Numerical Analyses On Multi-field Interactions In Soils During Marine Gas Hydrates Exploitation And Design Of Centrifuge Modelling Device

Energy shortage is a big challenge for global economic and social development.Natural gas hydrate(NGH)is an important alternative energy to solve the energy gap as a strategic step in global energy development in the future.The reserves of NGH are huge worldwide,only the reserves in the South China Sea reach 70 billion tons of oil equivalent,which is about half of the total oil and gas resources found in China.Studying the phase transition and evolution process of NGH exploitation to reveal its mechanism has become an important part of ensuring the utilization of NGH and predicting the corresponding geological hazard.It is of great significance for the security of energy supply and the sustainable development of economy and society in China.In this paper,the analytical model of thermo-hydro-mechanical coupling effect for the overlying layer is established and the influence of NGH exploitation on the overlying layer is studied.Based on the finite difference method,the numerical model of depressurization exploitation of NGH is developed to investigate the thermo-hydro-mechanical-chemistry coupling responses like temperature,pressure,saturation and displacement of the reservoir.Moreover,the physical simulation device under hypergravity for NGH exploitation simulation is designed based on the NGH exploitation simulation tests under constant gravity and the physical simulation similarity law.The main work and research results are summarized as follows:(1)In the overlying layer of NGH reservoir,the extra fluid pressure and the changes of temperature caused by NGH mining will lead to oscillating excess pore pressure accumulation at the bottom of the layer.Excess pore pressure will dissipate gradually after reaching the peak value,and tend to be stable finally.Heat transfer in the process of NGH exploitation causes an increasing expansion up to 0.5%of the soil layer,which has a great influence on the stability of the mining well.(2)Depressurization method induces severe multi-field interactions in NGH layer.The decomposition rate of NGH near the mining well is faster,leading to a decrease in the temperature due to the heat absorption of the reaction.At the radial direction,temperature decreases the most near the well.Gas accumulation rate is greater than gas discharge rate,thus the gas saturation increases rapidly and the NGH saturation decreases correspondingly.During NGH dissociation,the large seepage gradient and the sudden change of stress around the well will cause the main deformation of the layer(r/R<0.3).The strain continues to accumulate and finally tends to be stable.Axial strain in this area is about 2 times of the radial strain at the same position.The degradation of NGH causes the destruction of soil skeleton,redistribution of the particles and the increase of porosity.The whole process reflects that the effect of gravity can not be ignored.(3)In the physical simulation of NGH exploitation,effective simulation of the thermo-hydro-mechanical-chemistry coupling process should meet the key criterion of gravity similarity,which the existed simulation device under constant gravity cannot satisfy.With the hypergravity field created by centrifuge,the stress state in the model is similar to that of the prototype to ensure the effectiveness of stress field simulation during NGH exploitation.The main technical points of this physical simulation experimental device are the structural analysis of high pressure reactor under hypergravity,the servo control of high-pressure under hypergravity and the multi-mode mining simulation of NGH under hypergravity.