Study On Initiation And Propagation Of Multi-cluster Transverse Hydraulic Fracture Of Horizontal Well In Shale Reservoir

Shale gas has gradually become a hotspot in the development of China's natural gas,its high efficiency development is of great significance for the adjustment of energy structure and the insurance of energy safety,against backgrounds of the increasing demand for natural gas,vigorously promoting the "Green Development" and the "Shale Gas Revolution" in North America.However,due to the low porosity,poor permeability and strong heterogeneity of shale reservoir,it is necessary to transform the reservoir to form a complex volume network,which can improve the gas production capacity.It need deeper understanding for the depth affect reservoir reformation,propagation law of multi-cluster hydraulic fractures and fracture morphology,so as to improve the dynamic regulation of crack propagation and fracturing effect evaluation during fracturing construction.To solve the above problems,we carried out the analysis of the reservoir core,laboratory mechanics experiment,indoor large-scale physical simulation experiment and numerical simulation research in the paper,the present study are as follows aspects:(1)The mineral compontent and microstructure characteristics of shale were analyzed through X-ray diffraction(XRD)and scanning electron microscope(SEM).The results show that the brittleness index of the shale is between 55-76%,the average is 66%,the reservoir stimulation is suitable for the formation,as the brittleness of the reservoir is high.There is no obvious macropore,good cementability,without obvious fracture in the shale internal mineral particles,its own layered structure and lamellar mineral pyrite is the root of anisotropy of shale.(2)Based on the experimental results of uniaxial and triaxial compression tests,the physical and mechanical properties of different layers of shale were studied.The results show that the stress-strain curves of shale of each layer under uniaxial compression show significant brittleness,and the brittleness of each layer decreases with the increase of confining pressure;Look at the shale of each layer from top to bottom,the Young's modulus and Poisson of reservoir gradually become larger and smaller,and the reservoir brittleness gradually increases.(3)Laboratory horizontal well hydraulic fracturing physical simulation experient was carried out,single and double cluster cluster horizontal hydraulic fracture to the slotted ring through the prefabricated crack initiation and propagation.By prefabricating circumferential fracture,a single cluster and double cluster transverse hydraulic fracture initiation and propagation were realized.By means of acoustic emission location monitoring,pump pressure curve analysis and sample pressure cutting analysis,the method of identifying hydraulic fracture initiation and fracture was established,the net pressure curve is divided into: booster stage,hydraulic fracture initiation and crack propagation stage and seepage stage.Morphology and propagation characteristics(bifurcation,steering and intermittent expansion)of transverse hydraulic fracture under different displacement and cluster spacing was analyzed and summarized.There was a good consistency between the spatial distribution of acoustic emission events and the final distribution of hydraulic fractures of the samples.In this paper,a model of multiple hydraulic fractures geometry in shale reservoirs is proposed,which can be divided into 4 characteristic regions: region connecting the wellbore and fracture,near wellbore fracture region,far wellbore zone and interference region between two clusters.(4)According to reservoir characteristics and stress conditions of shale in Jiaoshiba area,Three-dimensional hydraulic fracturing model in anisotropic of shale was established.The effects of cluster spacing,in-situ stress and weak bedding face on fracture morphology and propagation are analyzed.The results show that the smaller the difference in ground stress and the greater the bedding strength become,the farther the main fracture and secondary fracture propagate,the more layers was communicated,and the more complex fracture was formed.From the perspective of cost savings and efficiency improvement,the fracture mode is more complex when the cluster spacing is 18 m,and the formation of fracture is more complex.In the same volume,the greater the density of bedding is,the smaller the fracturing fluid displacement and fracture energy is in the formation of complex fractures.In moderate the density of bedding,the greater the density of bedding is,the better the volume fracturing effect is.