A novel approach to enhance wellbore stability was put forth, based on the wellbore rock properties and instability mechanism of the hydrate reservoir, given the issue of wellbore instability when using water-based drilling fluids (WBDFs) in drilling operations, in weakly cemented muddy fine silt reservoirs of natural gas hydrates in the South China Sea. Three main strategies were used to increase the stability of reservoirs: enhancing the underwater connection between sandstone particles and clay minerals, preventing clay hydration from spreading and expanding, and strengthening the stability of hydration skeleton structure. An appropriate drilling fluid system was built with soil phase containing wellbore stabilizer. Sulfonic acid groups and electrostatic interaction were introduced based on the characteristics of underwater adhesion of mussels. Through the process of free radical polymerization, a zwitterionic polymer containing catechol groups named DAAT was prepared for application in natural gas hydrate reservoir drilling. DAAT is composed of tannic acid (TA), dimethyl diallyl chloride ammonium chloride (DMDAAC), 2-acrylamide-2-methylpropanesulfonic acid (AMPS) and acrylamide (AM). Experimental results from mechanical property testing reveal an adhesion force of up to 4206 nN between SiO2 and 5 wt % DAAT, demonstrating its ability to bind quartz sand particles effectively. The compressive strength and cohesion of the cores treated with DAAT increased by 58.33 wt % and 53.26 wt %, respectively, at -10 degrees C, compared with pure ice particle cores. This demonstrates DAAT can significantly enhance the compressive strength and cohesion of the core. Furthermore, the adhesion force between DAAT and hydrate particles reaches up to 344.4 mN/m, significantly improving the structural stability between hydrate particles. It demonstrates excellent adhesive properties to hydrate particles. In addition to adsorbing clay minerals, rocks, and hydrate particles, DAAT also forms hydrogen bonds with argillaceous fine silt particles with its low temperature cohesiveness characteristic. As a result, it improves the cohesion between core particles, and enhances the adhesion between hydrates and rocks, thereby enhancing the stability of hydrate reservoirs. In summary, DAAT is characterized by a simple preparation process, cost-effectiveness, and environmental friendliness. It is an innovative and practical material for enhancing wellbore stability in WBDFs for natural gas hydrate exploration in the South China Sea.

Methane gas hydrate-bearing sediments hold substantial natural gas reserves, and to understand their potential roles in the energy sector as the next generation of energy resources, considerable research is being conducted in industry and academia. Consequently, safe and economically feasible extraction methods are being vigorously researched, as are methods designed to estimate site-specific reserves. In addition, the presence of methane gas hydrates and their dissociation have been known to impact the geotechnical properties of submarine foundation soils and slopes. In this paper, we advance research on gas hydrate-bearing sediments by theoretically studying the effect of the hydromechanical coupling process related to ocean wave hydrodynamics. In this regard, we have studied two geotechnically and theoretically relevant situations related to the oscillatory wave-induced hydromechanical coupling process. Our results show that the presence of initial methane gas pressure leads to excessively high oscillatory pore pressure, which confirms the instability of submarine slopes with methane gas hydrate accumulation originally reported in the geotechnical literature. In addition, our results show that neglecting the presence of initial methane gas pressure in gas hydrate-bearing sediments in the theoretical description of the oscillatory excess pore pressure can lead to improper geotechnical planning. Moreover, the theoretical evolution of oscillatory excess pore water pressure with depth indicates a damping trend in magnitude, leading to a stable value with depth.

Permafrost regions of Qilian Mountains in China are rich in gas hydrate resources. Once greenhouse gases in deep frozen layer are released into the atmosphere during hydrate mining, a series of negative consequences occur. This study aims to evaluate the impact of hydrate thermal exploitation on regional permafrost and carbon budgets based on a multi-physical field coupling simulation. The results indicate that the permeability of the frozen soil is anisotropic, and the low permeability frozen layer can seal the methane gas in the natural state. Heat injection mining of hydrates causes the continuous melting of permafrost and the escape of methane gas, which transforms the regional permafrost from a carbon sink to a carbon source. A higher injection temperature concentrates the heat and causes uneven melting of the upper frozen layer, which provides a dominant channel for methane gas and results in increased methane emissions. However, dense heat injection wells cause more uniform melting of the lower permafrost layer, and the melting zone does not extend to the upper low permeability formation, which cannot provide advantageous channels for methane gas. Therefore, a reasonable and dense number of heat injection wells can reduce the risk of greenhouse gas emissions during hydrate exploitation.

The surface conductor is the first structural casing in deepwater natural gas hydrate (NGH) development, bearing the top load while suspending the casings of various layers. NGH decomposition leads to formation settlement, changing the mechanical properties of the formation and reducing the bearing capacity of the surface conductor, threatening the safety and stability of the wellhead. Understanding the bearing characteristics of the surface conductor in the hydrate formation can guide the safe drilling operation in the field. By introducing the negative skin friction theory of pile foundations and based on conventional bearing capacity models, a method for calculating the bearing capacity of surface conductors in NGH formations was developed. Using an NGH drilling simulation apparatus, the accuracy of the bearing capacity theoretical model was verified, empirical coefficients under different conditions were obtained, and the influence of soil parameters, hydrate saturation, and decomposition temperature on the bearing capacity of surface conductors was quantified. The results indicated that compared to clay, sandy soils have higher porosity and significantly weakened strength after the decomposition of NGH; when the hydrate saturation in the formation is 20%, the reduction in bearing capacity of the surface conductor in sand exceeds 30%, and in clay soils, it decreases by 25% after complete decomposition of NGH; as the hydrate saturation increases, the reduction in the bearing capacity of surface conductors after decomposition becomes more significant. Verified through Experimentation, the error of the hydrate-bearing strata-bearing capacity model is around 10%. For short-term test production operations of NGH in water, the design depth for surface conductors is around 100 m. These research results can provide a scientific theoretical basis for the design of conductor depth below the mud, and reduce operational risks.

High-latitude permafrost, including hydrate-bearing frozen ground, changes its properties in response to natural climate change and to impacts from petroleum production. Of special interest is the behavior of thermal conductivity, one of the key parameters that control the thermal processes in permafrost containing gas hydrate accumulations. Thermal conductivity variations under pressure and temperature changes were studied in the laboratory through physical modeling using sand sampled from gas-bearing permafrost of the Yamal Peninsula (northern West Siberia, Russia). When gas pressure drops to below equilibrium at a constant negative temperature (about -6(degrees)C), the thermal conductivity of the samples first becomes a few percent to 10% lower as a result of cracking and then increases as pore gas hydrate dissociates and converts to water and then to ice. The range of thermal conductivity variations has several controls: pore gas pressure, hydrate saturation, rate of hydrate dissociation, and amount of additionally formed pore ice. In general, hydrate dissociation can cause up to 20% thermal conductivity decrease in frozen hydrate-bearing sand. As the samples are heated to positive temperatures, their thermal conductivity decreases by a magnitude depending on residual contents of pore gas hydrate and ice: the decrease reaches similar to 30% at 20-40% hydrate saturation. The thermal conductivity decrease in hydrate-free saline frozen sand is proportional to the salinity and can become similar to 40% lower at a salinity of 0.14%. The behavior of thermal conductivity in frozen hydrate-bearing sediments under a pressure drop below the equilibrium and a temperature increase to above 0 C-degrees is explained in a model of pore space changes based on the experimental results.