Analysis of Gas Content Determination Methods for Coal Beds in TÜRKİYE

Methane is constituted more than 95% of the gases forms as either biogenically or thermogenically-derived gas in coal beds, and is stored as adsorbed gas and free-gas in the pores of the coal. The accurate determination of the coalbed gas content has a great importance for the estimation of the methane emissions that will occur in the underground coal mines, and accordingly for the planning of the mine-ventilation, as well as for the reservoir studies on the economic recovery of coalbed methane (CBM). Gas content determination techniques can generally be grouped into two categories as direct and indirect. In direct methods, coal samples taken from the coal seam are placed in sealed canisters and the volume of gas released from the sample is actually measured. Although different direct methods have been developed, such as quick-crushing and extended desorption methods, all direct gas content measurement methods are based on the same principles. The most commonly used methods subdivide the total gas content of a coal sample into three parts: lost, desorbed, and residual gas. Each of these parts is generally measured or estimated by a different procedure, and then combined to yield the total gas content of the sample. In the direct method developed by the US Bureau of Mines (USBM), which is widely used among the direct methods, core samples with a length of 50 cm are periodically collected and promptly placed in sealed canisters during drilling operation. The amount of gas released during sample collection process represents the lost gas component, which is not directly measured but can be estimated from desorption graphs. After the samples placed in canisters, the amount of gas released from core samples refers desorbed gas component, can be directly measured with a manometric gas volume measurement apparatuses. Finally, a small amount of gas still remains in the samples after the desorption is completed, known as the residual gas component. To measure residual gas component, samples are crushed to particle size below -200 mesh using a sealed mill. In indirect methods, the gas storage capacity of the sample can be determined by using various empirical equations or adsorption isotherms obtained in the laboratory. The USBM direct method was applied to coal samples taken from different coalbeds in Türkiye. In this paper, the results obtained from tests were analysed, application procedures of the USBM direct method were explained in detail and gas content determination methods were compared for different coalbeds from Türkiye.