# Sand Production Characteristics of Hydrate Reservoirs in the South China Sea
## 5 Study of Sand Emergence Phenomenon Based on In Situ Reservoir Samples
### Analysis of Results
Experimental Exploration of Quantitative Prediction Simulation of Natural Gas Hydrate Produced Water Out of the Sand  
At present, it has become a consensus in the industry that the hydrate test mining process will face the risk of sanding. However, the existing research on gas hydrate sanding is biased towards qualitative analysis and mechanism research. Therefore, the reliability of quantitative numerical simulation of sanding still needs to be verified, so it is impossible to obtain accurate quantitative sanding prediction results. In actual production, qualitative judgement can play an auxiliary role in engineering practice and often fails to provide effective guidance in regulating pump conditions, wellhead nozzle opening, separation methods, and so on. The quantitative prediction of gas hydrate water production and sand production can more effectively guide the production practice and optimise the production parameters, which is related to the long-term stable production of gas hydrate and reservoir stability. The amount of sand output from hydrate production is controlled by multiple factors such as man-made construction, pressure drop rate, reservoir properties, etc., which are difficult to simulate through experiments. Therefore, this experiment is intended to explore the feasibility of obtaining a quantitative sand output prediction model based on the experimental method by controlling a single variable: the pressure drop rate. This part of the study adopts the same sampling method as the previous experiment, the experiments were carried out using gas saturation for hydrate synthesis. and the following steps are taken:  
1. Pass 8 MPa methane gas in the thin-section model and leave it for 12 h;
2. Open the water cooling equipment to cool down the thin-section model and record the temperature and pressure changes in the thin-section model. After synthesising the hydrate, keep the low-temperature state and leave it for 24 h to ensure that the hydrate is fully generated;
3. Slowly inject cold water into the thin-section model body, discharge the excess methane gas and leave it for 12 h;
4. Adjust the outlet pressure using the backpressure valve, the pressure reduction rate was set to 300 min/MPa, 240 min/MPa, 180 min/MPa, 150 min/MPa, 120 min/MPa, 100 min/MPa, 90 min/MPa, 80 min/MPa, 60 min/MPa, 45 min/MPa, 30 min/MPa, 20 min/MPa, 10 min/MPa, 5 min/MPa. The hydrate will decompose to produce gas and water, accompanied by sand out;
5. The amount of water produced and the amount of sand produced after drying was measured separately after the experiment. The pressure-temperature curves for a single set of experiments are shown in Figure 1(a). During hydrate synthesis using in-situ reservoir samples, after the hydrate phase transition point is reached, the temperature changes are small, while the pressure plummets and then decreases slowly, as shown in Figure 1(b). Pressure change curves at different time intervals indicate hydrate generation and are not taken as data reflecting hydrate generation as the temperature is affected by the water bath cycle. In the microcosmic domain of pulverised/ultra-pulverised sands, hydrate generation is characterised by free water being generated first, while adherent water attached to the surface of the particles is gradually generated as the temperature decreases, resulting in a slow decrease in pressure as shown in Figure 11:5  
The amount of water produced and the amount of sand produced after drying were measured after the experiments, respectively, and are shown in Figure 11:6  
It can be seen from Figure 23 that, under different buckling rates, water production and sand production show a good correlation. In general, when the water production is  
Figure 11:7 State of particle-attached water and free water \(\rightarrow\) production of hydrate from free pore water \(\rightarrow\) gradual production of hydrate from particle-attached water \(\rightarrow\) total production of hydrate from particle-attached water.