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

Abstract

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 m 3 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 (n L ) 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 (n L and p L ) and a nominal density value for the adsorbed phase (ρ ads ). Two-parameter fits of n L and p L using different fixed values of ρ ads are discussed.

Molecular Simulation of Adsorption in Microporous Materials

“…This is consistent with experimental measurements [70]. Qualitatively, the maximum excess sorption of methane has been observed in studies on dried gas shales by Gasparik et al [71], as is shown in the right hand side of Figure 16; in their work, maxima in the excess isotherms were observed for all but the leastmature sample. However, it is possible that the adsorption on gas shales is more complex than adsorption on kerogen, as the clay minerals present a significant surface area in these samples.…”

High-Pressure Methane Adsorption and Characterization of Pores in Posidonia Shales and Isolated Kerogens

“…Within the phyllosilicates group, illiterich mixed layer illite-smectite is the most prominent component, followed by kaolinite, illite, muscovite and chlorite. In addition, there is a moderate content of quartz (8)(9)(10)(11)(12)(13)(14)(15)(16) wt.%) and minor contents of pyrite (4-9 wt.%), feldspars (1-5 wt.%) and dolomite (0.3-6.4 wt.%).…”

“…11 Although sorption data on shales are available, the absolute sorption capacity in shale remains poorly constrained. [11][12][13][14][15][16][18][19][20] Shales are complex heterogeneous materials with amorphous kerogen and inorganic phases. The amounts of methane adsorbed on shales are also very low and this is complicated by the fact that the minor kerogen component has a much larger adsorption capacity than the inorganic phase in the shale.…”

Geochemical and lithological controls on a potential shale reservoir: Carboniferous Holywell Shale, Wales

“…Montgomery et al, 2005;Gasparik et al, 2012;Rexer et al, 2013). Organic matter contents in the Holywell Shale only average 1.9 wt % and only a small portion of the Formation has TOC contents above 3 wt %.…”