The initial thickness of methane hydrate film was directly measured by suspending a single methane bubble in water at 274.0, 276.0, and 278.0 K. The results show that the initial hydrate film thickness decreases from tens of micrometers to about 10 mm with the subcooling increased from 0.5 K to about 3 K. When subcooling is higher than 1.0 K, all initial film thickness data measured under different temperatures vary inversely with the subcooling. Notable three-dimensional growths of hydrate crystals of different sizes and shapes at film front and emergence of new crystal were clearly observed at lower subcooling that resulting in the rougher surface of hydrate film and uncertainty of initial thickness measurement under lower subcooling. The hydrate film growth was dominated by film growth in thickness, not by lateral growth at low subcooling. The growth in thickness of hydrate shell covering one whole bubble surface was also investigated.

No abstract

The feasibility of evaluating the performance of anti-agglomerants from analyzing the variation of morphologies and chord length of particles was examined using an autoclave installed with particle video microscope and focused beam reflectance measurement probes. The experimental results from (methane + water + diesel oil) systemsshow that the agglomeration of hydrate can be judged directly from two laser probes. With the action of an effective anti-agglomerant, the size of droplets/hydrate particles is uniform and tends to that before hydrate formation when most of water converts into hydrate or the system stops stirring for 9 h, although the agglomeration occurs when water droplets initially convert into hydrates. The evaluation method was examined under different anti-agglomerant concentration, water cut, and subcooling for (water + diesel oil) systems.The action mechanism of anti-agglomerants was also proposed.

An experimental apparatus was built to measure P-wave velocity (v P ) of sandy sediment during hydrate formation from brine and free methane gas. The influences of hydrate saturation, initial brine saturation, conversion ratio of water to hydrate, and grain size of sand upon v P were investigated. The experimental results demonstrate that v P strongly depends on both hydrate saturation and initial brine saturation, whereas the influence of the grain size of sand is unremarkable. During the formation of hydrate for different experimental runs at identical initial brine saturation, v P increases with the increase of hydrate saturation; while it decreases dramatically with the increase of initial brine saturation for a given hydrate saturation. The correlation between v P and conversion ratio of water into hydrate was studied, and it was found that the relation between v P and the conversion ratio of water to hydrate is approximately the same for different experimental runs, regardless of how big the differences in initial saturation of brine and the size of sand grain are. v P was calculated for four different hydrate distribution models. The results suggest that low conversion ratio of water to hydrate, hydrate mainly acts as a load component of a dry frame matrix, whereas at higher conversion ratio of water to hydrate it partly acts as a cement that coats grains at grain contacts, and thereby leads to a dramatic increase in v P . Compared with the hydrate saturation, the conversion ratio of water to hydrate has a large role in distributing hydrate in the pores of sediments.

Abstract:The replacement of methane with carbon dioxide in natural gas hydrate-bearing sediments is considered a promising technology for simultaneously recovering natural gas and entrapping CO 2 . During the CH 4 -CO 2 replacement process, the variations of geophysical property of the hydrate reservoir need to be adequately known. Since the acoustic wave velocity is an important geophysical property, in this work, the variations of P-wave velocity of hydrate-bearing sediments were measured during different CH 4 -CO 2 replacement processes using pure gaseous CO 2 and CO 2 /N 2 gas mixtures. Our experimental results show that P-wave velocity continually decreased during all replacement processes. Compared with injecting pure gaseous CO 2 , injection of CO 2 /N 2 mixture can promote the replacement process, however, it is found that the sediment experiences a loss of stiffness during the replacement process, especially when using CO 2 /N 2 gas mixtures.