Study On Characteristics And Enhancement For High Pressure Combustion And Explosion Of Shale Gas In Horizontal Wells

In the critical period of energy transformation and green energy development,shale gas,as the largest and cleanest fossil energy,will develop rapidly in the future.However,because of the extremely small reservoir permeability,the artificial fracture network must be formed through fracturing modification to effectively exert the productivity of shale gas wells.The in-situ combustion and explosion fracturing technology of shale reservoir methane uses the high-temperature and high-pressure gas released by the in-situ combustion and explosion of shale gas in the shale reservoir to open multi-directional self-supporting fractures near the wellbore and builds a complex fracture network to achieve the purpose of stimulation.As the power source of combustion and explosion fracturing,the high-pressure combustion and explosion characteristics of shale gas in the wellbore are very important for efficient reservoir fracturing and wellbore safety.By means of theoretical analysis,numerical calculation and experimental research,this thesis mainly studies the combustion and explosion characteristics of shale gas under high pressure,the influence of wellbore model size and initial conditions on detonation parameters,and the enhancement effect of using pure oxygen as combustion promoter and setting strengthening ring on combustion and explosion.The main research works are as follows:(1)A physical and mathematical model for combustion and explosion in the wellbore was established.Ansys Fluent was used to numerically simulate the combustion and explosion process of methane in the wellbore model under highpressure conditions Detailed analysis was conducted on the flame propagation state,pressure wave and flame surface interaction at each stage,and the transition process from detonation to detonation;(2)The effects of wellbore size and initial conditions on the detonation characteristics of methane air premixed gas was studied using numerical simulation methods.The results show that with the increase of the wellbore length,the detonation distance becomes longer,the detonation temperature and pressure rise rate slightly increase,and the detonation temperature and detonation wave propagation speed basically remain unchanged.When the wellbore diameter increases,the detonation distance becomes longer,the detonation pressure,detonation temperature and pressure rise rate will decrease,and the detonation wave propagation speed will accelerate;When the initial pressure increases,the detonation pressure and the rate of pressure rise increase approximately linearly.The detonation pressure is basically 13～15 times of the initial pressure.The detonation temperature and the detonation wave propagation speed increase significantly,and the initiation distance shortens rapidly.When the initial temperature increases,the detonation distance decreases,the detonation pressure and the rate of pressure rise decrease,which is approximately inversely proportional to the initial temperature.The detonation temperature basically remains unchanged,and the detonation wave propagation speed increases slightly;(3)Using numerical simulation methods,the combustion and explosion strengthening effects of using pure oxygen as the combustion aid and setting strengthening rings were studied.As combustion aid,pure oxygen can significantly increase detonation pressure and temperature.The detonation pressure is 13-15 times the initial pressure.However,flame temperatures close to 5000 K place extremely high demands on the heat resistance of the wellbore.Setting reinforcement rings with different heights and spacing can significantly accelerate the development of flames,but both can promote the formation of detonation waves and enhance their impact on the wall.The initiation time and distance first shorten and then slightly increase with the increase of height,and are less affected by the strengthening distance.Due to the loss of momentum and energy caused by collisions,the average propagation velocity of detonation waves and the detonation pressure on the central axis gradually decrease with increasing height and decreasing spacing.However,the pressure attenuation near the wall is not significant,and a local high-pressure zone can be formed near the strengthening ring;(4)An experimental system for combustion and explosion of shale gas was established,and experimental research on methane combustion and explosion process was carried out.The results indicate that as the initial pressure increases,the detonation pressure gradually increases.The detonation pressure is 13-16 times the initial pressure,which is similar to the numerical simulation results.Setting a strengthening ring can significantly increase the detonation pressure.Within the height range of 0-1.5cm,the detonation pressure gradually increases as the height of the strengthening ring increases.