| As a new strategic resource,natural gas hydrate(NGH)has attracted much attention because of its huge reserves.The main methods of NGH exploitation are to break its phase equilibrium state by depressurization,heating,gas replacement or inhibitors injection,so as to decompose NGH into water and methane gas,and then extract and recover the generated gas.In the process of fluid extraction,the solid particles of sediment skeleton may fall off,migrate and produce due to the drag of the fluid,i.e.,the phenomenon of sand production.This will not only cause problems such as formation void and wellbore instability,but also may lead to the wear and clogging of electric submersible pump,wellbore and other production devices,thus restricting the continuous exploitation of NGH.Due to the shallow burial depth and poor cementation degree,the NGH reservoir is more prone to sand production.At present,almost all the NGH trial production projects around the world have encountered this problem,and some of them have even been terminated due to the uncontrollable sand production.The problem of sand production has become the main obstacle to the long-term safe and efficient exploitation of NGH.However,the mechanism of sand production during NGH exploitation is still not clear,and there is no corresponding numerical simulation program to quantitatively evaluate the sand production process and its influence on gas production performance.Therefore,it is very necessary to develop a numerical program capable of the simulation of sand production during NGH exploitation on the basis of clarifying the mechanism of sand production,so as to achieve the quantitative characterization of gas-water-sand production,and provide theoretical support for long-term safe and efficient NGH production.Based on the summary of the existing work,this study further clarifies the mechanism of sand production during NGH exploitation by theoretical analysis.Specifically,the detachment of solid particles is mainly affected by the formation deformation,fluid erosion and the formation strength weakening caused by hydrate decomposition.The migration process after the detachment of solid particles is not only affected by the fluid dragging capacity,but also controlled by the relationship between the particle size and the pore size of flow channel.By treating suspended solid particles as solutes in solution,and introducing the fluidization and deposition processes of particles into mass conservation equation of solute transport,the transport of solid particles was quantitatively described.On this basis,a mathematical model of solid particle shedding and migration in NGH bearing sediments is constructed and the corresponding numerical simulation module is developed.Then the newly developed module is overlapped with Hydrate Biot,the THM(Thermal-Hydrological-Mechanical)coupled program for NGH exploitation simulation,using the sequential coupling method.Therefore,Hydratesand,the first sand production simulation program embedded in the general simulation software TOUGH+Hydrate for NGH exploitation,is developed.The reliability of this code is preliminarily verified by comparing the simulation results with the experimental data.Due to the complexity of sand production,it is difficult to obtain an accurate analytical solution.In this paper,the reliability of the newly developed program was further verified by comparing the simulation results with the gas,water and sand production data published after the first NGH production test of Nankai Trough in Japan in 2013.On the other hand,the abnormal low gas production rate of well AT1-P3 during the second NGH production test in Nankai Trough in 2017 was explained by considering the sand production process and the plugging of sand control device caused by it,which significantly improved the problem that the simulated gas production rate was significantly higher than the measured value without considering the sand production process by previous researchers(the maximum error can be an order of magnitude).It is suggested that the reduction of depressurization amplitude and rate can alleviate sand production and the resulting blockage of sand control device.This can explain why Well AT-P2 can achieve an average gas production rate more than three times higher than Well AT-P3 in the second production test with a smaller maximum depressurization amplitude(i.e.,5MPa of Well AT1-P2 and 7.5 MPa of Well AT1-P3).Considering the huge NGH reserves and high risk of sand production in the argillaceous silt reservoir of the NGH production test site in Shenhu area,South China Sea,this study further selects the site as the research object to quantitatively analyze the sand production process of the argillaceous reservoir and its influence on gas production for the first time.It is suggested that the "three-phase zone" production requires less depressurization amplitude than traditional NGH layer production,which is conducive to mitigating sand production.However,The predicted content of solid particles in the produced fluid of argillaceous reservoir is still several to dozens of times higher than the upper limit of moderate sand production(0.05%).High performance sand control device(sand retention rate > 60%)should be adopted to ensure the safety of production.Meanwhile,the research results show that the use of sand control device with high sand retention rate will cause blockage around the well and decrease the gas production rate.For instance,in the middle and late stage of production(half a year to one year),when the sand retention rate is set at 60%,the gas production rate is only about 1/2 to 2/3 of that of 10% sand retention rate.How to achieve a balance between sand control and enhancing gas production is a major problem faced by NGH production in the future.This study can help to further understand the sand production process during NGH exploitation and its influence on gas production,so as to provide a theoretical basis for the formulation of production and sand control schemes in NGH trial production in the future. |
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