ACCELERATED ETHANE GAS HYDRATE FORMATION IN SEAWATER

Physical models based on chemical potentials indicate that the ethane hydrate phase boundary curve is shifted to higher pressures and lower temperatures in salt water compared to pure water. It is assumed by analogy that seawater, including the interstitial water in marine sediments, should be a more difficult medium for hydrate formation than fresh water. However, copious amounts of ethane hydrate have been produced experimentally at pressure conditions below those anticipated using raw seawater. Hydrate formation has begun in as little as 5 minutes with significant agitation and 40 minutes with limited agitation at 4 ^oC and 1.5 MPa. Hydrate formation has been achieved at elevated temperatures in a moderate time using classic supersaturation techniques. Extensive hydrate nucleation followed by agglomeration into robust growing phases occurs reproducibly. Sensors imbedded within the hydrate phase show a marked deviation from the expected clathrate hydrate/seawater phase boundary. Conditions in the mass show hydrate temperatures exceeding solution temperatures by a large margin, up to DT=7 ^oC. Varying the pressure results in a systematic variation of the temperature and has allowed the lower phase boundary to be mapped. We find that hydrate nucleation and growth appears to be significantly easier to achieve in natural seawater than calculations on idealized salt water would predict. The hydrate stability zone may be extended to lower pressures than anticipated owing to the presence of naturally occurring chemicals and particulates in seawater, which is a complex fluid that may not be easy to replicate by artificial fabrication under laboratory conditions. The position of hydrate phase boundaries in hydrates growing in seawater appears to be significantly different from the phase boundaries determined during dissociation, which takes place in essentially a fresh water environment.