Variable Gas Solubility Effect of the Gas Hydrate System and its Influence on Optimization of Gas Recovery
Authors: Max Michael , Holman Sarah 
Published: October 2008

Methane solubility in water is significantly lower in the presence of hydrate than in water where no hydrate is present. Experiments with hydrate present show a decrease in gas solubility with both decreasing temperature and increasing pressure when crystalline hydrate is introduced. The water around hydrate becomes supersaturated in methane and subsequently promotes additional hydrate formation without exsolution of free gas. Thus, heating of the water and gas produced from hydrate is recommended to insure the absence of unwanted hydrate formation and optimize flow assurance.

Introduction

Recovery of natural gas from gas hydrate will almost certainly necessitate dissociation of the hydrate in-situ, probably by a combination of thermal stimulation (introduction of warm fluids) and depressurization (reducing the pressure of gas in contact with gas hydrate (Max, 2000). During dissociation, a three-phase system of gas, water, and gas hydrate is in physical continuity. Complete dissociation of gas hydrate produces essentially pure water and hydrate-forming gas that is saturated with water vapor.

Although minor amounts of dissolved solids may be present following dissociation when the fresh water mixes with groundwater, it will form a groundwater mass that is substantially lower in dissolved material. The gas and water will tend to separate, but both gas and water masses that are produced will be saturated with the other so that conditions for reformation of gas hydrate may be present in both the water and gas masses.

One of the primary obstacles that must be overcome in recovery of natural gas from naturally occurring hydrate in permafrost, but especially in continental slope marine sediments, is prevention of the recrystallization of unwanted gas hydrate and maintenance of flow assurance while recovering the gas from the meta-reservoir formed by the dissociated hydrate. In order to prevent unwanted recrystallization of gas hydrate, as well as to facilitate the widespread dissociation of hydrate in an economic gas recovery scenario, it is important to appreciate the affects of solubility of the hydrate-forming gas in the pore water. It is becoming understood that solubility of hydrate forming gas in pore water can be affected by the presence or absence of hydrate.

When modeling natural gas recovery from gas hydrate, the effect of variable gas solubility in water is a factor to consider, along with temperature, pressure, and other variables such as salinity. A decrease in solubility accelerates the growth of hydrate by providing abundant hydrate-forming gas molecules. Managing the manner in which the variation of solubility of hydrate-forming gas occurs as part of a process of natural gas extraction from gas hydrate, may be one of the keys to successful gas recovery models.

Controls of hydrate development

Naturally occurring hydrate (which is mainly methane), is a relatively simple system consisting only of water molecules and the hydrate-forming gas. The most critical factor for hydrate growth is the existence of pressure-temperature conditions suitable for spontaneous nucleation and hydrate growth when an appropriate hydrate-forming gas is introduced to water.