# Sand Production Characteristics of Hydrate Reservoirs in the South China Sea
## 2 Research Status Worldwide
### Numerical Simulation Study of Sand Extraction from Gas Hydrate in the Sea Area  
Numerical simulation of hydrate mining sand output can play an increasingly important role in guiding production practice by obtaining the understanding and law of sand output at the ore body level based on the physical characteristics of the formation [13]. Currently, there are several simulation software based on heat transfer-fluid flow coupling, such as TOUGH + Hydrate v1.0 and v1.2 [14; 15], HydrateResSim [16; 17], CMG-STARS [18; 19], 3D DEM [20], etc., which can be used for the prediction of gas and water production during the hydrate extraction process. Some scholars have also developed simulation software such as TOUGH + Hydrate + FLAC3D [21], CMHGS [22], HydrateBiot [23; 24], etc., which can be used for the study of geotechnical problems related to the hydrate extraction process. The solid particle dislodgement and transport process was hitched to a coupled heat transfer-flow-geotechnical model for hydrate mining, and the critical hydraulic gradient was used to make a judgment on whether dislodgement of solids could occur, and the amount of particle dislodgement was correlated with the degree of shear damage to the sediment skeleton. It has been found that slowing the rate of buckling was effective in mitigating the sand output at a given mining volume [25]. In addition, the predictions obtained by using the recently developed simulation program fit the gas, water and sand output processes of the first hydrate test mining in Japan in 2013 fitted well [26; 27]. On this basis, a hydrate mining out of sand simulation procedure based on the continuous-discrete medium theory was constructed by integrating TOUGH + Hydrate, FLAC3D, and PFC3D multi-software, drawing on the previous work [21], and the hydrate mining simulation software TOUGH + Hydrate was integrated with the geotechnical analysis software FLAC3D, to achieve the coupled simulation of hydrate mining heat transfer-flow-geotechnical process. The hydrate mining simulation software TOUGH + Hydrate is coupled with the geotechnical analysis software FLAC3D to achieve the coupled simulation of heat transfer-flow-geotechnical process of hydrate mining, after which the information of stress state, hydrate saturation, fluid flow rate, etc. obtained from the simulation is transferred to the discrete element simulation software PFC3D as the initial and boundary conditions to carry out the microscopic sand emergence simulation. On the other hand, some scholars used TOUGH + Hydrate and finite element analysis software ABAQUS 2017 to establish a two-dimensional bare-borehole completion model, took the critical plastic strain as the judgement criterion of sand out [28], and qualitatively analysed the influence of hydrate decomposition, mining differential pressure, reservoir temperature, reservoir permeability and other parameters on sand out by solving the multi-field coupled equations of sand out from custom hydrate mining. The simulation results show that the stress concentration in the borehole and the reduction of formation strength caused by hydrate decomposition are the root causes of sand-out of the reservoir. In addition, a gas-water-sand multiphase seepage model during hydrate extraction was constructed by using CMG-STARS software without considering the geotechnical process [19], which was used to study the performance of gas production and the amount of sand output under different sand-proofing standards, and the results showed that there were large differences in the system gas production under different sand-proofing standards, and excessive sand-proofing precision (e.g., 2-6 \(\upmu\)m) would lead to clogging around the wellbore, which would seriously affect the efficiency of gas production.