Measurements of the Capillary Pressure-Saturation Relationship of Methane Hydrate Bearing Sediments

Abstract Methane hydrate present in permafrost and sub oceanic sediments has been identified as a potentially large energy source. Producing natural gas from hydrate results in the hydrate dissociating into gas and water, which then become distributed in the pore space according to gravitational, viscous, and capillary forces. Capillary pressure is a function of the medium (wettability, geometry) and the saturations of all phases (e.g. gas, hydrate, water) in the pore space. The presence of hydrate alters the geometry of the pore space, changing the capillary pressure-saturation relationship from the hydrate-free condition. Understanding the capillary pressure-saturation relationship of hydrate-bearing media is important for modeling the flow of gas and water through that medium, and predicting natural gas production from hydrate-bearing reservoirs. We have developed a method for measuring the capillary pressure-saturation relationship in methane hydrate-bearing sand, and our measurements and modeling aid in understanding the behavior of the gas and water in hydrate-bearing sediment. Our experiments involve hydrate formation in unsaturated sand, saturating the sample, followed by step-wise drainage from full water saturation to residual water saturation while measuring the pressure difference between the water and gas phases. During drainage, a number of intermediate static equilibrium conditions were established during which flow was discontinued. The static equilibrium observations provide water saturation vs. capillary pressure relations. For selected samples, drainage was followed by step-wise imbibition to full saturation. Introduction Natural gas hydrates are solid compounds in which a network of water molecules form clathrate structures around small gas molecules such as methane, ethane, and carbon dioxide. These compounds generally form and are stable at elevated pressures and low temperatures where water and the hydrate-forming compounds are available in sufficient quantities. Natural gas-hydrate-bearing deposits in sub oceanic sediments and permafrost regions are estimated to hold an enormous quantity of hydrocarbons, with the primary constituent being methane. Estimates of the volume of hydrocarbons contained in gas hydrates span a large range (from 1 to 5 × 1015 m3 (Milkov, 2004) to 1.2 × 1017 m3 at standard temperature and pressure (Klauda and Sandler, 2005)), and dozens of natural deposits have been encountered. It may be possible to develop some of these deposits to produce natural gas, providing a potential energy resource given that a sufficient amount of hydrate is present, permeability is adequate, the deposit is confined, and needed infrastructure is located nearby.