Natural gas hydrate is a potential future energy resource, and its development is now realistic; however, it is not clear how methane hydrate reservoirs accumulate. Few data are available on capillary pressure around gas hydrate reservoir, which is commonly used for reservoir characterization, because of the difficulty measuring it. Whereas conventional oil and gas reservoirs involve solid rocks, the layers containing concentrated gas hydrate consist of unconsolidated sediments, which are easily deformed during measurements. The pore-size distribution of Nankai Trough unconsolidated sediments was measured using mercury injection, which is commonly used to measure capillary pressure. After many measurements, the relationship between the injection pressure and injected volume of mercury was determined. The pore size at the time of injection was calculated from the mercury injection curves. After the measurements, we observed the microstructure of the sediments using field emission scanning electron microscopy (FE-SEM) and clarified the pore size. The pore sizes of sediments were compared to eliminate inferior data caused by sample deformation. We ultimately measured 99 pieces of unconsolidated sand, silt, and mud. The results of the entry pressure measurements of Nankai Trough sediments will help to determine the mechanism of methane hydrate storage in sediments and will contribute to methane hydrate research and development. The entry pressure that is an indicator of capillary pressure data will also be important reference data on unconsolidated sediments for assessing carbon dioxide capture and storage (CCS).
The mercury injection method is used to determine the physical properties of rock, such as porosity and pore size distribution, which are very important determinants of the fluid flow within rocks. By contrast, unconsolidated soft sediments contain much water, and their properties change with drying, which has prevented the use of mercury injection devices to measure their porosity.
