While the CH4 system that is used almost exclusively in gas production study is thought to be well understood and described, an important issue (with significant implications) that has yet to be investigated is the hysteresis between the P-T rela­tionships in a warming and cooling hydrate system. All predictions reported in the literature have relied exclusively on the warming P-T relationships, while the cool­ing P-T relationships have not even been quantified. The cooling P-T curve has a very different behavior (attributed to metastability) that is characterized by a long period of very slight pressure drop during continuous cooling, followed by a precipi­tous drop in P beyond a certain point. Because cooling and secondary hydrate forma­tion are quite common in the course of hydrate production [ 131, 132] , such P-T behavior can have a significant effect on production.

5.2.6 Fast P-T-X Parametric Relationships in Composite Hydrates

Even small amounts of a second hydrate-forming gas in addition to CH4 (a common occurrence in geologic GH deposits) can drastically alter the properties and behav­ior of hydrates. While statistical thermodynamics approaches [ 178 [ allow good descriptions of the composite system, these are cumbersome, slow, and unsuitable for use in numerical simulators. Thus, there is a significant need for fast parametric relationships describing the composite hydrate behavior over the P-T-X spectrum.