Research On Hydraulic Fracturing Tests And Mechanism Of Crack Extension Of Longmaxi Shale

In China,as coal,oil and other traditional energy supply are increasingly tight and the pressure of environmental protection is gradually increasing,shale gas,as a new type of clean energy,is playing a more and more important role.China is rich in shale gas reserves,with a wide distribution area and huge exploitation potential.However,shale gas reservoirs generally have the characteristics of "low porosity and low permeability".Only after large-scale hydraulic fracturing and reservoir volume transformation can we realize commercial exploitation.The geological conditions of shale gas reservoirs in our country are complex and changeable,with large burial depth.Besides,at present,it is impossible to effectively monitor the expansion process of hydraulic fractures and their morphologies in deep formation,coupled with the lack of in-depth understanding of the reservoir volume transformation mechanism,although some shale gas wells have been commercially exploited,the construction process is still blind,resulting in a certain amount of waste of resources.Therefore,it is of great significance to grasp the law of hydraulic fracture propagation in shale gas reservoirs,study the formation mechanism of hydraulic fracturing network and reveal the main factors that affect the expansion of cracks.Taking the outcrop of Longmaxi Formation shale in Shizhu County,Chongqing as the research object,the basic mechanical properties of shale,constitutive model,reservoir compressibility evaluation,hydraulic fracturing physical simulation experiment in large chamber and their related theoretical researches were carried out.The main contents and conclusions are as follows:(1)The anisotropy characteristics of shale: Through SEM(scanning electron microscope)and X-ray diffraction,the pore structure and mineral component characteristics of shale were analyzed.It is found that shale is a kind of fine-grained sedimentary rock with bedding structure,contains a large number of microcracks and micropores,is dominantly composed of brittle minerals(such as quartz),whose content is up to 75.42%,and has high friability.Based on the wave velocity tests,uniaxial and triaxial compress experiments,three-point bending tests and Brazilian disk experiments of shale under different bedding angles,it is found that all the wave characteristic,stress-strain curve,deformation behavior,compressive strength,elasticity modulus,failure mode,tensile strength,fracture toughness and other physical-mechanical properties of shale have obvious anisotropy characteristics.It is considered that the existence of layered sedimentary structures and weak cementation between bedding planes are the sources of anisotropy.(2)Constitutive model: Based on triaxial compress experiment of shale under different strain rates,taking full account of the evolution of the shale damage variable and taking the yield stress as the cut-off point,a new measure of micro-mechanical strength was proposed and a statistical strain-rate dependent damage constitutive model for shale was established with physically meaningful model parameters,which are verified and discussed.(3)The brittle characteristics of shale: Based on the stress-strain curve,the brittleness-drop coefficient,the stress landing coefficient and softening modulus were used to establish the evaluation index of brittleness of shale,whose suitability was verified by doing uniaxial and triaxial compress experiments.It is found that shale is brittle and belongs to the highly compressible formation.The brittleness index of shale decreases with the increase of confining pressure,and it drops more quickly under low confining pressure than under high confining pressure.The brittle characteristics of shale have obvious anisotropy characteristics.Under the same confining pressure,the brittleness index of shale with the bedding inclination of 0 ° and 90 ° are the highest,followed by 60 ° and the smallest is 30 °.(4)Hydraulic fracturing experiment: Based on true triaxial simulation experimental system,servo control system of hydraulic fracturing equipment,acoustic emission positioning system,adding tracer in the fracturing fluid before the experiment,cutting the specimen of shale and performing a large industrial CT scan after the experiment,a large-scale true triaxial physical simulation test on hydraulic fracturing of shale and hydraulic fracture characterization method were established.The extension and spatial distribution of hydraulic fractures in shale samples under vertical and horizontal well fracturing were analyzed.The formation mechanism of fracture network was discussed.It is found that the position of outlet between the end of wellbore and bedding plane,the degree of cementation of bedding plane and the development of natural fractures are closely related to the formation of fracture network.It is generally believed that strata with medium level of cementation bedding plane are most conducive to the formation of fracture networks.Under the condition of horizontal well fracturing,the expanding regularities of hydraulic fractures in outcrops of shale,tight sandstone and caustic limestone were comparatively analyzed.The influences of lithology,bedding plane,natural fractures,horizontal stress difference,fracturing fluid displacement and viscosity on the on the law of hydraulic crack propagation were studied.(5)Crack propagation fracture mechanics model: The law of crack propagation of plate shale,containing two prefabricated closed collinear fractures,was analyzed through uniaxial compression test.From the perspective of fracture mechanics,the crack initiation sequence was analyzed and the brittle fracture criterion of plate shale was derived.Combining with the large-scale true triaxial physical simulation test on hydraulic fracturing of shale,from the aspects of fracture mechanics and elasticity mechanics,the initiation pressure of hydraulic main fractures under horizontal well fracturing was calculated.The interaction between hydraulic fractures and natural fractures(bedding plane)is quantitatively calculated,and their corresponding criterions were given.The analysis of the mechanism of the crack propagation and the interference mechanism between the fractures provide the theoretical basis for the hydraulic fracturing field.