found that when the shale failure mainly occurs in the bedding,the shale failure strength is linearly related to the change of the joint strength,and this correlation will not change due to the variation of confining pressure.Besides,the shear-to-tensile strength ratio of bedding has greater influence on the macroscopic fracture morphology and micro-crack types of shale than joint strength.(2)A hydraulic fracturing model was established for laminated shale formation.The influence of joint strength parameters on the interaction between hydraulic fractures and beddings are systematically analyzed.The roles of the joint strength and shear-to-tensile strength ratio on the evolution of hydraulic fractures are explored.The fractal dimension is introduced to quantitatively evaluate the contribution of each kind of micro-cracks to the complexity of the final fracture network.When the joint strength is relatively high,the induced hydraulic fractures are simple and mainly extend along the direction of major principal stress.When the shear-to-tensile strength ratio is relatively low,the induced hydraulic fractures become complex and mainly extend radially around.Besides,reducing the joint strength increases the fractal dimension of bedding cracks,while reducing the shear-to-tensile strength ratio increases the fractal dimension of matrix cracks.(3)The mineral heterogeneity of shale is included in the developed hydraulic fracturing model.The impacts of the mass fraction and elastic modulus of low brittle minerals on the fracture initiation pressure,breakdown pressure,fluid injection volume,induced fracture permeability are analyzed.It is found that under the same fracturing conditions,the shale gas reservoirs with a large percentage of low brittle minerals and low elastic modulus are conducive to the fracture initiation and fluid flow through hydraulic fracturing.This can evaluate the difficulty and effacity of hydraulic fracturing based on the mineral composition of shale.(4)Three kinds of initial near-wellbore fractures were designed to comparatively study the effects of the initial fracture length,fracture branch orientation,and symmetry of near-wellbore fractures on the induced fracture propagation path,pore water pressure distribution,and critical fracturing pressure.It is found that different initial near-wellbore fractures have different effects on the fracture permeability and stimulation reservoir volume.Under the same fracturing conditions,initial X-shape fracture induces more complex final fracture network,while initial symmetric radial fracture induces larger fracture aperture.These research results can deepen our cognition on the initiation and propagation of cracks in laminated shale formation during hydraulic fracturing.It also provides a feasible tool for the selection of hydraulic fracturing location and the evaluation of effective fracturing based on bedding strength parameters,mineral heterogeneity as well as initial near wellbore fractures.There are 96 figures,24 tables and 191 references.