CBM gases in coal has different properties with gases in the conventional reservoir. Because of the contradiction in mechanism of storing gases in the coal, the Formation Evaluation Mehod in the conventional oil and gas industry is unapplicable for CBM reservoir to calculate the gas volume in it. Gas content determination method had been developed for calculating the gas volume which is adsorbed on mictopore of the gas reservoir.
Tool for calculating gas volume usinh USBM direct determination method
Gas content calculation method which common to be used is Total amount gas content divided by 3 part, i.e lost gas, desorbed gas, and residual gas. Every part is calculated using different procedure, then they are concucted to know the total gas volume. Generally, each gas content determination method uses the same terminology for indivual component of the gas content, however each individual component in every different method is uncomparable, because the differences of the pocedure condition which is done for every method.
3.1. Lost Gas
Lost gas is total or proportion of the total gas volume come out from the sample when the sample taken and accepted before placed in desorption box airthight. Lost gas volume can not be calculated directly, so that it should be estimated from the gas volume data that has been measured before (after?). Because the lost gas volume is an estimated quantity, so generally it dianggap as the fewest component to be trusted total gas content.
Lost gas content when the coal sample has not been entered on to desorption box generally depends on the duration of the accepted sample, physical properties of the sample, fluid drilling type, and the water saturatiom / total free gas content.
Estimation Lost Gas Volume Graphic with direct estimation USBM method
3.2. Desorption Gas
When the coal sample entered on to desorption box, desorption gas will be accumulated and can be calculated directly. This direct calculation done by water movement that is described by Bertard (1970) and completed by Kissell et al., (1973). There is more accurate direct method developed by USBM for calculating few gas volume in oil shale sample (Schatzel et al., 1987), this method based on contrast pressure calculation in the box when the gas released, then using Ideal gas rule for calculating desortption gas volume..
3.3. Residual Gas
Volume of gas adsorbed from coal sample decrease seiring bertambahnya waktu. Adsorption calculation untuk teknik penyerapan tambahan dihentikan pada suatu titik ketika rata-rata penyerapan yang rendah telah dicapai. Rata-rata ini dapat dicapai dengan waktu berhari-hari untuk sampel batubara yang rapuh dan waktu yang berbulan-bulan jika sampel batubara tersebut cukup keras. Generally, when the adsorption average has been achieved the point di mana akan dihentikan, so some gas volume persistent in the coal. Bertard et al., (!970) and Levine (1992) supposed that residual gas may not depends on the difusion, however residual gas is in equilibrium condition in the pressure around 1 atm in the adsorption box.
Residual gas volume can be calculated by break down the coal sample in the airtight container and calculate the gas volume which come out sama seperti when calculate the gas adsorbed by the coal (Diamond et al, 1986). Diamont et all, 1986 had done gas content analysis from 1500 coal sample of 250 coal bed. The analysis result proved than in the low grade coal, residual gas is 40 up to 50% from the total gas content in the coal. However, in the high grade coal whis is brittle can has residual gas less than 10% of total gas content volume.