Dynamic evolution of pore structures of hydrate-bearing sediments induced by step-wise depressurization

Pore structures are of great importance to study gas production from natural gas hydrate reservoir. The dynamic evolution of pore structures over the course of methane hydrate dissociation and their association with particle size distribution of hydrate-bearing sediments remain poorly understood. In this study, a combination of X-ray computed tomography (CT) observation and pore network modeling was applied to exploring the dynamic evolution of pore structures with hydrate dissociation induced by depressurization in two initially brine-unsaturated sediments with different particle size distributions. Methane hydrates were synthesized in two silica sands, each of which is with different particle size distributions, at 2.0 degrees C and 8.5 MPa. The reaction process continued nearly three weeks until the pore pressure and temperature inside the reaction vessel did not have significant changes for at least 7 d, to provide sufficient time for methane hydrate in sediment pores to reach steady spatial distribution. The X-ray micro-focus CT observation of methane hydrate dissociation, which was induced by step-wise depressurization, was further performed, to obtain a series of high-resolution CT images of hydrate-bearing sediments at different degrees of hydrate saturation. The corresponding pore structures of the two sediments with different hydrate saturation are studied by the digital image processing methods, including image filter, 2-D threshold segmentation of binarized CT images, 3-D digital core reconstruction, and topological extraction of actual pore structure. To overcome the obstacle caused by the similar X-ray attenuation coefficients of methane hydrate and deionized water, the phase-contrast between methane hydrate and pore water was enhanced by the addition of NaCl into pore water. To have a better configuration about the distributions of materials in sediments, the bulky patchy-like hydrate adhering to the inner wall of the reaction vessel or occupying macroscopic pores was used to calibrate the CT values of sand, water, gas, and methane hydrate in pore space. Based on the above results, the dynamic evolution of sediment pore structures with hydrate dissociation, which was induced by step-wise depressurization, and their association with particle size distribution were investigated. The results indicate that the morphology of gas hydrate in sediments is largely heterogeneous, and the dissociation of methane hydrate generally starts from the contacting place between gas and hydrate. In the early stage of hydrate dissociation, several types of hydrate morphology appear, including pore-filling and grain-cementing, while in the late stage of hydrate dissociation, grain-cementing hydrate is dominant. As methane hydrate dissociates, the average pore throat radius, porosity, absolute permeability and two-phase percolation zone in the sediments will gradually increase, but the coordination number, shape factor, and irreducible water saturation will change in an opposite trend. Due to the complex kinetic behavior of hydrate dissociation, the average pore radius usually gradually decreases in the early stage of hydrate dissociation, followed by a sharp increase when the hydrate saturation is below a critical value of <10%. However, for the sediment with a narrower grain size distribution, the average pore radius, average pore throat radius, porosity, and absolute permeability become higher at the same hydrate saturation. As revealed by the evolution of coordination number and shape factor in the two sediments studied, the original particle size distribution, hydrate morphology, and pore-scale spatial distribution have a synergetic effect on the pore structure of hydrate-bearing sediments.