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.

A series of tri-axial shear tests were carried out to determine the stress and strain characteristics, as well as the volume deformation of methane hydrate-bearing sediments during gas hydrate dissociation. An innovative type of depressurization was adopted with a high-pressure and low-temperature tri-axial apparatus. Results show that: (1) decrease in pore pressure during the shear process may result in the failure of hydrate-bearing sediments, but they did not collapse completely due to high effective confining pressure; (2) depressurization leads to the contraction of volumetric strain and the ultimate deformation shows no difference compared to that prior depressurization; (3) high saturation hydrate-bearing sediments were more sensitive to depressurization, which could be due to the methane hydrate acting as a skeleton structure at some sites when the pore hydrates’ saturation is high.

Silty-clay reservoirs are a weak point in sand production and sand control studies due to their low economy. However, China’s marine natural gas hydrates (NGH) mostly exist in silty-clay sediments, which restrict the sustainable and efficient development of NGH. In order to study the sand production of hydrate silty-clay sediments, hydrate production experiments in vertical wells and horizontal wells were carried out using a self-developed hydrate sand production and sand control simulation device. The results showed a great difference between the hydrate silty-clay sediments and hydrate sand sediments. The significant differences in production pressure and production temperature between the different layers indicated the low permeability and low heterogeneity of the hydrate silty-clay sediments. The sliding settlement of the overall depression in the horizontal well and overall subsidence in the vertical well of the hydrate silty-clay reservoir would easily lead to silty-clay flow and large-scale sand production. When water rates decreased, the property of “silty-clay sediment filtration and wall building” was found, which formed a “mud cake” around the wellbore. The good strength of adhesion and fracture permeability of the “mud cake” provided ideas for reservoir reformation. This study further discusses sand production and sediment reformation in hydrate silty-clay sediments.

The effects of sediments with different clay contents on the mechanical properties of hydrate deposits were studied using a high-pressure, low-temperature triaxial apparatus with in-situ synthesis, as well as the mechanical properties of self-developed hydrate sediments. Through multi-stage loading, triaxial compression tests were conducted by adding quartz sand with different clay contents as the sediment skeleton, and the stress–strain relationship of the shearing process and the strength of sediments with different clay contents were determined. Volumetric changes were also observed during shearing. The results show that the strength of hydrate sediments decreases with the increasing clay content of sediments; in the processes of depressurization and shearing, the hydrate samples exhibited obvious shear shrinkage, regardless of the sediment particle size.