Gas Loss Characteristics of Coalbed Methane-Containing Coal Sampled Using a Freezing Method

To estimate accurately the coalbed methane content, a pressurized sampling method with the aid of frozen sealing is proposed. In addition, the influences of adsorption gas pressure, freezing temperature, soaking time in drilling fluid, drilling fluid type, and exposure time in the atmosphere on the gas loss rate and the gas desorption characteristics of frozen samples were evaluated. Under low gas pressure, the gas in adsorption and free state in the coal sample can be trapped completely by the freezing sampling method. However, an appropriate freezing temperature is necessary to limit gas loss caused by frost cracks. The addition of polymers to the drilling fluid can enhance gas solubility and improve the gas trap capacity of the freezing sampling method. Although the freezing method can seal the gas perfectly in a coal sample, gas loss caused by gas diffusion through the frozen body is still difficult to avoid. Therefore, reducing the sampling time can limit effectively the gas loss rate. The freezing sampling method was confirmed to be an efficient method for obtaining gas-containing geological samples in an in situ state. The gas loss characteristics of coalbed coal sampled using the freezing method can provide theoretical support for reducing and estimating gas loss during the freezing sampling process.