A simulation study on the formation mechanisms of gas hydrates in pores of gas hydrate-bearing sediments in the South China Sea

The formation of high-quality gas hydrates in the pores of the sediment matrix serves as the premise and material basis for experimental research on gas hydrates, thus providing theoretical guidance for the exploration of deepwater hydrocarbons and gas hydrates in China. Based on the geological conditions of gas hydrate-bearing sediments in well GMGS2-07 in the South China Sea, this study conducted numerical simulations and laboratory experiments using the TOUGH+HYDRATE code and a self-developed device for gas hydrate reactions and generation, respectively. Firstly, this study verified the accuracy and reliability of the numerical simulation method. Then, it investigated the quality of gas hydrates generated under different thermal conductivities and water saturation of sediments to determine their influencing mechanisms. The results are as follows: (1) Both numerical simulations and laboratory experiments yielded consistent variation trends and nearly identical characteristic values of the temperature, pressure, and three-phase materials in the process of gas hydrate formation. Therefore, the numerical simulation method used in this study is accurate and reliable; (2) A higher thermal conductivity was associated with faster gas hydrate formation, as well as the higher saturation and more uniform distribution of the final gas hydrates. However, the correlation between the thermal conductivity and the final gas hydrate saturation depended on the position relative to critical boundaries, which were determined at a distance of 1.8 cm from the upper and right boundaries of the tank reactor used in this study. There was a positive correlation between the thermal conductivity and the gas hydrate saturation within the critical boundaries. Otherwise, they were negatively correlated. The distances between the critical boundaries and the boundaries of the water bath increased with an increase in the tank reactor size but were not affected by the formation permeability; (3) As the formation water saturation increased, the saturation of the final gas hydrates first increased and then decreased, and the peak water saturation was less than the theoretical gas-water ratio under the initial pressure. The gas hydrates showed the highest saturation and the most ununiform distribution when the initial pressure was 7.8 MPa and the water saturation was about 22.23%. Therefore, the sediment matrix prepared using high-thermal-conductivity materials, the initial water saturation lower than the theoretical gas-water ratio, and the initial temperature and pressure falling to the left of the phase equilibrium curve are conducive to the formation of high-saturation, uniformly distributed gas hydrates. This study will provide a sound material basis for well cementing in deep-water sediments bearing hydrocarbons and gas hydrates, as well as the drilling and exploitation of gas hydrate resources. Moreover, it will contribute to the technical preparation for the commercial recovery of gas hydrates. © 2023 The authors.