Nanopore characterisation and gas sorption potential of European gas shales

Abstract

An inter-laboratory study of high-pressure gas sorption measurements on two carbonaceous shales has been conducted to assess the reproducibility of sorption isotherms on shale and identify possible sources of error. The measurements were carried out by 7 different international research laboratories on either in-house or commercial sorption equipment using manometric as well as gravimetric methods. Excess sorption isotherms for methane, carbon dioxide and ethane were measured at 65°C and at pressures up to 25 MPa on two organic-rich shales at dry conditions. The inter-laboratory reproducibility of the methane excess sorption isotherms was better for the high-maturity shale (within 0.02 – 0.03 mmol g-1) than for the low-maturity sample (up to 0.1 mmol g-1), which is in agreement with results of earlier studies on coals. The procedures for sample conditioning prior to the measurement, the measurement procedures and the data reduction approach must be optimized to achieve higher accuracy. Unknown systematic errors in the measured quantities must be minimized first by applying standard calibration methods. Furthermore, the adsorption of methane on a dry, organic-rich, high-maturity Alum shale sample was studied at a wide temperature range (300 – 473 K) and pressures up to 14 MPa. These conditions are relevant to gas storage under geological conditions. Maximum methane excess uptake is 0.176 – 0.042 mmol g-1 (125 - 30 scf t-1) at 300 - 473 K. Supercritical adsorption was parameterized using the modified Dubinin-Radushkevich and the Langmuir equations. Gas in shales is stored in three different states: adsorbed, compressed (free) and dissolved; quantifying each underpins calculations of gas storage capacity and also the mechanisms by which gas must be transported from pore (surfaces), to fracture, to the well. While compressed gas dominates in meso- and macropores, it is often assumed that (a) sorbed gas occurs mainly in micropores (< 2nm) and (b) micropores are mainly associated with organic matter. In the third part of this thesis, those ideas are tested by characterising the porous structure of six shales and isolated kerogens from the Posidonia Formation in combination with high pressure methane sorption isotherms at 45, 65 and 85°C. Together, these data help us to understand the extent to which (a) small pores control CH4 sorption and (b) whether “sorption” pores are associated with the organic and inorganic phases within shales. Samples were selected with vitrinite reflectance of 0.6, 0.9 and 1.45%. Pore volumes – named sorption pore volumes here - were determined on dry shales and isolated kerogens by CO2 isotherms measured at -78°C and up to 0.1 MPa. These volumes include micropores (pore II width < 2nm) and narrow mesopores; according to the Gurvitch Rule this is the volume available for sorption of most gases. Sorption pore volumes of Posidoniashales range from 0.008 to 0.016 cm3 g-1, accounting for 21 - 66% of total porosity. Whilst sorption pore volumes of isolated kerogen are much higher, between 0.095 – 0.147 cm3 g-1, normalization by TOC shows that only half the sorption pore volume of the shales is located within the kerogen. Excess uptakes on dry Posidonia shales at 65°C and 11.5MPa range from 0.056−0.110 mmol g-1 (40−78 scf t-1) on dry shale, and from 0.36−0.70 mmol g-1 (253−499 scf t-1) on dry kerogen. Enthalpies of adsorption show no variation with TOC and maturity, respectively. The correlation between maximum CH4 sorption and CO2 sorption pore volume at 195 K is very strong and goes through the origin, suggesting that the vast majority of sorbed CH4 occurs in pores smaller than 6 nm. Approximately half the sorption pore volume and thus CH4 sorption potential of these dry shales is in organic matter, with the rest likely to be associated with clay minerals. Sorption mass balances using isotherms for kerogen and clay minerals do not always account for the total measured sorbed CH4 on dry shales, suggesting that some sorption may occur at interfaces between minerals and organic matter.