ABSTRACT
With huge reserves, marine natural gas hydrate (NGH) is an important potential efficient and clean resource to replace oil and gas. The interaction between drilling fluid and NGHs in the drilling process such as mass and heat transfer is likely to induce the decomposition of NGHs in the reservoir, which leads to a decrease in reservoir strength and wellbore instability. In order to know the decomposition characteristics of hydrates during drilling fluid invasion, a simulation and evaluation apparatus for hydrate formation and decomposition was independently assembled to study the impacts of drilling fluid temperature, NaCl, and nano-SiO2 on the phase stability of hydrates in a porous medium. Study results show that the decomposition of hydrates in a porous medium can be divided into three stages during drilling fluid invasion. Hydrates are very sensitive to the change in drilling fluid temperature. The higher the drilling fluid temperature is, the faster the hydrate decomposition rate is. 5.0% NaCl can inhibit the decomposition of hydrates in local and accelerated decomposition stages, and the acceleration of hydrate decomposition rate in stable high-speed decomposition stage promotes hydrate decomposition. Nano-SiO2 can hardly affect the phase stability of reservoir hydrates at low concentrations. However, when nano-SiO2 concentration is equal to or greater than 3.0%, the decomposition rate of hydrates increases, and gradually accelerates with the increase in nano-SiO2 concentration, due to the impact of thermal conducting capacity of the fluid system enhanced by it. These findings will help guide the design of drilling fluid systems for marine NGHs and provide a scientific basis for safe and efficient drilling.
