Hydrate formation and distribution within unconsolidated sediment: Insights from laboratory electrical resistivity tomography

Laboratory visual detection on the hydrate accumulation process provides an effective and low-cost method to uncover hydrate accumulation mechanisms in nature. However, the spatial hydrate distribution and its dynamic evolutionary behaviors are still not fully understood due to the lack of methods and experimental systems. Toward this goal, we built a two-dimensional electrical resistivity tomography (ERT) apparatus capable of measuring spatial and temporal characteristics of hydrate-bearing porous media. Beach sand (0.05–0.85 mm) was used to form artificial methane hydrate-bearing sediment. The experiments were conducted at 1°C under excess water conditions and the ERT data were acquired and analyzed. This study demonstrates the utility of the ERT method for hydrate mapping in laboratory-scale. The results indicate that the average electrical conductivity decreases nonlinearly with the formation of the hydrate. At some special time-intervals, the average conductivity fluctuates within a certain scope. The plane conductivity fields evolve heterogeneously and the local preferential hydrate-forming positions alternate throughout the experimental duration. We speculate that the combination of hydrate formation itself and salt-removal effect plays a dominant role in the spatial and temporal hydrate distribution, as well as geophysical parameters changing behaviors during hydrate accumulation.