In-situ Gas Content of Coal
DIRECT METHOD FOR ESTIMATION OF IN-SITU GAS CONTENT OF COAL
Method
In this method the gas content of coal is determined by collecting and measuring the volume of gas released from a sample of coal core taken out from the coal-seam by drilling.
Immediately after drilling, the recovered coal-core sample is placed in a specially designed canister provided with sealing arrangement and is sealed. The atmospheric pressure (at the surface) being lower than the underground reservoir pressure, the coal-core sample starts releasing gas through desorption inside the closed canister. This can be seen from the increase in pressure in the pressure gauge provided at the top of the canister. At required intervals, the gas is taken out from the canister through outlets and its volume is measured by gas measuring devices. This process is allowed to continue till the release of gas comes to an end.
The gas content of coal sample is determined from the measured volume of total gas released from the sample and the weight of the coal-core sample.
After drilling, the actual release (desorption) of the total volume of gas is estimated in stages, which comprises the following components:
A.Measurement of Desorbed Gas (Q1)
B.Determination of Residual Gas (Q2)
C.Estimation of Lost Gas (Q3)
D.Total Gas Volume (Q)
A. Measurement of Desorbed Gas ( Q1 )
The volume of desorbed gas released from the coal-core sample kept in a desorption canister is measured about every five minutes (depending on number of active canisters) and the data for short-term desorption period is collected; which are used for estimating the “Lost Gas Volume” desorbed from the same sample during its retrieval and handling at the surface.
After the first hour of gas volume measurement, the frequency of measurement is decreased depending on gas desorption rate from the canisters.
Such measurement are continued till the volume of gas released falls below the resolution at which it can be measured accurately (say 0.1 to 1 cc).
During measurement the following data are to be noted carefully.
(ii) Head Space Volume (CC)
(iii) Canister Temperature (C°)
(iv) Ambient pressure (Psig/Kg/Cm2)
(v) Periods of excessive pressure (>10 psig) in the desorption canister(s)
(vi) Desorbed Gas Sample collection intervals (Minute)
(vii) Desorbed Gas composition as a function of time
(viii) Length of desorption test (Minute)
(ix) Sorption Time (Minute)
(x) Coal-core sample weight. ( g )
After collection of first few desorption data (every five minutes interval) during the first hours, the frequency of data collection is reduced to once or twice every hours for the first 24 hours and then to once or twice every day till the rate of released gas volume comes to 0.05 ml/g/day for several consecutive days. Thereafter, when the coal sample ceases to release any more gas effectively, the sample is then sent for determination of Residual gas.
The changes in ambient temperature and pressure are recorded at the time of taking each reading for the correction of gas volumes to Standard Temperature and Pressure (STP).
The cumulative value obtained from measurement of desorbed gas volumes give the “Desorbed Gas” (Q1) Value for the coal-core sample.
B. Determination of Residual Gas (Q2)
Method
After determination of Q1, the core-coal sample is weighed and a known weight of this sample is transferred into an air-tight steel vessel of specific dimension. The vessel contains several brass rods inside it which grind the core sample placed inside the vessel on rotating the vessel by the help of a set of rubber rollers driven by a motor. The vessel is rotated at a particular RPM rate for a specific period of 40 Minutes.
[ The present coal crushing Mill used for pulverization of coal sample is similar in design as was used by Diamond and Levine (1970) and it is found that it crushes the coal sample down to a fine powder of below 200 mesh size in about 40 Minutes rotation of the vessel.]
The vessel is also equipped with separate gas inlet and outlets and prior to start grinding, the vessel is purged with an inert gas (N2 or Argon) to avoid adsorption of atmospheric oxygen (initially present inside the vessel) by the crushed coal.
The volume of residual gas released after crushing the coal is measured by water displacement method in a calibrated burette.
From the measured gas volume obtained from a portion of the coal-core the total Residual gas (Q2) is calculated for the total weight of the original core-coal sample used in the desorption test earlier.
C. Estimation of Lost Gas (Q3)
The moment the coal-core sample is recovered from the well bore, desorption of gas from the sample gets started and continues till the actual process of gas content measurement can be started inside the canister. The volume of this unrecoverable portion of gas is termed as “LOST GAS” (Q3)
Method
The Lost gas is calculated by a graphical method which is based on the volume of gas emission during first few hours data (obtained from desorption test) and square root of desorption time data.
The drilling medium used has a great influence on the amount of Lost gas and the following data are required to estimate it from the equations (a) and (b) given below and to construct the Lost gas graph.
Experimental data required
- Drilling Medium (Air/Mist/Water/Mud)
- Time coal-bed encountered (A)
- Time core started out of borehole (B)
- Time core reached surface (C)
- Time core sealed in canister (D)
- Lost gas time calculation is done as below by using equations;
D .Determination of Total Gas Volume (Q)
The total gas volume obtained by addition of Q1,Q2 and Q3 gives the in situ gas content of the coal – core sample.
Where W is the weight of coal-core sample taken in canister.
