Undersaturation in Coals: How Does it Happen and Why is it Important
By combining gas content data with an adsorption isotherm, it is possible to determine the gas saturation condition of a coal sample. Fully saturated coals are ideal from a CBM perspective because they will produce gas immediately as soon as water is produced from the reservoir. As the gas saturation level decreases, more water needs to be produced in order to reduce the reservoir pressure to the critical desorption pressure when gas will start to desorb from the coal.
In highly undersaturated coal reservoirs, many months to years may be required to sufficiently dewater the coals to allow the desorption process to begin. This long dewatering time can ultimately result in an uneconomic prospect due to long period of little or no cash flow accompanied by ongoing operating expenses.
Determination of the gas saturation condition is relatively easy and inexpensive. By collection this data early in the life of a CBM project, much time, effort and money can potentially be saved.
Understurated coal reservoirs may eventually produce large volumes of gas once the reservoir pressure has been reduced to the critical desorption pressure. However, the economics of these reservoirs may be marginal at best. Unfortunately, data collected from many Rocky Mountain CBM prospects indicate that the coals are significantly undersaturated. Examples of some of these prospects, with associated gas and water production data, will be presented.