COALBED METHANE
 
With the increase of the world energy demand, coalbed methane becomes more attractive as a new energy alternative. Indonesia is known to have contained enormous coal resources. This is why LEMIGAS established a new laboratory dedicated in coal gas analysis

Methane in coal is an inherent by-product gas of the natural process of coalification during coal formation. It is desorbed and produced from deep coal seams and generated either from a biological process as result of microbial activity or from thermal process as result of increasing heat with depth of the coal. Generally, the higher the coal rank the more methane produced in the coal

Currently, LEMIGAS provides several analyses in coal gas. They are gas content, absorption-isotherm, gas composition, proximate, ultimate and analyses.
 
         
  • GAS CONTENT OF COAL  
  Gas content of coal is the most important parameter needed for the evaluation of the extent of the gas reservoir. There are two methods for determining the gas content in the laboratory.

The first method is based on the slow, natural desorption of gas from coal. In this method, the coal sample, which is a core, or lump samples from coal face, is sealed in canister and the gas evolved from coal over times collected in an inverted measuring cylinder by water displacement in the cylinder.

The second method relies on finely crushing the coal to speed up the gas desorption rate. Using the method, the seam gas content can be determined in a space of 2 to 3 hours. The significant advantages of this fast desorption method include:
  
 
» Very fast evaluation of the total gas content of the coal at a particular test site in the rig
» Significant reduction in the chances of the gas leakage, loss of any CO2 and oxidation of the coal during the measurement procedure. The latter gives rise to abnormally high levels of nitrogen in the evolved seam gas.
» Quick turnarounds in freeing up the available field and laboratory equipment that can then be reused for other tests.
» Reduction in laboratory cost because of shorter time for monitoring, less frequent gas volume and composition measurements and better resources utilization
 
  LEMIGAS uses two different size stainless steel canister, 60 cm and 20 cm long with 65 mm in diameter, to collect coal samples. The canisters can filling up 1.5 kg and 0.5 kg of coals, respectively. They are tested for leaks before being dispatched to the field. The pre-dispatch check consists of pressuring the canisters with nitrogen up to 500 kPa and checking for bubbles using an appropriate liquid leak test (snoop). The leak test is also undertaken once the canisters containing the coal are returned to the laboratory.

Field Measurement of Gas Content ang Gas Composition:
If the transportation of coal to the gas laboratory might result in gas leaking or the urgency to get a quick result of gas content, LEMIGAS uses a CBM MOBILE LABORATORY which is a truck vehicle equipped by the coal gas laboratory instruments. The mobile laboratory is capable to measure the gas content and the gas composition. By this truck, the laboratory can be located at drilling sites, therefore, eliminating the time required for transporting the coal canister to the laboratory and reducing the risk of gas leakage during transportation.
  
     
  • MEASUREMENT OF GAS ADSORPTION ISOTHERM  
  Coal is porous and adsorbent rock and has significant storage capacity for methane and carbon dioxide gases. The gas adsorption storage capacity of coal is measured in terms of adsorption isotherm. The adsorption isotherm expresses the relationship between the mass/volume of gas adsorbed and free gas pressure. Adsorption isotherm as gas diffusivity and permeability is a one of the fundamental reservoir properties of coal. The adsorption isotherm depends on coal rank, moisture content and the isothermal temperature at which measurement is undertaken.
We refer to CSIRO in applying volumetric method: In this method, the equibilirium mass of adsorbed gas is indirectly measured by measuring the variation of gas pressure in a cell before and after saturation of coal by gas.
The volumetric isotherm determination system uses the pressure-volume method to determine the adsorption-isotherm. The system can operate to a gas pressure of up to 16 Mpa. The apparatus is fully automated, with computer controlled gas injection, pressure measurement and data acquisition.		  
  The volumetric adsorption isotherm system has three parts:  
 
» A computer unit where the software for the pressure valves operations and data logging is installed. The application sofeware (LabView) consists of a user interface which allaws the control the operation of the equipment and record of the pressure data.
» An electrical console which contains the relays for computer control of the system, using solenoids to switch compressed or bottled air, which in turn switch the valves on the gas system. It also has switches for manual operation of the valve for some operations, testing and maintenance. It also receives pressure signals from the transducers on the gas system, converts them to a standart form (Mpa), provides a visual display and transfers the data to the computer module.
» The water bath unit containing the gas system, consisting of a trolley, which supports the hardware – reference and sample cells, pressure transducers, valves and conecting tubing to supply gas, and a water bath to control the isothermal temperature.
 
     
 
• MEASUREMENT OF GAS ADSORPTION ISOTHERM
Ultimate analysis is a method to show components of coal by ignoring the presence of complexed compound and determine only the essential component. The organics compounds of coal mainly contain carbon and hydrogen. Additionally, coal has anorganics compounds such as nitrogen, sulphur and oxygen.

LEMIGAS uses Elemental Analyzer 1112 of Eager 300 to measure Nitrogen, Carbon, Hydrogen, Sulphur, and Oxygen. It can measure automatically up to 36 samples. The resulted data, especialy carbon content, is used to support rank coal analysis. Eager 300 displays a chromatogram where the identified peaks marked by the recognized elements including with real times and percentages. The apparatus also measures Net Calorific Value and Gross Calorific Value
 
• PROXIMATE ANALYSIS
Proxymate analysis of the coal sample is required in order to have an appropriate interpretion for absorption-isotherm analysis. Normally, the absorption-isotherm analysis is expressed on a dry ash-free basis (daf). Correlation may also be made on the basis of coal rank, which may be indicated, depending on the coal has origin, by the volatiles and/or the fixed carbon content of the coal sample.

We refer to Australian Standart Method AS1038.3 and AS426.1, to obtain value for air-dry moisture, volatile matter, ash content, and fixed carbon.
 
       
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