Investigation On Exploitation Of Marine Gas Hydrate By Water Jetting Combined With Co₂/N₂ Replacement

As an internationally recognized new type of clean energy,natural gas hydrate(NGH)is widely distributed and has huge reserves.It has great commercial development prospects and is expected to become an important substitute of conventional energy in China.NGH mainly occurs in marine clayey sediments,and the hydrate reservoir has the characteristics of non-diagenesis,weak cementation and poor permeability.The NGH mining methods of depressurization,thermal stimulation and chemical inhibitor injection based on the principle of phase equilibrium"destruction"face potential risks of engineering geological disasters and environmental problems.The solid mining method(water jet mining)with the core technologies of water jet erosion and pipeline transportation as well as the CO₂ replacement method based on the principle of molecular exchange can avoid these problems and maintain the stability of the formation.Moreover,the latter can realize CO₂ geological storage.In addition,the NGH production field test in Shenhu area of South China Sea and Alaska frozen soil area of the United States also confirmed the feasibility of these two exploitation methods.Thus,water jet mining and CO₂ replacement are suitable for the development of China's marine NGH because of their safety and environmental protection characteristics.However,research on water jet erosion and fragmentation of hydrate reservoir is in the initial stage,and how to make good use of the goaf in the reservoir has not yet formed a unified and effective understanding.For CO₂ replacement,the poor permeability of hydrate reservoir will greatly inhibit the diffusion of displacement medium,and the limited contact area will reduce the exploitation efficiency.Existing research focuses on pure hydrate and hydrate in sandy sediments,and the mechanism of molecular exchange in muddy sediments has not been revealed.Therefore,considering the complementary effects of the two mining methods,this paper proposed a combined mining method in which pure CO₂ fluid or CO₂/N₂ is injected after the water jet erodes NGH reservoir to form a goaf.Focusing on the two methods,researches on water jet erosion of hydrate bearing sediment(HBS)and molecular exchange between CO₂/N₂ and NGH were carried out.This paper first explored the coupling inteaction between HBS and water jet as well as the failure mechanism of HBS under low pressure and high pressure water jet impingement.The results of low pressure water jet impingement indicate that the coupling between the jet fluid and the pore fluid is the main reason for the formation of the damage.Low pressure water jet can cause erosion damage and seepage damage to the HBS with low hydrate saturation,in which seepage damage is the main form,but it can not effectively damage the HBS with high hydrate saturation.The interaction between high pressure water jet and HBS is dominated by the coupling between jet fluid and two intersecting interfaces.Moreover,compared with the erosion damage,the damage caused by stress wave propagation in HBS is dominant.Based on the ALE algorithm and ANSYS/LS-DYNA software,the numerical simulation of low pressure water jet erosion of hydrate bearing sediments was carried out.Taking erosion depth and erosion volume as evaluation indexes,the sensitivity of three jet parameters(including jet velocity,target distance and nozzle diameter)was investigated.The results demonstrate that erosion depth is more sensitive to jet velocity and target distance,and erosion volume is more sensitive to jet velocity and nozzle diameter.The results of single factor analysis indicate that the erosion efficiency of marine HBS increases with the increase of jet velocity because of its low cementation strength and easy to be broken,and relatively high erosion efficiency can be obtained within 3 mm target distance.Moreover,the nozzle diameter of 1.8mm corresponds to the largest erosion depth,and erosion volume can be greatly improved by the increase of nozzle diameter.Meanwhile,the nozzle diameter less than 1.2 mm mainly causes vertical corrosion,on the contrary,radial corrosion is more common.In addition,the increase of jet angle(angle between jet direction and normal direction of HBS top surface)can improve the radial erosion effect,and the jet angle within 10°can achieve larger erosion volume.A high pressure water jet crushing system which can create a low temperature environment was developed independently.It consists of five subsystems,which can realize the high-pressure water jet crushing of various materials.The water jet pressure can reach 200 MPa,and the crushing process can be observed in real time.The experiments of high pressure water jet crushing HBS with different hydrate saturation were carried out based on the system.The breaking effect parameters and the failure mechanism of HBS with low/high hydrate saturation were studied.The results indicate that the volume fragmentation of HBS can be caused by the impact of high pressure water jet.There are central fracture pits,top annular failure zone and lateral cracks on the outside of HBS,while circumferential cracks,radial cracks and conical cracks appear inside.With the increase of water jet pressure,the top damage area and the number of side cracks first increase and then decrease,the average inclination angle of cracks decreases linearly,and the average length of cracks increases gradually.After the impact of high pressure water jet,the huge pressure formed on the top surface of HBS will propagate in the form of stress wave.The tensile stress caused by Rayleigh wave propagation on the top surface of HBS will lead to the formation of irregular annular failure surface.The shear stress and tensile stress caused by the propagation of P-wave and S-wave are the fundamental causes of circumferential crack,radial crack and conical crack.The interference between stress wave and reflection wave can strengthen the damage.After the internal cracks of HBS connect and propagate to the boundary,the macroscopic side cracks are formed.The kinetic characteristics of CH₄ extraction from hydrate by CO₂/N₂ replacement in different kinds of clayey sediments were investigated.The main foundings are as follows:(1)Kaolinite or illite in HBS have little effect on the CH₄ replacement rate.However,for the porous media containing montmorillonite,it has a significant inhibition on the production of CH₄ due to the strong water absorption and expansion of mineral particles.Hence,the CH₄ replacement efficiency is low.Moreover,it may also be caused by the limited migration of CO₂/N₂ in the interlayer of montmorillonite particles as well as the thermodynamic hindrance of the molecular exchange between CO₂/N₂ and CH₄ hydrate.Montmorillonite in HBS can significantly reduce the free water content in porous media and the formation of CO₂/N₂ mixed hydrate,resulting in almost constant pressure in montmorillonite containing sediment and significantly lower CO₂ sequestration rate than kaolinite and illite containing sediment.(2)The increase of montmorillonite content in HBS is unfavorable to CH₄production and CO₂ sequestration,but the increase of CO₂/N₂ injection in the system will increase the driving force of replacement reaction,so CH₄ replacement rate will be increased.(3)In hydrate bearing clayey sediments with different hydrate saturation,the increase of hydrate saturation can reduce the CH₄ replacement efficiency,but it can increase the CH₄ replacement efficiency in pure sand sediment or sandstone.The diffusion of CO₂/N₂ in the HBS is the main factor that affects the change of CH₄production with hydrate saturation.A small hydrate saturation can significantly reduce the diffusion barrier of displacement medium.On the other hand,CO₂ sequestration is little affected by the variation of hydrate saturation.At hydrate saturation ranged in5.17%to 22.6%,the CO₂ sequestration ratio varies from 61%to 66%.Finally,this paper analysed the engineering feasibility and advantages of the combined mining method in which pure CO₂ fluid or CO₂/N₂ is injected after the water jet erodes NGH reservoir to form a goaf,and a comparative experimental invetigation on hydrate exploitation by CO₂/N₂ injection in NGH reservoir with goaf and intact reservoir was carried out.The findings indicate that the combined mining method is practicable in engineering.For the pure sand reservoir with low hydrate saturation,the presence of goaf can promote the production of CH₄ and increase the replacement efficiency,but the CO₂ sequestration rate is lower than that of complete reservoir.With respect to to the clayey reservoir and the pure sand reservoir with low hydrate saturation,the existence of goaf is conducive to the diffusion of CO₂/N₂ in the early stage.Moreover,under the synergistic effect of higher hydrate molar ratio of CO₂/N₂ injection to initial CH₄ hydrate,the CH₄ production rate and replacement efficiency in the reservoir with a goaf are significantly higher than those in the intact reservoir.In addition,the CO₂ sequestration ratio in the reservoir with a goaf is higher than that in the intact reservoir since the replacement reaction of CO₂/N₂ and CH₄ hydrate plays a leading role in carbon sequestration.The combined mining method proposed in this paper can provide a new strategy for the development of marine NGH in China.Meanwhile,the obtained findings can provide theoretical basis and technical support for marine NGH exploitation by water jetting or CO₂/N₂ replacement.