Core Desorption

Gas Content From Core Desorption

Sigra provides a comprehensive service to measure the gas content of coal core retrieved by wireline. This is a service that is unique in its accuracy and completeness. It provides solutions that are useful in the assessment of outburst risk and the prediction of gas release from in-situ and cut coal. Our procedures are outlined below.

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Sigra core desorption canister connected to automatic desorption monitor and gas collection bag sitting on controlled temperature bath in which the canister would be placed.

Procedure
  • To time the core cutting and retrieval process.
  • To place all core containing more than 300 mm of coal into a canister ( 1 m or 3 m) without delay for any logging.
  • To automatically monitor the rate of gas desorption in approximately 20 ml steps each of which has a time stamp.
  • To place the core in a temperature bath held at seam temperature.
  • To collect all gas released and to re-measure its volume to check the consistency.
  • To continue desorption until approximately 80% of initial gas has been released.
  • To analyse for gas composition with multiple samples to detect compositional change.
  • To log the core for coal type and cleat infill and measure apparent relative density of each ply group identified.
    • If the core has apparent cleat carbonates, to test these with acid for reactivity – this may then lead to a more precise measurement of the effect of acid leaching, which has a direct influence on permeability.
  • To sub sample all of the core to build up a sample for residual gas determination (Q3) by crushing. This crushed sample then becomes a representative sample for supply of crushed material for sorption isotherms. Desorption of the crushed material is undertaken in two stages
    • To one atmosphere of partial pressure of seam gas
    • To a very low partial pressure of seam gas
Analysis

Sigra’s sophisticated analysis involves:

  1. Accounting for water vapour as a component of gas volume release from canisters. This can produce a 7% error at high seam temperatures.
  2. Assessing the initial gas loss during retrieval of the core and on surface prior to the commencement of desorption rate measurements. This is done by:
    • Using the standard method based on the square root of time plot of early desorption and an assumed start to desorption. As part of this test the Apparent Diffusion Coefficient (DA) is calculated. This is a calculation of the diffusion coefficient based upon the initial slope of  cumulative desorption versus square root of time plot and the total gas content. The calculation is based on the core diffusing as though it were a uniform cylinder. As no coal core is without some inhomogeneity and fractures DA is in fact a combination of the both the diffusion coefficient and the fracturing within the core. It is useful in modelling the gas release of cut coal and as an indicator of outburst proneness.
    • By a more rigorous approach which models the actual process of desorption from drilling right through to crushing for residual gas. In this a numerical model of diffusion based upon the diffusive behaviour of a fractured core is fitted to the measured core desorption. One of the inputs to the model comes from logging the core in the form of choosing a suitable core fracturing model. The software then refines this to arrive at an improved fracture distribution and the associated diffusion coefficient. Using this information with the volume of the remainder of gas collected during crushing, and either the measured or estimated sorption isotherm, enables a far more accurate estimate of lost gas to be made through a numerical model. This actually models desorption through the changing pressure in the borehole as the core is retrieved and during the period on surface when gas is being lost from it.
  3. Characterising the shape of the desorption curve both in terms of the standard desorption time (tau) for 2/3 of total gas desorption and for departure from standard Fickian diffusion.
  4. Calculation of the gas content of the carbonaceous fraction of the core coals on a mineral matter free basis.
  5. Calculation of the average gas content of the seam including multiple canisters and core loss.

Sigra’s estimate of gas content through these procedures far exceeds the AS 3980-1999 Standard.

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