The low permeability of clayey-silt hydrate reservoirs in the South China Sea affects the thermal and pressure conductivity of the reservoir, which is difficult to spread to the far end of the wellbore and achieve commercial gas production. In this respect, enhancing the permeability to assist depressurization is necessary. Hydraulic fracturing is a promising reservoir stimulation method for gas hydrate reservoirs. Up to now, majorities of research focus on the fracability of hydrate-bearing sandy sediments, but the studies rarely involved fracture propagation characteristics of clayey-silt sediments in the hydrate dissociation area. In this paper, three sets of hydraulic fracturing experiments under different confining pressure were carried out using the clayey-silt sediments in the Shenhu Area. Computed tomographic (CT) images indicated that clayey-silt sediments could be artificially fractured, and the fracturing fluid could induce tensile fractures and local shear fractures. A multimorphological fracture zone occurred near the borehole. Furthermore, the greater the confining pressure imposed, the greater the breakdown pressure was, and the microfracture arose more easily. The fractures at the top were generally wider than those at the bottom with the same confining pressure. The experimental results could reveal the fracture initiation and propagation mechanism of clayey-silt sediments and provide theoretical support for hydraulic fracture in the hydrate dissociation area.

As a dynamic research method for molecular systems, molecular dynamic (MD) simulation can represent physical phenomena that cannot be realized by experimental means and discuss the microscopic reaction mechanism of things from the molecular level. In this paper, the previous research results were reviewed. First, the MD simulation process was briefly described, then, the applicability of different molecular force fields in the natural gas hydrate (NGH) system was discussed, and finally, the application of MD simulation in the formation and decomposition law of NGH was summarized from the perspective of NGH mining. The results show that the selection of water molecular force field has a great influence on the simulation results, and the evaluation of water model applicable to the simulation of NGH under different thermodynamic states is still an open research field that needs to be paid attention to. The effect of surface properties of porous media (such as crystallinity and hydrophilicity) on hydrate needs to be further studied. Compared with thermodynamic inhibitors, kinetic inhibitors (such as amino acids) have more promising research prospects, and further research can be carried out in the screening of efficient kinetic inhibitors in the future.

Summary: The depressurization method for natural gas hydrate exploitation encounters the difficulties of low productivity and sand production. Enhancing hydrate production efficiency and reducing massive sand production are the key issues to be solved in hydrate development. This paper proposes a new approach to hydrate reservoir stimulation and sand control by liquid-solid phase change proppant (LSP) filled in radial well (SCRP). A multicomponent flow model of sand-water-gas-hydrate is established and applied to evaluate the effect of SCRP on hydrate development. The results obtained show that the application of SCRP realizes an integrative effect of both reservoir stimulation and sand control. Radial wells increase the sweep area of pressure drop and promote hydrate decomposition. Solid particles, which are formed by LSP, not only fill and support radial wells but also act as sand control medium, preventing sand from flowing into the production well. It is found that reducing the production pressure difference can effectively reduce sand production. To achieve a large-scale stimulated reservoir volume, it is necessary to increase the length of a single radial well, the number of radial wells in the same layer, and the number of radial well layers. The spiral layout of radial wells has a better effect on improving gas production. A single radial well achieves good sand control effect with the application of the LSP system, but an increase in radial well number leads to an increase in sand production, resulting in massive amounts of sand gathered in the production well. As a result, in the case of multiple radial wells, other sand control technologies, such as mechanical sand screening, should be used along with SCRP to further reduce sand production.

Many hydrate-bearing sediments in the Shenhu area of the South China Sea are featured with unconsolidated clayed silt, small particle size, and high content of clay, which can pose a great challenge for gas production. In order to investigate the gas-water relative permeability in clay-silt sediments, through a radial flow experiment, samples from the target sediment in the Shenhu area were selected and studied. The results show that the irreducible water saturation is high and the influence of the gas-water interaction is obvious. The relative permeability analysis shows that the two-phase flow zone is narrow and maximum gas relative permeability is below 0.1. The flow pattern in clay-silt sediment is more complicated, and the existing empirical models are inadequate for flow characterization. The depressurization method to extract a hydrate reservoir with clay-silt sediments faces the problem of insufficient production capacity. Compared with the ordinary hydrate reservoir with sandstone sediment, the hydrate reservoir with clay-silt sediment has a low permeability and poor gas flow capacity. The gas-water ratio abnormally decreases during the production. It is urgent to enhance production with cost-effective measures.