As China’s economy turns to a stage of high-quality development,the demand for oil and natural gas continues to grow,while the requirements for environmental protection are also increasing.In recent years,shale gas,as a kind of abundant,efficient and clean unconventional natural gas resource,has been paid more and more attention by the national energy department,which is known as the new hope of China ’s energy development.The exploration and development of shale gas is of great significance to ensure Chinese energy sources security,improve the energy structure and promote the scientific and technological progress of China’s oil and gas industry.The "Thirteenth Five-Year Plan" of the Ministry of Science and Technology proposes the basic research project of the transformative technology of in situ combustion and explosion fracturing of methane in shale reservoirs.The efficient development of shale gas is expected to reach a new level.However,no matter what fracturing method is,it depends on the downhole perforation of shale reservoir.The perforation effect will directly affect the fracturing effect,and then affect the production efficiency of shale gas.In this paper,the engineering problem of shaped charge perforation in deep shale reservoir is taken as the research object.Through the combination of theoretical analysis and numerical simulation,the jet formation characteristics of perforating bullets with different charge structures,the perforation and fracturing effect of perforating bullet penetrating into shale,and the influence mechanism of high ground stress on the perforation and fracturing effect of shale are systematically studied.The main research contents are as follows:(1)Based on the theory of quasi-steady jet formation,the theoretical calculation of perforating bullet and caseless charge structure is carried out.The parameters such as collapse velocity,collapse angle and ejection angle in the process of jet formation are obtained,and the jet velocity and mass distribution are further obtained.The LS-DYNA software is used to simulate the process of jet formation and compare it with the theoretical results to verify the accuracy of numerical simulation and reveal the enhancement mechanism of the shell on the shaped charge effect.(2)To study the influence mechanism of shaped charge liners on the damage and cracking effect of perforating bullet penetrating into shale,a three-dimensional model of "perforating bullet-air-shale" was established.The cone angles of shaped charge liners were set to 50 °,60 °,70 °,and 80 ° respectively.The wall thicknesses were 0.5,1.0,and 1.5 mm,and the materials were copper,steel,titanium,and tungsten,respectively.The numerical simulation was conducted using LS-DYNA software,and the systematic analysis was carried out from the jet velocity and shape,perforation effect and the fracture extension characteristics of shale.(3)Based on the RHT constitutive model,the parameters of constitutive model under different ground stress were fitted and calibrated based on a large number of experimental data,and the dynamic relaxation method was used to apply prestress loading to shale.The finite element model of jet penetrating shale with different ground stress was established and the numerical simulation was carried out.The influence of ground stress on the penetration of shaped charge jet into shale reservoir is analyzed from the perforation fracturing effect,pressure and damage distribution.Through the pressure change of monitoring points,the stress wave transmission law in the perforation process is explored,and the inhibition mechanism of high ground stress on the fracturing effect of shale perforation is revealed.(4)Based on the analytical solution of the uniform load problem of thick-walled cylinder,the theoretical analysis and numerical simulation of the stress distribution of "casing-cement ring-shale" are carried out.The theoretical values and the simulated values are in good agreement.On this basis,the downhole perforation operation model of deep shale reservoir is constructed,including perforating bullet,perforating gun,air and "casing-cement ring-shale".Taking the ground stress of 20 MPa as an example,the numerical simulation study is carried out to analyze the response of various downhole media during the perforation process,including equivalent stress,equivalent strain,displacement and pressure distribution.This thesis includes 58 figures,15 tables,and 136 referances. |