Sand production is the process in which formation sand and gravel would migrate into the wellbore by the flow of reservoir fluids. This is a significant problem that endangers the safety of hydrate exploitation. The aim of this study is to understand sand production during hydrate exploitation. A novel experimental apparatus was constructed to examine sand production in the hydrate layer by using the depressurization method. Hydrate production was divided into three periods: water, gas with water drops, and gas. We detected sand production in the first two periods: fine sand in the first period and sand grains in the second. The temperature related characteristics of the hydrate layers and the rates of sand production differed during different stages of hydrate production. The unique sputtering occurring owing to the decomposition of the hydrate might have provided the driving force for sand migration, and water gas bubbles or gaseous water drops from the decomposed hydrate might have enhanced sand carrying capacity. The subsidence of hydrate-bearing sediments was influenced by sand production, whereas the maintenance of crustal stress possibly influenced the rate and magnitude of subsidence. Future experimental and numerical research into the dynamical thermal properties and material balance of the hydrate layer production must consider its dynamic subsidence.Energies 2018, 11, 1673 2 of 17 prevent, solve and overcome sand control problems and their prediction [14][15][16]. Sand production is a severe problem affecting the safe and efficient hydrate exploitation [4,5,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. Sand production has been reported in the cases of Messoyakha terrestrial hydrate reservoir [18,19], Mallik terrestrial hydrate production tests in 2007 and 2008 [17,20,31], Alaska's North Slope terrestrial hydrate exploitation [31] and Nankai offshore hydrate production tests in 2013 and 2017 [22,23,33,34]. In May 2017, China performed two types of hydrate production tests in the Shenhu and Liwan areas of the South China Sea using methods of sand control and sand separation, respectively [26,28,29]. The sand control demonstrated by these production tests may not be sufficient for large-scale commercial hydrate exploitation with larger production rates and times. Therefore, research on the sand production mechanism in hydrate exploitation is essential.In experimental investigations, the importance of fine sand production during gas production owing to hydrate-bearing sediments has been noted, even when the content of fine sand is relatively low [36], but these tests did not involve wellbores or temperature gradient. Sand production occurs during the depressurization process, when hydrates are unstable. The driving force here is not dissociated gas flow but the water flowing through pores; the water flow rate determined the occurrence of sand production [37], and the effects of the wellbore and layered temperature were not discussed in these tests. Fine sand invasion was obs...