The role of pre-adsorbed water on methane sorption capacity of Bossier and Haynesville shales

•Moisture equilibrated methane sorption isotherms were determined for two American gas shales.•We examined the impact of water on the sorption capacity of shales and their components.•Pre-adsorbed water decreases CH4 sorption capacity linearly up to the critical moisture content.•The residual methane sorption capacity is purely a function of organic matter in overmature shales.•Sorptive gas storage will be overestimated by over 50% when based on dry methane sorption isotherms.

Methane in unconventional shale gas reservoirs is partially stored as sorbed gas. Water, which is omnipresent in gas shales, decreases methane sorption and gas storage capacity. However, the exact controls and mechanism for this are still not sufficiently understood. Therefore, we measured high-pressure methane sorption isotherms of Bossier and Haynesville shales as a function of pre-adsorbed water content at 318 K (45 °C) and 348 K (75 °C) and pressures up to 25 MPa.Pre-adsorbed water content significantly decreases methane excess sorption capacity. Dry maximum methane sorption capacities of 0.110 mmol/g for the Bossier Shale and 0.104 mmol/g for the Haynesville Shale at 318 K (45 °C) decrease by 78% and 68% upon moisturizing the sample to 97% relative humidity (RH). Decrease in methane excess sorption capacity linearly correlates with increasing water content up to a critical threshold of 0.6–1 mmol/g (app. 50–75% RH). Additional pre-adsorbed water content has no further impact on methane excess sorption.Dry methane sorption isotherms can be approximated as the sum of the sorption behavior of individual constituents. The main sorption sites are located on clays (mostly illite) and organic matter in approximately equal proportions.Illite clays are responsible for the main loss in methane sorption with increasing water content. All of the methane sorption sites of clay minerals are lost at water saturations of 32 and 50% for the Haynesville and Bossier shales, respectively. This makes the clays methane excess sorption on the reservoir scale negligible, where comparable or higher water saturations prevail. Consequently, the hydrophobic part of the organic matter alone makes up the residual methane excess sorption.The reduction in methane excess sorption for the analyzed samples can be as high as 54 and 72% for reservoir water saturation values for Haynesville and Bossier shales, respectively. This research demonstrates how methane storage capacity calculations for the reservoir scale might be grossly over-estimated when the significant impact of water and its interaction with the mineralogical and organic parts of the shale is not properly accounted for.