Natural Gas Hydrates

Gas clathrates (commonly called hydrates) are crystalline compounds thatoccur when water forms a cage-like structure around smaller guest molecules. Gas hydrates of interest to the natural gas hydrocarbon industry are composedof water and eight molecules: methane, ethane, propane, isohutane, normalbutane, nitrogen, carbon dioxide, and hydrogen sulfide. Hydrate formation ispossible in any place where water exists with such molecules-in natural orartificial environments and at temperatures above and below 32 degrees F whenthe pressure is elevated. Hydrates are considered a nuisance because they blocktransmission lines, plug blowout preventers, jeopardize the foundations ofdeepwater platforms and pipelines, cause tubing and casing collapse, and foulprocess heat exchangers, valves, and process heat exchangers, valves, andexpanders. Common examples of preventive measures are the regulation ofpipeline water content, unusual drilling-mud compositions, and large quantitiesof methanol injection into pipelines. We encounter conditions that encouragehydrate formation as we explore more unusual environments for gas and oil, including deepwater frontiers and permafrost regions. permafrost regions. Hydrates act to concentrate hydrocarbons; 1 ft 3 of hydrates may contain asmuch as 180 scf of gas. Large natural reserves of hydrocarbons exist inhydrated form, both in deep oceans and in the permafrost. Evaluation of thesereserves is highly uncertain, yet even conservative estimates indicate thatthere is perhaps twice as much energy in hydrated form as in all otherhydrocarbon sources combined. While there is one commercial example of gasrecovery from hydrates, the problems of in-situ hydrate dissemination indeepwater/permatrost environments will prevent their cost-effective recoveryuntil the next century.

Hydrates normally form in one of two small, repeating crystal structures, shown in Fig. 1. The two hydrate structures are formed from a basic" building block" water cavity that has 12 faces with 5 sides per face(512). Linking the vertices of the 512 cavities results in Structure 1, withinterstices of large cavities composed of 12 pentagons and 2 hexagons (512 62). Linking the faces of the 512 cavities results in Structure 2, with intersticesof large cavities composed of 12 pentagons and 4 hexagons (512 64). Details ofstructure are given in a recent monograph. Structure 1, a body-centered cubicstructure, forms with natural gases containing molecules smaller than propane;consequently Structure 1 hydrates are found in situ in deep oceans withbiogenic gases containing mostly methane, carbon dioxide, and hydrogen sulfide. Structure 2, a diamond lattice within a cubic framework forms when naturalgases or oils contain molecules larger than ethane but smaller than pentane;Structure 2 represents hydrates that commonly occur in production andprocessing conditions, as well as in the cases of gas seeps in shallow oceanenvironments. The newest hydrate, Structure H, is neglected in this overview;it is yet to be found outside the laboratory. Inside each structure cavityresides a maximum of one of the eight guest molecules. The cavity occupied is afunction of the size ratio of the guest molecule within the host cavity. To afirst approximation, the concept of "a ball fitting within a ball" isthe key to understanding many hydrate properties. Much more certainty existswith respect of properties. Much more certainty exists with respect of themolecular structure of hydrates than to the kinetic mechanism of hydrateformation; hydrate kinetics are currently at the forefront of research. On amacroscopic level, hydrate formation and dissociation may be considered using aphase diagram, such as Fig. 2. In this figure, pressure is plotted vs. temperature, with gas composition as a parameter, for methane/propane mixtures. Consider a gas of any composition given on a line in Fig. 2. At conditions tothe right of the line, a gas of that composition will exist in equi-ibrium withliquid water. As the temperature is reduced (or as the pressure is increased), hydrates will form from the gas and liquid at the line, so that three phases(hydrates/gas/liquid water) will be in equilibrium. With further reduction oftemperature (or increase in pressure), the fluid phase that is not in excess(normally water) phase that is not in excess (normally water) will beexhausted, so that to the left of the line the hydrate will exist with theexcess phase (normally gas). phase (normally gas). Note that all the conditionsgiven in Fig. 2 are for temperatures above 32 degrees F, and that pressuresalong the lines vary exponentially with temperature. This figure alsoillustrates the dramatic effect of gas composition on hydrate stability; as anyamount of propane is added to methane, the structure changes (Structure l Structure 2) to a hydrate with much wider stability conditions. Note the markeddecrease in pressure (or increase in temperature) needed to form hydrates, whenas little as l% propane is in the gas. Hydrates can result from eithersaturated gas or liquid hydrocarbon, without a free-water phase; thesetwo-phase conditions are relatively rare and, thus, not addressed in thisoverview. Three common methods are used to predict the three-phase conditionslike those of Fig. 2. They are, in order of increasing accuracy, thegas-gravity chart,2 the Katz Kj charts,3 and a statistical thermodynamicsmethod.4 The first two calculation methods may be done by hand; a computer isrequired for the third method, which may be used to predict most threephaseconditions with acceptable accuracy.

Inhibition/Dissociation. The four common means of inhibiting/dissociatinghydrates are (1) removing one of the components, either the hydrocarbon orwater; (2) heating the system beyond the hydrate formation temperature at apressure; (3) decreasing the system pressure below hydrate stability at atemperature; and (4) injecting an inhibitor, such as methanol or glycol, todecrease hydrate stability conditions, so that higher pressures and lowertemperatures will be pressures and lower temperatures will be required forhydrate stability. These techniques are called thermodynamic inhibition becausethey remove the system from thermodynamic stability by changes in composition, temperature, or pressure. As long as the system is kept pressure. As long asthe system is kept outside thermodynamic stability conditions,

JPT

P. 1414