| Natural gas hydrates are white and pale yellow,solid,ice-like cage type crystalline compounds;formed by small-molecule gases such as light hydrocarbons,carbon dioxide,and water under low-temperature and high-pressure conditions.These are also known as“combustible ice”.It is estimated that 90%of the world oceans are the potential areas of submarine gas hydrates formation,total organic carbon reserves of natural gas hydrate on the earth is about twice the sum of the oil,natural gas and coal.Due to its high energy density,wide distribution range,large resource reserves,shallow burial depth and small environmental pollution,the research of natural gas hydrate is highly regarded by many countries in the world.In 2007,NGH samples were successfully drilled from the SH7 site of GMGS-1 research area in the Shenhu area,in the northern South China Sea.The subsequent exploration results show that the natural gas hydrate resources in the South China Sea have great potential for development.From May to July in 2017,China Geological Survey and Guangzhou Marine Geological Survey jointly several units implemented gas hydrate trials exploitation in the Shenhu area in the South China Sea,and the first time to combined wells and soft soil foundation transformation techniques with gas hydrates exploration and development.The combination of development has achieved remarkable results.Despite this,the current marine gas hydrate exploitation technology is still not mature enough to ensure the economic and safety of mining,which is also the main factors restricting the commercial exploitation of natural gas hydrates.Based on the characteristics of marine natural gas hydrate reservoirs and considered the deficiencies of existing hydrate production methods.A new idea for cutting and crushing marine gas hydrate deposits using high-pressure water jets is proposed.Together with the reverse circulation"real-time"center sampling method and the hydro-transporting hydrate slurry method,it is expected to form a new method which suitable for the exploitation of marine gas hydrates.In addition,the theoretical analysis,numerical simulation and laboratory tests of the process and mechanism of crushing hydrate sedimentation under high pressure water jets were carried out.The theoretical analysis results show that under the action of high pressure water jet,the crushed effect of hydrate sediment is between rock and sand,and the depth and diameter of the crushed crater is smaller than that of rock,slightly larger than that of sand.The crushing mode is mainly affected by the hydrate saturation.When the saturation is low,the crushing mode is mainly the scouring failure;when the hydrate saturation is high,the crushing mode is mainly the stretching action and the shearing action produced by the jet striking action,that is,the stretching action,the shearing action and the scouring action together leads to sediment crushed.In order to grasp the significances and influence rules of crack flow velocity,spray distance,and confining pressure on the crushing effect of hydrate sediments under high-pressure water jets,a numerical simulation study of high-pressure water jet fragmentation of hydrate deposits was conducted.The simulation results showed that jet velocity has the most significant effect on the crushing volume and crushing depth of hydrate deposits.The spray distance has no obvious effect on the crushing depth,but has a significant influence on the crushing volume.Confining pressure has little effect on the crushing effect.And the higher jet flow results in the deeper crushing depths,larger crushing volumes and crushing pit diameters.When high-pressure water jets are used to simulated hydrate reservoirs and exploited hydrate combined with depressurization or other methods,larger spray distances can reduce the crushing volume and reduce the impact on reservoir stability,while satisfying the crushing depth.When the high-pressure water jet is used for cutting and crushing the hydrate reservoir,the smaller spray distance has a larger production volume,and the mining efficiency is higher.By fitting the relationship between the crushing depth and the jet velocity,the critical velocity of crushed hydrate sediment under high pressure water jet is v_c=18.15 m/s.Then,a self-developed simulation test device for multi-function rock crushed used for high pressure water jet breaking simulation test research of hydrate sediments.The effects of jet flow rate,spray distance,and hydrate saturation on the hydrate sediment deposition were analyzed.Affecting the significance and the law of influence,optimizing the combination of jet parameters,determining the critical velocity of crushing,and initially exploring the crushing process and mechanism of the hydrate-bearing sediments by high-pressure water jets.The simulation test research results showed that the critical velocity of crushed hydrate sediment under high pressure water jet is v_c=16.84 m/s,which is consistent with the simulation results.Jet flow,spray distance and hydrate saturation have an important influence on the crushing effect of hydrate deposits.The jet flow is the most important factor affecting the crushing volume,crushing depth and diameter of the crushing pit.The spray distance mainly affects the crushing volume and the diameter of the crushing pit.The hydrate saturation determines the crushing mode of hydrate deposit.The higher jet flow can obtain the deeper crushing depth,larger crushing volume and diameter of crushing pit;for spray distance,crushing volume and bottom diameter of crushing pit increase with increasing spray distance.For hydrate saturation,when the hydrate saturation is lower,the crushing mode is similar to that of sand.The crushed is dominated by scouring action,supplemented by tensile-shear failure,and the bottom crush radius is larger;when the hydrate saturation is higher,at this time,the crushing mode is similar to that of rock,and is dominated by tensile-shear crushing,supplemented by scouring damage,deeper crushing depth,and smaller change in crushing pit diameter.In this paper,we used hydraulic fracturing and water jet slotting techniques as a reservoir stimulation method to increase the local permeability of hydrate reservoirs,and used TOUGH+HYDRATE to study the exploitation efficiency of marine gas hydrates after stimulation,and investigate the influences rules of variable-stimulation effects on the production efficiency of marine gas hydrate under different reservoir conditions.The simulation results showed that the reservoir stimulation can promote the decomposition of natural gas hydrate and increase the decompression efficiency of marine natural gas hydrate.In the five-year exploitation of 7MPa exploitation pressure,compared with the unstimulated marine hydrate reservoir,the gas production volume of the stimulated marine hydrate reservoir can increase by 36.23%.Increasing the number of cracks can improve the decompression efficiency of gas hydrates.The rate of gas production increasing with the increase of the number of cracks,but the increasing rate decreases.There is a good value for crack spacing,when the crack spacing is 3m,the hydrate decompression efficiency is better.When the permeability of hydrate reservoir is low,the decompression efficiency of densely fractured network is higher.When the hydrate reservoir has high permeability and high saturation condition,the production efficiency of the sparsely fractured network is ideal.In this paper,the method of numerical simulation is used to systematically analyze the influence rules of different reservoir transformation effects on the production efficiency of marine gas hydrate under different reservoir permeability and saturation conditions,which provided a certain reference for the research on the production technology of marine gas hydrates.Analyzed the crushing rules of hydrate deposits with different saturation degrees under different jet conditions.And the crushed process and mechanism of hydrate deposits under the action of high pressure water jets were briefly described.Provided a theoretical basis for jet parameter design and improve the crushing efficiency of the jet. |
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