Study On Mechanism Of Ice Formation-Migration-Jamming In Gas Hydrates Exploitation By Depressurization

Natural gas hydrate(NGH)has huge reserves and is environmentally friendly.It is one of the potential energy resources in China.Depressurization is considered to be the most effective method for hydrates exploitation.The process of hydrates exploitation by depressurization involves a variety of processes,such as phase change,multiphase flow and heat transfer,and reservoir pore structure changes.The rapid dissociation of hydrate is the first step to realize the efficient development of NGH.Since hydrate decomposition is an endothermic process,it may lead to the formation of ice phase.At the same time,under the action of fluid dragging force,hydrate phase and ice phase particles will migrate in the pore channel,and then lead to the blockage of local flow channel,which will seriously affect the gas and water production in hydrate mining process.Therefore,the influence of ice formation-migration-blockage mechanism on hydrate exploitation efficiency cannot be ignored.Based on hydrate decomposition dynamics and computational fluid dynamics,this study focused on the effects of ice formation,hydrate and ice migration and blockage on the depression-production characteristics of methane hydrate,aiming to improve the numerical model of hydrate production and better guide the actual production.Firstly,the model of hydrate exploitation using depressurization method was improved to reveal the influence of in-situ ice formation on the gas generation process of hydrate dissociation.Based on the assumption that the new ice phase is attached to the hydrate surface,a calculation model of the surface area of the hydrate covered by the ice phase is proposed,so as to accurately describe the effect of the ice relative hydrate decomposition rate.The in-situ ice phase was introduced into the absolute permeability model and the mass and energy conservation equation.It is found that considering the ice phase,although the ice phase is generated intensively in the area close to the mining well and plays a role in inhibiting the gas generation rate of hydrate decomposition and reservoir permeability,the reservoir temperature remains stable when it drops to near the freezing point,so that the hydrate can continue to decompose and the gas production of the reservoir increases.At the same time,it is found that the lower the well pressure is,the more the ice formation is,and the more significant the influence is on the hydrate mining process.The initial temperature of different reservoirs has little effect on ice formation.Secondly,the jamming mechanism of fluid-driven particles under different fluid-solid density differences was clarified through experimental research,and the governing equation for the formation of jamming in the flow channel of hydrate phase and ice phase particles was established.Experimental study shows that with the fluid-solid density difference increasing,the flow channel jamming rate increases first and then decreases.This trend is caused by the change in the effective channel diameter.In the case of small fluid-solid density difference,the effective channel diameter is equal to the natural diameter of the channel,and the jamming rate increases with the increase of the density difference.In the case of large density difference(more than 0.13 g/cm3),the effective channel diameter is less than the natural channel diameter,and decreases with the density difference,which leads to the jamming rate decreases.Based on this,a calculation model of transient jamming rate of flow channel is established for water-methane-hydrate-ice system.According to image analysis,when the jamming rate exceeds 0.6,the jamming occurs.Finally,based on the theory of Two-Fluid Model,a model of gas hydrate exploitation by depressurization was established,which considered the formation of ice phase,hydrate phase and ice phase migration and jamming.It is found that the hydrate phase and ice phase migrate to the area close to the mining well and form accumulation,which leads to the slow growth of local reservoir permeability and the decrease of temperature,leading to the decrease of gas production rate of the reservoir.Compared with the solid-phase static model,the calculation results of the two-fluid model fit better with the experimental data.The solid phase migration increases the amount of ice generation and makes its distribution more concentrated in the near-well area.The influence of hydrate phase and ice phase migration can be amplified by lower well pressure.In addition,when the particle size of solid phase is large,the channel will be blocked in the near-well area,resulting in a significant drop in permeability and temperature.However,when the particle size is small to a certain extent,the change of particle size has little effect on permeability and gas production.Different particle size does not affect the change of reservoir pressure field.Therefore,in the process of hydrate exploitation,it is an effective means to reduce the migration velocity and particle size of hydrate phase and ice phase in the reservoir,especially in the near well area.At the same time,appropriate well pressure should be set according to reservoir conditions.