Effect of shale properties on CH4 and CO2 sorption capacity in Czech Silurian shales

Abstract

The study involved an experimental assessment of the CO2 and CH4 sorption capacity of seven dark Silurian shale samples from three formations in the Barrandian Basin (Czech Republic). The shale samples had dominant inorganic matter consisting mainly of clay minerals, quartz, calcite and plagioclase, and very low organic matter. Most of the organic particles were the remains of zooclasts, especially graptolites and bitumens. The graptolite reflectance (Rr) reflecting the degree of thermal maturation of the shales ranges between 0.56% in the Kopanina Formation and 1.09% in the Litohlavy Formation, with the exception of the probably thermally altered part of the Motol Formation, which has reflectance higher than 1.70%. The total organic carbon (TOC) content was found to be in the range of 0.34–2.37 wt%. The porosity values were from 4.6% to 18.8%. In all shale samples, the high pressure excess sorption isotherms of CO2 and CH4 were measured at a temperature of 45 °C for both gases at pressures up to 8 MPa for CO2 and up to 15 MPa for CH4 on dry samples, using a manometric sorption apparatus. The Gibbs equation was used for evaluation of the maximum excess sorption (nmax,CO2 and nmax,CH4). The nmax,CO2 values ranged from 0.118 mmol/g to 0.244 mmol/g, while the nmax,CH4 values were more than twice lower (0.050 mmol/g - 0.088 mmol/g). The xCO2/CH4 ratio ranged from 1.47 to 2.98, with a median value of 2.54, calculated without the sample from the Kopanina Formation (4.36). The influence of the shale parameters on the CO2 and CH4 sorption capacity was determined. No relation has been proven between the value of Rr and the TOC-normalized CO2 and CH4 sorption capacities. The CO2 sorption capacity showed a negative correlation with TOC and with clay minerals but a positive correlation with quartz, while the behaviour of CH4 was opposite in these cases. A strong positive correlation was found between the volume of micropores and the CO2 excess sorption capacity, and negative dependence was found for CH4. The CO2 sorption capacities always depend on the micropores content, and the CH4 capacities depend on the shale components.