Abstract
Methane hydrate is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of methane hydrates from the reservoir has been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of methane hydrates that is transformed into methane methane and water. Therefore, it is very important to clarify the physical phenomena of gas-water multiphase flow in porous media and the properties of formation and dissociation of methane hydrates.
We have carried out the experimental study on hot water injection as one of the thermal stimulation methods. From the results, it was found that (1) when temperature in downstream zone of sand column was lower than equilibrium condition, additional hydrate growth was promoted at downstream zone due to migration of cooled water and dissociated gas, (2) as a result, differential pressure increased exponentially, and water permeability of sand column decreased drastically.
For inhibition of hydrate growth and improvement of permeability in hydrate reservoir, we conducted further experimental work on the simultaneous injection process of nitrogen and hot water. Nitrogen has the effect as an inhibitor as well as methanol and salts. In this experiment, firstly, nitrogen was injected into sand column to displace free methane gas, and then hot water injection was started. In the progress of dissociation, temperatures in the sand column and differential pressures, production rate of dissociated gas were measured. Additionally, based on measuring data, water permeability in dissociation process was estimated. Due to the inhibitor effect of nitrogen, it was possible to continue water injection without permeability reduction. Thus dissociated gas production was completed earlier in comparison with normal hot water injection process.
