Hole-bottom freezing method for gas hydrate sampling

Abstract

The analysis of core samples is an essential component in the evaluation of natural gas hydrate (NGH) reserves. To prevent NGH volatilization in the sampling process, the commonly employed pressure-temperature preservation technique is used to maintain the temperature-pressure condition of the NGH core at its in situ value. However, the structure of a typical pressure-temperature preservation sampler is notably complicated, the diameter of the core is small, and the insulation effect is poor. To solve these problems, we propose a hole-bottom freezing sampling technique that uses liquid nitrogen as a cold source to decrease the temperature of the NGH core, which reduces its critical breakdown pressure and promotes the self-preservation of NGHs. We also propose two different types of hole-bottom freezing samplers, which are denoted as the cold-source built-in freezing sampler and the cold-source external freezing sampler. Both experimental tests and numerical simulation of the heat-transfer process during core freezing were conducted to evaluate the freezing efficiency of the new method. We calculated and compared the liquid-nitrogen consumption of the two different types of samplers in the process of NGH sampling. The results demonstrate that greater efficiency is obtained using the proposed cold-source external freezing sampler.