HUTECH

Abstract Natural gas hydrate (NGH) is widely considered a potential alternative to traditional energy resources. Some countries are trying to exploit and utilize NGH. However, due to the sensitivity of the hydrate, and a lack of exploitation experience with the NGH, drilling in hydrate reservoirs is considered risky. In order to understand the disturbance effects of drilling fluid with different circulation rates on the hydrate-bearing sediments during the drilling process, a novel hydrate drilling experimental apparatus was used to study the drilling process in the hydrate-bearing sediments. Four drilling experiments were carried out in consolidated sediments containing Tetra-n-butyl ammonium bromide (TBAB) hydrate. The drilling fluid injection rate ranges from 500 to 1800 g/min. The results show that hydrate dissociation can be induced by the circulation of relatively high-temperature drilling fluid when drilling in hydrate-bearing sediments, and that the hydrate dissociation rate is greatly influenced by the circulation rate of the drilling fluid. The drilling fluid invasion area has a maximum as the experiment progresses. Increasing the drilling fluid circulation rate can enhance the sediment temperature rise rate, enlarge the hydrate dissociation region, and increase cumulative TBAB production. Drilling fluid with a relatively low circulation rate is beneficial to wellbore stability. These findings will provide help with drilling design suitable for hydrate-bearing sediments, and provide a scientific basis for achieving safe drilling in hydrate-bearing sediments.

Abstract Gas hydrate is considered as a new promising energy resource, and it is extensively distributed in offshore sediments and permafrost region. Two important factors affecting the gas hydrate dissociation are the water saturation in the sediments and the particle size of the sediments. In this study, the influences of the water saturation on the hydrate dissociation by depressurization in the sediments with different particle sizes were analyzed. It was found that there could be an optimum water saturation (Sw,op), at which the hydrate dissociation rate reached to the maximum value. When the water saturation (Sw) was smaller than Sw,op, the hydrate dissociation was mainly limited by the heat transfer rate of the sediments, and the hydrate dissociation rate increased with the increase of the water saturation. When the water saturation (Sw) was larger than Sw,op, the hydrate dissociation rate was limited by the mass transport rate of the gas released from the hydrate dissociation, and the hydrate dissociation rate decreased with the increase of the water saturation. The experimental results showed that for the sediments with the particle size of 40–60 mesh, the optimum water saturation is smaller than 0.042. For the sediments with the particle size of 325–400 mesh, the optimum water saturation is larger than 0.415. For the sediments with the particle size of 105–125 mesh, the optimum water saturation is closed to 0.413. Therefore, the optimum water saturation for hydrate dissociation increased with the decrease of the particle size of the sediments, and it was inferred that the optimum water saturation was related to the irreducible water saturation of the sediments. It was a new knowledge about the influence of the water saturation on the hydrate dissociation in the sediments with different particle sizes, and it could provide some guidance for the production method of gas hydrate in different types of hydrate reservoirs.