Gas Types

Gas is stored in coals by a mixture of means which is related to the method by which the gas reached the coal. Gas in lower rank coals is generally biogenic, meaning that it is derived organically by bacteria during the breakdown process. The gas in higher rank coals is frequently, but not exclusively, of thermogenic origin, meaning that it is derived from the process of being heated in the ground. Coals are formed in a water saturated environment and are usually wet, though in some cases a gas cap may exist. Coals will have generated a lot of gas as part of its formation process and only a fraction of that gas will remain in the coal. Coals are a source rock for many petroleum reservoirs. The main gas generated in coals is methane although trace amounts of other gases have been recognised. In addition, other gases may have been introduced, usually from igneous sources. Other gases are, most commonly, carbon dioxide then nitrogen but may include some noble gases, notably argon.

Gas Storage

Gas is stored in coal within the micropores and in multilayer adsorption with possibly some  chemisorption into the coal structure itself. Each gas type and the water in the coal compete for storage sites. This is most notable where hydrothermal sweeps of carbon dioxide rich fluids have affected the coal.

Where methane has not been totally replaced and there is a mixture of both carbon dioxide and methane in the coal, common theory suggests that methane would evolve first as it is not as well bonded to the coal as carbon dioxide. In fact this is frequently not the case and carbon dioxide is preferentially released. The explanation for this is that the carbon dioxide replaces the methane on less favourable sites where it is not bonded so tightly to the coal.

Water should also be considered as a competitor with gases for storage sites. This becomes clear when sorption isotherms are measured in both dry and wet coals - wet coals adsorb less gas. The sorption isotherm is by definition the relationship between the volume of gas stored in coal and the pressure of that gas at a fixed temperature. This is usually measured at the seam temperature as the value varies with temperature. Sorption isotherms are measured by forcing gas back into powdered coal and then releasing the gas in pressure steps, while measuring the coal weight or the volume of gas released.

This is, however, not a simple process as the result depends on a choice of the moisture content of the coal for re-adsorption. In coal seams, the water is present initially, then methane is generated. It is therefore reasonable to argue that methane displaces water from potential surface sites. In the case of mixed gas isotherms it is not sensible to simply expose the coal to a mixture of gases and to let it adsorb, the complex process of adsorbing one gas and then another must to be adopted.

Sorption isotherms are conventionally described by the Langmuir equation, which suffices in most cases but has no sound theoretical basis as it is written for monolayer adsorption.

In the field, Sigra samples and measures the gas release from core or cuttings to obtain gas volume.
The gas is sampled and analysed for gas type. Varying gas types during production display the gas generation and sorption history of the seam.

In some cases, in a mixed CO2/CH4 seam gas environment, the CO2 is initially released at a higher rate than the overall gas composition. This is indicative of the manner by which the CO2 was introduced to the seam.