Mobilization of water affecting formation and dissociation of hydrate in clay sediment

Abstract

Under in situ conditions, common occurrences of liquid water within hydrate-bearing reservoirs have been confirmed during recent field studies. However, the potential influences of such dynamic process – migration of liquid water on formation and dissociation of gas hydrate have still never been investigated. By using brown clay as storer of mobile liquid water, in combination of ice segregation, gradient temperature fields and plane observation windows, and in addition to magnified ‘climbing effect’ under gradient temperature fields, these potentials were investigated. Non-occurrence and even dissociation of hydrate within stability region of methane hydrate are identified; and in comparison with generality of P and T, liquid water migration is responsible for such heterogeneities and localities during hydrate formation and dissociation. For the sample with height of 22.5 cm, upper limit of hydrate converging section does not exceed 15.1 cm and the lower one is above 3.4 cm. I.e. hydrate appearance mainly distributes in the middle and lower section of sample with a range of 6.5 ∼ 9.1 cm and the maximum utilization ratio of sample in vertical direction does accordingly not exceed 45 %. Moreover, ‘breathing’ and ‘water ripple’ vibrations along the bottom of solid as-formed hydrate structures are observed, implying a mechanical driving force to suck up liquid water and then convey away to foster hydrate growth elsewhere. A novel layer-by-layer accumulation growth pattern of hydrate is accordingly proposed, providing a new perspective for the future applications and developments relevant to natural gas hydrate, e.g. resource evaluation of hydrate, over-optimistic comments on current hydrate stability, reason of hydrates exploration technology losing its efficacy, plugging mechanism of hydrate inside pipelines, etc.