Gas Content: direct method
The direct method of estimating the content of gas in a body of coal consists of drilling a core
sample from an area of the coal seam and enclosing this sample as quickly as possible in an airtight
bomb. It is customary to use the full length of the core sample as variations can occur depending on
the quality of the coal seam. The coal bomb is then connected to a water displacement apparatus and
the quantity of gas released with time is measured and recorded as Q2. This method is
continued until the gas emitted from the sample is less than 0.05 cm3/gram per day for five
consecutive days (McCulloch et al, 1975) or the gas emitted is less than 10 cm3/day for seven
consecutive days (Diamond & Levine, 1981).
After the gas emission has reduced to the required rates, the mass of the sample is measured and the sample is
placed once again in a sealed bomb and crushed using steel rods or balls to release any remaining gas. This gas
is measured once again using the water displacement method and the quantity is measured and recorded as Q3.
To account for the quantity of gas lost between drawing the sample from the coal seam and placing the sample in the
sealed bomb, the results of gas emitted vs. the square root of time are plotted on a graph. By making the assumption
that gas release follows the square root law, the release at time zero can be found by extrapolating the other results.
Q1 can then be found by noting the time taken to seal the sample in the bomb and recording the gas content.
The lost time should be taken as from the start of drilling to the time of sealing in a horizontal drill hole, or for
vertical drill holes lost time is the time taken for the pressure in the core to equal pressure in the hydraulic head.
The total gas content of the sample can be calculated as
Q = Q1 + Q2 + Q3
Direct Estimation
Indirect Estimation
Probability Estimation
