4. Sorption of CH₄ and CO₂ on Belgium Carboniferous Shale Using a Manometric Set-up

Shale gas resources are globally abundant and the development of these resources can increase CH₄ production. It is of interest to study the possibility of enhancing CH₄ production by CO₂ injection (Enhanced Gas Recovery—EGR). Some studies indicate that, in shale, five molecules of CO₂ can be stored for every molecule of CH₄ produced. The technical feasibility of Enhanced Gas Recovery (EGR) needs to be investigated in more detail. The amount of extracted natural gas from shale has increased rapidly over the past decade. A typical shale gas reservoir combines an organic-rich deposition with extremely low matrix permeability. One important parameter in assessing the technical viability of (enhanced) production of shale gas is the sorption capacity. Our focus is on the sorption of CH₄ and CO₂. Therefore we have chosen to use the manometric method to measure the excess sorption isotherms of CO₂ at 318 K and of CH₄ at 308, 318 and 336 K and at pressures up to 105 bar on Belgium dry black shale from a depth of 745 m. The shale was obtained from a former coal mine in Zolder in the Campina Basin (North Belgium), which contains Westphalian coal and coal associated sediments of Northwest European origin. We derive the equations for excess sorption in the manometric set-up. Only a few measurements have been reported in the literature for high-pressure gas sorption on shales, and interest is largely focused on shales occurring outside Europe. The excess sorption isotherm shows an initial increase to a maximum value of 0.175 ± 0.004 mmol/g for CO₂ and then starts to decrease until it becomes zero at 82 bar and subsequently the excess sorption becomes negative. Similar behaviour was also observed for other shales and coal reported in the literature. The experiments on CH₄ show, as expected, decreasing sorption for increasing temperature. We apply an error analysis based on Monte-Carlo simulation. It shows that the error is increasing with increasing pressure, but that the manometric set-up can be used to determine the sorption capacity of CO₂ and CH₄ on the black shale with sufficient accuracy.