Model Studies Of The Effect Of Physical Properties Of Methane-Hydrate-Bearing Reservoirs On Dissociation Of Methane Hydrates

Natural gas hydrate (NGH)(mainly methane hydrate) is featured with high power density, worldwide distribution, enormous reserves, no pollution and abundant organic carbon content and, hence, possesses important commercial values and broad application prospect. Exploitation of NGH deposits has become one of major development areas of energy industry. Depressurization is regarded to be the most economical one among NGH dissociation methods. Understanding the influence factors of the method of depressurization and their influencing rule can provide practical guidance for exploitation of the NGH deposits.This paper quantitatively analyzes the effect of pore-water salinity, the thermal conductivity of reservoir rocks and particle size of porous media on depressurization dissociation of methane hydrate based on multiphase seepage and heat transfer models of formation and dissociation of methane hydrate. Results show that:(1) High pore-water salinity contributes to the drop speed of system temperature and, hence, accelerates dissociation of hydrate; however, higher pore-water salinity (≥17%) leads to a rapid decline of system temperature, which inhibits hydrate’s dissociation;(2) Gas production rate and hydrate dissociation rate increase with increasing thermal conductivity, whereas the increasing pore-water salinity can weaken this rule;(3) Large particle size can slower the dissociation rate of methane hydrate; whereas the change of particle size doesn’t affect the final quantity of dissociated hydrate; in addition, the change of specific surface area caused by changing particle size is the main reason that particle size affects hydrate’s dissociation.In addition, the effect of permeability and capillary pressure of porous media on the method of depressurization is also analyzed. Results show that permeability heterogeneity and anisotropy are important factors that affect front location of hydrate’s dissociation. In case of lacking of highly permeable channels, permeability heterogeneity can slow the migration of gas and water in the vertical direction, hydrate’s dissociation slows down, the time needed to reach steady-state for dissociation extends. Permeability anisotropy affects fluid flow in the vertical direction, dissociation rate of hydrate changes significantly. In contrast, capillary pressure has a little effect on hydrate’s dissociation process, gas production and water production of wellhead, which can be negligible.