Editing Coalbed methane extraction (section)

== Measuring the gas content of coal ==
Coal bed gas content measurements are commonly used in mine safety as well as coal bed methane resource assessment and recovery applications. Gas content determination techniques generally fall into two categories: (1) direct methods which measure the volume of methane released from a coal sample sealed into a [[desorption]] canister and (2) indirect methods based on empirical correlations, or laboratory-derived [[sorption isotherm]] methane storage capacity data. Laboratory sorption isotherms provide information about the storage capacity of a coal sample if these are measured under geological realistic pressure and temperature conditions. Thus, the maximum gas content that can be expected for methane recovery can be assessed from such laboratory isotherm measurements.<ref>{{Cite web |url=http://www.rubotherm.com/investigation-of-enhanced-coal-bed-methane-ecbm-processes.html |title=''Investigation of Enhanced Coal Bed Methane (ECBM) Processes - Gravimetric Adsorption Measurements under Realistic Conditions'' |access-date=2013-07-24 |archive-url=https://web.archive.org/web/20140423050310/http://www.rubotherm.com/investigation-of-enhanced-coal-bed-methane-ecbm-processes.html |archive-date=2014-04-23 |url-status=dead }}</ref>

The total gas content by the indirect methods is based on the [[empirical formula]] given by Meisner and Kim. The quantity of gas is determined by the Meisner and Kim formula using the moisture content, volatile content, the volume of methane adsorbed on wet coal, fixed carbon, thickness of coal, and temperature.
<br />Meisner (1984) observed that the amount of methane gas (VCH4) is related to volatile matter (daf).

<br />V<sub>CH4</sub> = −325.6 × log (V.M/37.8)

<br />The estimation of the ''[[in situ]]'' gas content of the coal will be evaluated by using Kim's (Kim 1977) equation 

<br />V = (100 −M − A) /100 × [ Vw /Vd ]  [K(P)<sup>N</sup> - (b × T)]
<br />Where,
<br />V = Volume of methane gas adsorbed (cc/g) 
<br />M = Moisture content (%) 
<br />A = Ash content (%).
<br />Vw/Vd =   1/(0.25 ×M + 1)
<br />Vw = Volume of gas adsorbed on wet coal (cc/g) 
<br />Vd = Volume of gas adsorbed on dry coal (cc/g)
<br />The values of K and N depend on the rank of the coal and can be expressed in terms of the ratio of fixed carbon (FC) to Volatile matter(VM)
<br />K = 0.8 (F.C /V.M) + 5.6
Where
<br />F.C = Fixed carbon (%)
<br />VM = Volatile matter (%)
<br />N = Composition of coal (for most bituminous coals, N = (0.39 - 0.013 × K)
<br />b =Adsorption constant due to temperature change (cc/g/◦C).
<br />T = Geothermal Gradient × (h/ 100) + To
<br />T = Temperature at a given depth
<br />To = Ground temperature
<br />h = Depth (m)

<br />'''Estimation of methane content in coal seams by Karol curve'''

<br />In the absence of measured methane content of coal beds, and production data from coal bed methane wells, gas content can be estimated using the Eddy curve. Eddy and others constructed a series of curves estimating the maximum producible methane content of coal beds as a function of depth and [[coal rank|rank]].
<br />The estimation of the methane content of a coal bed is determined from the Eddy curve by locating the average depth of each coal seam on the depth axis. A normal line is extended upward from the depth axis (feet) to intersect the specific coal rank curves. A line from the point on the curve is extended normally to the lost and desorbed gas axis (cm<sup>3</sup>/gm). The intersection of the line and the axis is the estimated methane content of the coal seam.