Three-phase relative permeability of hydrate-bearing sediments

During production of methane gas from hydrate-bearing sediments, gas, brine, and hydrate may all be present in the sediment. In such a situation, the relative permeability of each phase is the key parameter affecting flow. Thus far, the majority of relative permeability measurements made for hydrate bearing sediment are for gas in the presence of hydrate (no water) or water in the presence of hydrate (no gas). This thesis presents methodology and preliminary results from conducting three-phase relative permeability measurements on hydrate bearing sediment. The experiments took place in a cold lab kept at a hydrate-forming temperature (6°C). A sandstone core that was representative of Gulf of Mexico sediment and was 22” in length and 1.5” in diameter was used. The core holder had pressure taps along its length that are connected to differential pressure transducers to measure pressure drops and minimize sensitivity to capillary end effects. The core or sandpack was partially saturated with DI water to a water saturation of approximately 40% by injecting a specified volume of water into the core or mixing the sand with DI water before packing. The sample was then brought to hydrate forming pressure (1250 psi) by injecting methane. To determine the hydrate saturation, a pump monitored the volume of methane injected into the core in order to maintain 1250 psi. Once hydrate formed, hydrate stability was controlled by pressure, temperature, and salinity. At constant pressure and constant temperature, changes in salinity moved the stability conditions and at certain salinity, the system reached three-phase equilibrium. Three-phase brine was injected to allow flow through the core without destroying or forming any additional hydrate while continuously monitoring the pressure drops. After steady state was reached, as indicated by the pressure drop and saturation remaining constant with time, varying rates of gas were injected to change the gas saturation, and again obtain steady-state pressure drops. This process was completed for representative saturations of gas and water to create a full relative permeability curve. A procedure has been developed for forming hydrates in a pressure vessel and relative permeability values have been measured for samples with hydrate saturations of 24% and 35%. The methodology developed in this thesis will be used to conduct numerous three-phase relative permeability measurements on hydrate bearing sediment.