Print this pagePSUnder-Saturation in Coals: How Does It Happen and Why Is It Important*
*Oral Presentation at Rocky Mountain Section AAPG Annual Meeting, Jackson, Wyoming, September 24-26, 2005. Appreciation is expressed to Lyn George, Technical Program Chair, and Don French, for encouraging the author to submit this presentation.
Click to view presentation in PDF format (2.6 mb).
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 under-saturated 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.
Under-saturated 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 under-saturated. Examples of some of these prospects, with associated gas and water production data, are presented.
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Why is Under-Saturation Important?
Why are Some Coals Under-Saturated?
BUT
AND
Why Are Some Coal Reservoirs Fully-Saturated? Coals can become fully saturated when more gas is added due to secondary biogenic gas generation and migration of biogenic and thermogenic gases (Figure 3).
Eastern margin of Washakie Basin (Figure 4), where hydrodynamics is partially responsible for the high gas content, with downdip flow from meteoric recharge and updip flow from hydrocarbon recharge. South flank of Uinta Basin (Figure 5), where thermogenic gas has moved updip. Northwest flank of San Rafael Swell (Figure 6), where biogenic gas has migrated updip.
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