GAS HYDRATES DEFINITION, PRODUCTION AND FORMATION CONDITIONS. Gas hydrates are crystralline compounds, formed by the chemical combination of natural gas and water under pressure at temperatures considerably above the freezing point of water. In the presence of free water, hydrate formation is often confused with condensation, and the distinction between the two must be clearly understood. Condensation of water form a natural gas under pressure occurs when the temperature is at or below the dew point at that pressure. Free water obtained under such conditions is essential to the formation of hydrates that will occur at or below the hydrate temperature at the same pressure. Hence, the hydrate temperature would be below, and perhaps the same as, but never above the dew point temperature.
In solving gas production problems, it becomes necessary to define, and thereby avoid, conditions that promote the formation of hydrates. Hydrates may chokew the flow string, surface lines, and well testing equipment. Hydrate formation in the flow string would result in a lower value for measured wellhead pressures. In a flow rate measuring device, hydrate formation would result in lower flow rates. Excessive hydrate formation may also completely block flow lines and surface equipment.
Conditions promoting hydrate formation are:
- Gas at or below its water dew point with » free» water present.
- Low temperature.
- high pressure.
EQUILIBRIUM CONSTANT GAS-CONDENSATE SYSTEMS:
As the exploration for natural gas is extended to deeper horizons, more reservoirs
containing gas condensates are discovered. The gas may be in the gaseous phase at liquid reservoir conditions but may condense to form some liquid at some point in the path to the separator. Engineering design of these production systems requires some understanding of phase behaviour.
PHASE BEHAVIOUR:
Phase behavior is simple for single component systems but becomes more complicated as more components are added to the system. A discussion of the simplest system will lead to an understanding of the more complex systems.
Single Component Fluid: The phase behaviour of a fluid can be described by determining its response to pressure and temperature changes. In a liquid the molecules are very close together, but in a gas the molecules are widely separated.
Certain forces exist that tend to either confine or disperse the molecules. Confining forces are primarily pressure and molecular attraction. Dispersing forces are kinetic energy and molecular repulsion. The relative magnitudes of the confining and dispersing forces dictate wheter the fluid is a liquid or a gas. An increase in temperature increases the kinetiic energy of the molecules and thus the dispersing forces, while an increase in pressure goes up the confining forces.
Multicomponent Fluids: When more than one fluid is present, the difference in molecule size and energy has an influence on the phase change.There is no sharp transition from liquid to vapor or from vapor to liquid, but the molecules are able
to escape from the liquid or gas at different pressures and temperatures because of molecular attraction. The locus of all points where the first bubble of gas appears in a liquid as pressure and temperature conditions are changed is called the bubble point line. The locus of all points where the first droplet of liquid appears as the conditions for a gas are changed is the dew point line. The highest pressure at which a gas exist is called the cricondenbar while the highest temperature at which liquid can exist is the cricondentherm.
TYPES OF GAS RESERVOIRS
Reservoir that yield natural gas can be classified into essential four categories, these are:
DRY GAS RESERVOIR DEFINITION:the fluid exists as a gas both in the reservoir and the reservoir and the piping system. The only liquid associated with the gas from a dry gas reservoir is water. A phase diagram is illustrated in the next picture.
WET GAS RESERVOIR DEFINITION: the fluid initially exists as a gas in the reservoir and remains in the gaseous phase as pressure declines
at reservoir temperature. However, in being produced to the surface, the temperature also drops, causing condensation in the pipig system and separator.
RETROGRADE CONDENSATE GAS RESERVOIR DEFINITION: the fluid exists as a gas at initial reservoir conditions, As reservoir pressure declines at reservoir temperature, the dew point line is crossed and liquid forms in the reservoir, Liquid also forms in the piping system and separator.
ASSOCIATED GAS RESERVOIR: many oil reservoirs exist at the bubble point pressure of the fluid system at initial conditions. Free gas can be produced form the gas cap of such a system. Gas which is initially dissolved in the oil can also be produced as free gas at the surface. The phase diagram of such a system will depend on the properties of the oil associated with the gas.
The fact that hydrocarbon liquids are frequently in contact with natural gas makes it imperative that methods be available to calculate the volumes or masses of each phase and also the composition of each phase existing at various conditions of pressure and temperature.
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