# 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  
1. Analysis of the impact of displacement flow on particulate transport 20 mL/min, 40 mL/min, and 60 mL/min (pure gas) were used to flow through the simulated reservoir, and the sand production morphology was observed by microscope. When the pumping flow rate was 20 mL/min, N\({}_{2}\) was used to carry out the pure gas-phase replacement experiments, the experimental temperature was 15 \({}^{\circ}\)C, and the replacement time was 20 min. The whole process was recorded by the micro camera system, and the flow rate and pressure were recorded by the data acquisition software. From the video recording of the whole replacement process, it can be seen that when using low-flow pure gas replacement, a few small non-uniform seepage channels will be formed around the simulated wellbore at the beginning, and with the passage of time. After extending outward for a certain distance, these small sand channels won't collapse and expand, showing a relatively stable \"fine flow\" sand production pattern, and the final equilibrium state is shown in Figure 11.  
At a pumping flow rate of 40 mL/min, N\({}_{2}\) was used to conduct pure gas-phase replacement experiments at a temperature of 15 \({}^{\circ}\)C for 20 min, and the replacement time was 20 min. It can be seen from the video recording of the whole replacement process that at this flow rate. At the initial stage of the replacement (Figure 12a), the sand started to come out from around the near-well, and the small, irregular channels were expanded to gradually form the \"umbrella\" sand channels (Figure 12b). Over time, the \"umbrellas\" of several umbrella-shaped channels converge near the wellbore and eventually flow into the wellbore (Figure 12c), and the replacement will finally reach a stable equilibrium state, and the channels will no longer be expanded. When the pumping flow rate is 60 mL/min, N\({}_{2}\) is used to carry out pure gas-phase replacement experiments with a temperature of 15 \({}^{\circ}\)C, and a replacement time of 20 min. It can be seen from the video of the whole replacement process that, when the high-flow replacement is carried out, the collapse phenomenon of the near-well zone will evolve very soon around the well (which is a reference for the impact caused by non-uniform gas production during exploitation), and then the gas flow will be gradually stabilized. Once the weakly cemented part of the reservoir starts to produce sand, a dominant channel will be formed; as the macro  
Figure 11: Photo of 20 mL/min pure gas expulsion experiment. reaches a stable equilibrium condition, the fluid will have a greater drag force on the formation particles. This forms a dominant channel with greater fluid drag on formation particles; a stable equilibrium state will be reached macroscopically as expulsion proceeds, as shown in Figure 13.  
Figure 12: Variation of photo in 40 mL/min pure gas expulsion experiments. (**a**) Initial status. (**b**) Umbrella seepage state. (**c**) Umbrella handles convergence state. Analysis of factors affecting different flow rates under pure liquid drive conditions  
Considering the random alternation of water and gas production in the actual production process, at the end of the above gas drive experiments, the simulated reservoirs were driven using pumping flow rates of 20 mL/min, 40 mL/min, and 60 mL/min (pure liquid phase),  
Figure 13: Pure gas 60 mL/min expulsion photo. (**a**) Near-well zone collapse. (**b**) Dominant channel emergence. (**c**) Expansion of the dominant channel. respectively, without replacing the samples, and the morphology of sand out of the simulated reservoirs was observed microscopically.