Comparisons of Geophysical Measurements During Methane Hydrate Formation and Dissociation in Unconsolidated Sediments

Laboratory acoustic and electrical experiment has been proved to be a practical and powerful approach to study the geophysical properties of methane hydrate-bearing unconsolidated sediments, which is of great significance for seismic exploration and resource evaluation of marine gas hydrate. To better understand the differences of hydrate occurrence states in the pore space and process in hydrate formation and dissociation, we measured P-wave and S-wave velocities and electrical resistances of methane hydrate-bearing unconsolidated sediment along hydrate saturation variations. The experiments were conducted through the newly designed experimental apparatus. The sediment sample, made by 0.25-0.425mm quartz sand, was wrapped by a single layer waterproof and breathable membrane to keep sample water-saturated while allowed the methane pass through from all side directions. Thus, the methane hydrate was formed by the dissolved methane in pore water as the reactor temperature dropped to a certain value via the cooling system. The hydrate saturation was calculated by methane consumption amount as referring to the gas pressure change in the reactor, which was then related to the corresponding ultrasonic velocity and electrical resistance measurements. After hydrate formation reached a high saturation and nearly stopped, we increased the reactor temperature to dissociate the hydrate. The experiment was completed after the hydrate fully dissociated, and the gas pressure returned to the initial value when experiment started. Analysis of the experiment results indicated that the relationship between the measured geophysical parameters and hydrate saturation presented a stage change pattern associated with dominant pore occurrence states (suspend in pore fluid, adhere to particles to partially support sediment frame, and cement particles) of hydrate in the various periods. As comparison, the dissociation process ran much faster than the formation process. And the P-wave velocity decreases drastically and even lower than the water velocity because of the free gas released, while the S-wave velocity almost remained the same with a tiny diminution.