Effect of volcanic intrusions and mineral matters on desorption characteristics of coals (Case Study)

Abstract

In this study, coal seams properties in two coal core wells are studied to identify dominant parameters controlling desorption characteristics of coals. Available published data from desorption canister tests including sorption time and gas content (lost gas, desorbed gas, and residual gas) are employed. Proximate analysis data assists in coal characterization and data from high pressure adsorption tests are used to investigate the methane adsorption on coal as a function of pressure. For fracture and cleat analysis, gamma ray, density, and acoustic image logs are used. All data and analysis combined with regional geology assist to study desorption properties of different coal seams in these two wells. Kalbar-1 and Peebs-1 are the two core wells in this study. Entirely different patterns (gas content versus depth) are observed which can be explained by their different geological setting. A major volcanic intrusion of approximately 60m in thickness and a few minor intrusions in the area explain abnormally high gas content of shallow coal seams. The methane Langmuir adsorption isotherm for shallow coals (composite sample) is steep and has higher gas adsorption capacity compared to other seams. The shallow coal seams have been cooked by volcanic intrusions and have higher than expected gas content. For Peebs-1, the composite plot of gas content, sorption time, and ash content combined with gamma ray, density, and acoustic image logs provides a useful set to study desorption properties of coal seams. The fracture and cleat analysis reveals that low sorption time generally belongs to coal seams with higher cleat and fracture density. The existence of open cleats and fractures might facilitate gas release from coal matrix. The inverse correlation observed between sorption time and ash content while maceral composition is fairly similar suggests that desorption properties might be controlled by mineral matters in this well.