The Impact of Undersaturation on Coal Gas Economics

Abstract

Many coal deposits throughout the world are undersaturated to some degree. With actual gas contents less than expected from reservoir pressure and the sorption isotherm, these coals require dewatering before gas desorbs. An operator must bear water production and disposal costs, frequently for an extended period of time, prior to realizing any revenue from gas sales. Gas and water profiles for wells completed in Powder River and Greater Green River coals were simulated assuming saturated coals and selected degrees of undersaturation. Base case economics were constructed by combining the simulated gas and water profiles from saturated coals with typical capital costs and well operating expenses for each basin. Gas and water profiles from simulation of ever deeper undersaturation were used to drive additional economic analyses. From this suite of economic analyses, the impact of undersaturation on economic parameters such as net present value, time to payout, and breakeven gas price was assessed. Introduction Gas content of a coal deposit is the result of gas generation and gas escape over geologic time. Coal gas is generated by thermogenic and biogenic processes and is often a blend of both types. Coal gas is released from the coal matrix whenever reservoir pressure is insufficient to retain it and often migrates out of the seam. A coal holding the maximum possible amount of gas for current reservoir pressure and temperature is said to be "saturated" whereas a coal holding less than this theoretical maximum is characterized as "undersaturated". Undersaturated coals are always water filled and require depressuring by water production to initiate gas release from the matrix into the cleats. The pressure at which gas first appears is defined as the "desorption pressure". Slightly undersaturated coals behave similar to saturated coals with only a short delay prior to first gas production followed by a steady, strong, rising gas production rate. Deeply undersaturated coals behave quite differently and require extensive dewatering prior to initiation of gas production. With a desorption pressure significantly less than initial reservoir pressure, peak gas rate of a deeply undersaturated coal can be significantly less than that of an equal but saturated coal. Utilizing actual gas contents measured from desorption tests, the Langmuir sorption isotherm, and initial reservoir pressure, the degree of undersaturation can be described two ways. An undersaturated coal can first be characterized by comparing saturated and measured gas contents. Assuming a single component gas, saturated gas content of a coal seam can be calculated with Langmuir's isotherm: Equation 1 The degree of saturation can be expressed as the ratio of measured to saturated gas contents. Assuming a San Juan basin coal with an initial pressure of 1800 psia and gas storage described by the Northeast Blanco Unit (NEBU) isotherm published by Mavor, et al1 (VLis = 756.4 scf/ton, pL = 309 psia), the saturated gas content is 645.6 scf/ton. If the gas content measured from desorption tests is 581.0 scf/ton, the ratio of the two gas contents gives Equation 2 That is, the coal is 90% saturated with gas. Alternatively, the coal is 10% undersaturated in gas.