High pressure methane sorption characteristics of lacustrine shales from the Midland Valley Basin, Scotland

Abstract

Three different oil shales of lacustrine origin from the Midland Valley, Scotland were investigated to assess their methane sorption capacity, the influence of pre-adsorbed water on methane sorption as well as the contribution of their individual constituents (minerals and organic matter) on water and methane sorption characteristics.Measurements were conducted using a manometric sorption device. In total, 15 isotherms were measured at 318K (45°C) at 5 different moisture states (dry, and moisture equilibrated at 11, 33, 53 and 97% relative humidity) on a set of samples in the light oil/wet gas maturity zone (3, 8, 21% TOC).Water uptake behaviour during moisture equilibration is controlled by total clay content whereas methane sorption capacity (0.015–0.18mmol/g) is controlled by TOC. Pre-adsorbed water significantly decreases methane sorption capacity in a non-linear manner until a critical moisture content of 0.5–0.7mmol water/g is reached. The impact of water on methane sorption capacity is strongly controlled by clay and TOC content, with water having strong influence on methane sorption capacity of clays and little influence on methane sorption capacity of early mature, kerogen type I organic matter. Our results are consistent with previous findings of methane sorption on organic matter and water interaction with clay minerals observed on marine source rocks.Shale gas potential of the West Lothian Oil-Shale Formation was critically evaluated on the basis of a recent reservoir estimation by the British Geological Survey. The organic matter in these lacustrine shales show three times lower sorption capacities in comparison to marine organic matter, but more data are needed and maturity effects have to be taken into account in future studies.