Energy Reports (Nov 2021)
Study on the growth habit of methane hydrate at pore scale by visualization experiment
Abstract
The growth habit of natural gas hydrate in formation pores directly influences its occurrence and distribution, which are fundamental factors for macroscopic reservoir properties. In order to simulate the hydrate growth at pore scale and demonstrate the growth process directly, microscopic visualization experiments were carried out using glass etching model and high-resolution video microscope in this work. The crystal growth behavior of methane hydrate was observed clearly in real time, and fluid state and system pressure were analyzed. On this basis, hydrate growth patterns and corresponding control mechanisms were investigated. The results show that for a static methane–water system, the hydrate crystal grew rapidly about 10 min after perturbation. The solution gas crystallized with water, forming colorless, transparent and polygonal crystal with sharp edge and smooth surface. The free gas crystallized with water at the gas–water interface first, resulting in a quick formation of hydrate crust. Then water passed through the hydrate crust slowly and crystallized with the free gas trapped inside hydrate crust. The resulting hydrates seemed to rough and honeycomb aggregations of tiny and tightly packed crystal particles. The two kinds of hydrate with different morphologies represent two typical hydrate growth patterns. For the hydrate growth pattern of solution gas–water, solution gas, which migrates to the crystal growth front driven by the concentration gradient, is in direct contact with water, leading to a fast crystal growth rate. For the crystal growth pattern of free gas–water, the preferentially formed hydrate crust at the gas–water interface not only separates the free gas from the water phase, but also selectively obstructs the passage of gas and only allows water to pass slowly, which slows down the hydrate growth process. This research is helpful to further understand the experimental phenomena at the core and field scale, explain the existing problems, and provide support for the research on the dissociation and exploitation of natural gas hydrate.
