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Abstract
A new general correlation for estimating coal adsorption isotherms using log data has been developed. The "Langmuir Rank Equation," developed with the same dataset used for deriving Kim's equation, provides an improved fit with the measurements. The Langmuir constants are derived from a coal rank indicator and adjusted for temperature. At elevated pressures, such as those in the San Juan basin, this approach may offer a substantial improvement in gas content estimates, as indicated in tests with a limited number of isotherms.
The new correlation, adjusted for the effect of moisture, could be used with well logs alone in estimating gas content (saturated conditions) in coalbeds where no core samples are available. Since the equation provides the entire isotherm, it could also be used in the estimation of reserves. It could also be used when quality checking isotherms. It is important to note that both log-based models (Kim and Langmuir Rank) were developed based on isotherm data, and as such, assume the coal is saturated and cannot be used to identify undersaturated coals.
Introduction
Isotherm data must accurately represent the coal reservoir. This is essential whether making simplified reserve calculations or more complex model-based projections. Properly done, this requires obtaining a number of isotherm samples statistically sufficient to characterize the coal seams of interest both vertically and laterally. Depending on isotherm variability, this number can be considerable. Two factors influencing isotherm character arc coal rank and ash content. Coal rank can vary vertically, from seam to seam at a given location. Laterally, maps of this parameter for a single coal seam show significant variation from township to township, and occasionally within a township. Ash content often shows significant vertical foot-to-foot variations within a well, particularly in the San Juan basin.
Coal quality (as indicated by rank and ash content) is known to vary both vertically and laterally and it is expected that isotherm character will also exhibit variations. Therefore, gas resource evaluations would be improved if measured isotherm data were available for every foot of every coal seam in each well in the area to be evaluated. In practice, such extensive coring and testing are not done for operational and economic reasons; instead, the collection of measured isotherms is comparatively sparse. On the other hand, well log coverage is much more plentiful, and logs, particularly the bulk density log, have shown excellent correlations with the components of proximate analysis: ash content, fixed carbon, and volatile matter. A general correlation between proximate analysis and measured isotherms, if such could be found, could be used together with well logs to estimate isotherms where measurements are lacking. Such a correlation would also be useful in quality checking measured isotherms since there are currently no accepted industry standards for making these measurements.
The two most common methods of determining gas content from logs are 1) relating ash content to gas content, and 2) the Kim equation. The ability to relate ash content to core-determined gas content has been documented by other authors. Although relationships based on ash content can provide reasonable estimates of gas contents, they are site specific and will vary from area to area. The Kim Equation is based on relationships among saturated gas content, coal rank (carbon/volatile ratio), pressure, and temperature.
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