# Sand Production Characteristics of Hydrate Reservoirs in the South China Sea
## 1 Introduction  
Presently, more than 90 per cent of China's proven reserves of fossil energy resources are coal. The per capita oil reserves represent about 11 per cent of the world's average, while the natural gas is only about 4.5 per cent. The shortage of oil and gas resources has become an important factor restricting the sustainable development of China's economy and society. Natural gas hydrate (commonly known as combustible ice, hereinafter referred to as hydrate) has advantages in large reserves, and wide distribution, and therefore has been considered a potential alternative energy source in the future by the world's major oil and gas companies, and research institutions [1, 2].  
Hydrates in nature are cage-like crystalline compounds formed by natural gas molecules and water under certain temperature and pressure conditions and are widely distributed in shallow sediments on the seafloor and in permafrost on land [3]. According to conservativestatistical analysis, the global hydrate resources are about 2 \(\times\) 10\({}^{5}\) billion tonnes of oil equivalent, which is about twice the total carbon of global conventional fossil fuels [4], and more than 90% of them are endowed in the marine area. China's sea area is also rich in hydrate resources. The South China Sea alone is estimated to have 74.4 billion tonnes of oil equivalent hydrate. Till now, Canada, the United States, Japan and China have conducted many onshore and offshore hydrate test mining operations [5; 6], while South Korea and India have also formulated detailed hydrate test mining plans [7; 8]. Particularly, the results of Japanese offshore test mining and South China Sea Shenhu Sea test mining are gratifying, and global hydrate industrialisation is progressively advancing.  
However, to further realize the utilisation of hydrates, it is necessary to solve the technical bottlenecks for its safe and efficient exploitation. Hydrates in nature are mainly distributed in onshore permafrost and deep-sea sediments with harsh environments and a severe lack of infrastructure, leading to more difficulties and greater safety risks in exploration and development compared to conventional oil and gas. In particular, the hydrate-bearing strata in the sea itself have a poor degree of consolidation [9]. It is easy to induce a series of engineering problems such as reservoir subsidence, instability out of the sand, and so on, and the latter in particular has become one of the bottlenecks restricting the safe, efficient, long-term and controllable extraction of hydrates. The Mallik test mining well in Canada in 2007, the CO\({}_{2}\) replacement mining test well in Alaska, USA in 2012, and the hydrate test mining wells in the South China Sea Trough in Japan in 2013 and 2017 all had serious sanding problems, which greatly affected the efficiency of gas production and even led to the test mining having to be terminated prematurely [10]. Although the sand production from the two rounds of trial production in the South China Sea muddy silt hydrate reservoir is relatively small [11; 12], the capacity decay caused by the reduction of the permeability of the sand-proof medium also restricted the stable production cycle.  
The main cause of the above phenomenon is a lack of clear understanding of the risks and patterns of sand production in unconsolidated muddy silt reservoirs, resulting in the failure to adopt effective sand control techniques. In order to clarify the sand production risk of the South China Sea hydrate reservoir, to elucidate the pore change characteristics of the reservoir under gas/water/sand migration, and to explore the quantitative prediction method for water and sand production, this paper conducted relevant research using real logging data and reservoir samples.