High-Pressure Methane Sorption Isotherms of Black Shales from The Netherlands

High-pressure methane sorption isotherms were measured
on one Paleozoic
and five Mesozoic shales, considered as targets for shale gas exploration
in The Netherlands. The samples varied in mineralogy, organic richness,
and thermal maturity. Four of the samples were clay-rich (total clay
content 60–71 wt %), one contained equal amounts of clays and
quartz (36 wt % and 33 wt %, respectively) and one was a marl sample
(clays 34 wt %, carbonates 49 wt %). The total organic carbon contents
(TOC) ranged from <1 wt % to 10.5 wt %, and the thermal maturity,
as inferred from Rock-Eval analysis, from immature to overmature.
Excess (Gibbs) sorption isotherms for methane were measured at 65
°C on dry samples up to 25 MPa. The maximum excess sorption capacities
within this pressure range varied from 0.05 to 0.3 mmol/g (1.1–6.8
m3 STP/t). No correlation of excess sorption capacity with
TOC was found. Low-TOC, clay-rich shales had comparable or even higher
methane sorption capacities per unit rock mass (mmol/g) than organic-rich
shales, and a positive correlation was found between the maximum Langmuir
capacity (nL) and the clay content. This
observation supports the notion that clay minerals can contribute
significantly to the sorption capacity of shales. Furthermore, we
demonstrate that significant errors in TOC-normalized sorption capacities
may result from the uncertainties in TOC contents, especially at low
TOC values. A comparison between the immature and the overmature sample
(both organic-rich with equal clay contents) did not show any enhancement
of the sorption capacity with thermal maturity. However, the excess
sorption isotherm of the overmature sample had a distinct maximum,
while no maximum was observed for the immature sample in the experimental
pressure range. A Langmuir-type absolute sorption function, with a
term taking the volume of the adsorbed phase explicitly into account,
gave a good representation of the measured excess sorption isotherms.
The three-parameter fit yielded the Langmuir parameters (nL and pL) and a nominal density
value for the adsorbed phase (ρads). Two-parameter
fits of nL and pL using different fixed values of ρads are
discussed.