# 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
From the video recording of the whole replacement process, it can be seen that the perturbation phenomenon is stronger than that of the single-phase replacement and the majority of the gas passes along the contact surface between the visible glass during the two-phase replacement, due to the presence of gas and water at the same time. In addition, the simulated reservoir forms a shallower bubble-like sand outlet pore and the sand outlet pore expands from near the wellbore to the well circumference at a certain radius. Most of the gas passes along the contact surface between the visible glass and the simulated reservoir, forming a shallow bubble-like sand outlet hole, and the sand outlet hole expands from the vicinity of the wellbore to the periphery of the well with a certain radius. However, as the repulsion progresses, the channel soon stops expanding and eventually forms a stable state, as shown in Figure 11:1  
When the pumping flow rate is 40 mL/min, the gas-liquid two-phase replacement experiment is carried out by mixing N\({}_{2}\) and water, the experimental temperature is 15 \({}^{\circ}\)C, and the replacement time is 20 min. From the video recording of the whole replacement process (Figure 18), it can be seen that the gas-liquid two-phase seepage is strong, and it carries the weaker part of the formation sand to be produced after intruding into the simulated stratum; due to the flow rate increase and the formation of the channel, the flow rate will be increasingly fast and the formation sand will be carried out to form a slump after being carried out to a weak part of the wellbore. As the flow rate increases, the flow rate will become faster and faster after forming a channel, and the formation of sand will be carried out and form a collapse in the weak part, and the collapsed part extends from the wellbore to the far end. Figure 11:2 Gas-liquid 20 mL/min mixed replacement photo. (**a**) Initial disturbed state of expulsion (**b**) Stable state at the end of expulsion. When the pumping flow rate is 60 mL/min, the gas-liquid two-phase replacement experiment is carried out after mixing with N\({}_{2}\) and water, the experimental temperature is 15 \({}^{\circ}\)C, and the replacement time is 20 min. From the video of the whole replacement process (Figure 19), it can be seen that, at the beginning of the replacement process, due to the larger flow rate and the stronger gas-liquid two-phase seepage, it can be seen that obvious infiltration can be seen in the whole process of the reservoir; then the weakly cemented stratum sands are carried and transported, and the dominant channel rapidly collapses to form a strong impact; finally the fluid continues to flow along the dominant channel, resulting in a shortfall. Then the weakly cemented formation sand is carried and transported, and the dominant channel collapses rapidly, forming a strong impact and causing a deficit; eventually, the fluid continues to flow along the dominant channel. In this gas-liquid two-phase drive process, the initial loading of sand samples was 150.8 g. After collection, precipitation, drying, and weighing, the final drive out of sand was measured to be 48.4 g, with a sand production ratio of 32.1%. Overall, when the gas-liquid two-phase drive, even if the water content is lower, it will still form a larger advantageous seepage channel, which will eventually lead to the emergence of the deficit phenomenon; when the pump injection volume increases, the gas-water ratio decreases, and the liquid-phase sand-carrying effect is enhanced, and the  
Figure 11:3 Gas-liquid 60 mL/min mixed replacement photo. (**a**) Initial permeability state of replacement (**b**) End deficit state of replacement. Figure 11:4 Gas-liquid 40 mL/min mixed replacement photo. (**a**) Initial state of expulsion (**b**) Collapse state at the end of expulsion. degree of sand release and the disturbance of the formation is strengthened, which indicates that the liquid-phase is the main factor influencing the sand release, and the gas-phase is a secondary factor.