Hang Bian

The occurrence of mountainous geological hazards, primarily including rockfalls, landslides, and debris flows, is frequently influenced by multiple environmental factors and exhibits significant spatial heterogeneity and cumulative effects. To address the need for regional-scale susceptibility assessments within complex geological settings, we propose a novel geological hazard susceptibility assessment model based on conditional probability. This study establishes a dual-module evaluation framework incorporating certainty factors (CFs) and weights (W), in which the CF quantifies the contribution of each factor class to hazard occurrence, while the weights reflect the relative importance of the conditioning factors, thereby improving the model’s capability to characterize multifactorial coupling effects. Using three representative mountainous regions in Xinjiang, China—the Ili Valley Region (IVR), the Northern Piedmont of the Tianshan Mountains (NPTM), and the Kunlun–Altun Mountain Region (KAMR)—we integrate 7938 historical hazard points and 11 conditioning factors within a GIS environment to conduct the assessment. The results reveal regional differences in the weights of conditioning factors: IVR is primarily controlled by Elevation (0.184), Urban-Critical Infrastructure Density (0.163), and Annual Precipitation (0.156); NPTM is dominated by Annual Precipitation (0.153), Urban-Critical Infrastructure Density (0.145), and Road Density (0.136); and KAMR is governed by Elevation (0.197), Seismic Acceleration (0.167), and Hydrogeological Type (0.134). In IVR, NPTM, and KAMR, the Very-High and High susceptibility zones occupy 37.10%, 34.86%, and 26.23% of the land area, respectively, and contain 78.18%, 77.24%, and 82.10% of the identified geological hazards. The region-specific ROC-AUCs are 0.8536 (IVR), 0.8545 (NPTM), and 0.8775 (KAMR), indicating good predictive capability across sedimentary basins and tectonically active zones. This study provides methodological and data support for quantitative risk assessment of geological hazards at the regional scale under complex geological conditions.

Summary: Depressurization (PD) and thermal stimulation (TS) are the primary methods for producing gas from natural gas hydrate (NGH) sediments. Fluid flow properties of the hydrate sediment, such as permeability, are fundamental parameters for simulating both processes. Most of the existing formulated permeability models are based on the numerical or experimental investigation of hydrate morphology evolution without considering the decomposition methods. In this study, we investigate the hydrate-decomposition-methods (PD and TS processes)-dependent fluid flow properties of hydrate sediments, which is achieved by microcomputed tomography (micro-CT) scanning of hydrate morphology evolution during PD- and TS-induced decomposition, as well as pore-scale modeling of fluid flow in the extracted 3D fluid-rock-hydrate images.We find that the decomposition behavior during TS is much more complicated than that during PD. The retardation zone in the PD sample increases the heterogeneity of the pore structure, while the secondary hydrates generated during TS cause even more heterogeneity in the pore space. The better facilitation of the TS method on hydrate split is favorable for the continuity of the gas phase. The pore-scale fluid flow simulation shows that the modified Kozeny-Carman (K-C) model is the best to describe the evolution of the normalized permeability with hydrate saturation during PD. However, a single model is not sufficient to describe the normalized permeability during TS decomposition due to the stronger heterogeneous porous structure reformed by the local accumulation of secondary hydrates. The two-phase flow capability is best at the initial stage of PD decomposition, while the two-phase flow region becomes wider as TS decomposition progresses. These results provide significant references for the simulation of the natural hydrate extraction process using different decomposition methods.

Natural gas hydrate is a kind of environmentally friendly unconventional energy with large reserves, which attract increasing attention in recent years. The microscopic pore structure and mineral composition of natural gas hydrate reservoir have a great influence on its seepage characteristics during exploitation. In this study, computed tomography and scanning electron microscope are used to obtain digital images of three clayey-silt natural gas hydrate reservoir samples in the Shenhu area of South China Sea, and then, the pore structure and the mineral composition of the samples are obtained after image processing. The result indicates that the clayey-silt samples show strong hydrophilic characteristics, small particles, good sorting properties, variable pore distribution, small average pore and throat radius, large porosity, and a large content of submicron pores. The mineral composition of the reservoir is mainly quartz and contains a certain amount of clay minerals, mainly illite, while the mineral composition of reservoir rich in microbial fossils is mainly carbonate. The results indicate that the pore structure and mineral composition of natural gas hydrate reservoirs in Shenhu area lay a foundation for the efficient development of the clayey-silt reservoir.

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.