Search

Natural gas hydrates (NGHs) are widely distributed in marine and permafrost regions with huge reserves, which are considered one of the important potential sources for future clean energy. At present, China, Japan, the United States, Canada, etc. have conducted several trials; however, they all face varying degrees of challenges, such as low gas production efficiency and discontinuous production periods. In the oil and gas industry, hydraulic fracturing is a mature and highly efficient method for enhancing production through pressurization. Therefore, the successful application of fracturing technology to the NGH reservior is an urgently needed solution and could be a potentially revolutionary technology. This study summarizes the main recent fracturing advances in the hydrate field; it outlines the existing fracturing equipment for the NGH reservoir that differs from traditional oil and gas reservoir development, discussing the more efficient numerical simulation methods from the unit cell, experimental scale, to field scale. Additionally, it investigates the main controlling factors of fracturing behavior, such as the effects of fracturing fluid (viscosity and injection rate) and sample conditions (saturation, stress anisotropy, matrix, and natural fractures). The relationships and mechanisms proposed herein can provide new insights for understanding the fracturing behavior during hydrate exploration and constructing safe fracturing and extraction technologies.

Natural gas hydrate is a strategic energy source with great potential in the future. Elucidating the seepage and thermal conductivity characteristics of hydrate reservoirs is critical for evaluating economic acceptability. However, limited research considers the existence of fracture in natural and artificial hydrate reservoirs. In this study, a digital core generation method for fractured hydrate-bearing sediments (HBSs) based on CT images was proposed; 120 models with different crack openings, inclination angles, and growth ratios were reconstructed. The main findings were as follows: as fractures mature, the permeability of the reservoir will increase, and the effective thermal conductivity will greatly decrease. The increase in the fracture inclination angle will increase the permeability of HBS reservoirs and reduce the effective thermal conductivity. The inclination angle and growth ratio of fractures have the most significant impact on the permeability characteristic, and the growth ratio has the most significant impact on the thermal characteristic. In terms of predicting the permeability and thermal conductivity of fractured HBS, the back propagation and support vector machine are the relatively optimal neural networks compared with random forest and long–short-term memory.

Gas hydrate resources are abundant in the South China Sea (SCS), primarily occurring in clayey-silty reservoirs. However, the process of natural gas hydrate (NGH) extraction can lead to the destruction of cementation structures, the reduction of the bearing capacity, and the deformation of both the hydrate reservoir and its overlying soil layers (OSLs), ultimately causing engineering geological hazards such as seabed landslides. Therefore, before achieving commercial exploitation of NGHs, a comprehensive grasp of the mechanical characteristics of NGH reservoirs and their OSLs is imperative. However, most of the research on the safety of hydrate exploitation focuses on the mechanical properties of the hydrate reservoirs while overlooking the stability of OSLs, especially the undrained mechanical properties of OSLs. Based on this, this study investigates the undrained mechanical characteristics of the OSLs of NGH reservoirs in SCS under the influence of multiple parameters. The findings suggest that the strength and modulus of the OSLs of NGH reservoirs in the SCS are positively correlated with effective confining pressure, shear rate, and overconsolidation ratio but negatively correlated with porosity. Moreover, an increase in the effective stress and porosity enhances the strain-hardening and shear-contraction characteristics of the OSLs. Therefore, during hydrate extraction, locations within the OSLs characterized by a shallower burial depth, lower OCR, and higher porosity are more susceptible to failure, and it is advisable to select the NGH reservoir with a deeper burial depth, smaller porosity, and higher OCR. The conclusions of this study will contribute to the establishment of the constitutive relationship of the OSLs of NGH reservoirs in the SCS and provide a theoretical basis for reservoir stability during hydrate extraction processes.

Short-term trials of natural gas hydrate (NGH) have demonstrated that the efficient and safe exploitation of NGH is impeded by sand production. Considering the highly argillaceous characteristics of NGH reservoirs, a refined sand control method has been proposed. The study investigates how to reduce the impact of mudcake plugging on the permeability of the sand control screens. The proposed gradient sand control method has been experimentally validated for its feasibility in reducing mudcake plugging. The optimal combination involves gradient sand control elements with sand control accuracies of 60 and 40 μm. By implementing this optimized approach, the sand production rate can be reduced by 78.1% while only causing a minor decrease of 9.1% in fluid production. This signifies a significant improvement in the sand control efficiency for NGH reservoirs.