GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 93-2
Presentation Time: 8:30 AM


XIONG, Fengyang, School of Earth Sciences, The Ohio State University, 125 South Oval Mall, Columbus, OH 43210; School of Earth Sciences, The Ohio State University, 125 South Oval Mall, Columbus, OH 43210, ROTHER, Gernot, Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, 1 Bethel Valley, Oak Ridge, OH 37831-6110, TOMASKO, David, William G Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 121 Hitchcock Hall, 2070 Neil Ave, Columbus, OH 43210 and MOORTGAT, Joachim, School of Earth Sciences, College of Arts and Sciences, 125 South Oval Mall, COLUMBUS, OH 43210

Sorption is a complex and important process in surface chemistry. Understanding its behavior will help to better characterize the performance of engineered materials (e.g., catalyst) and protect personal health (e.g., respiratory protection). Black shale, a heterogenous geomaterial, has played an increasingly important role in the global energy market in the past decade. A significant fraction of gas in such shales is adsorbed on the surfaces of the exceedingly small pores. To estimate the total initial gas-in-place, experimentally measured ‘excess’ sorption is utilized to determine the absolute, or ‘real’, amount of sorbed gas. In this process, the density of the sorption layers is required. The pressure dependence, composition, and volumes of sorption layers in shales, however, remain poorly constraint. In this work, we propose a novel and practical experimental procedure to estimate the dynamic effective densities of sorbed methane.

To do so, a series of high-pressure CH4 sorption isotherms were conducted with several Lower Permian Shanxi shale core samples from the Ordos Basin, NW China and one Poshidonia shale sample from Germany using a gravimetric method at three temperatures of 65, 75, and 95 °C. Further, organic geochemistry, X-ray diffraction mineral composition, and analyses of specific surface areas were carried out to shed light on the distribution of sorbed methane molecules on different shale minerals. We compared the estimated effective densities of sorbed methane at different pressures and temperatures under assumptions of monolayer and binary layer with cubic and hexagonal close packing. The results show that at the same temperature and low pressure, the effective density of sorbed methane increases with pressure within a monolayer, while at high pressure, sorbed methane will begin to accumulate beyond monolayer. The pressure at which the binary sorption occurs is determined by the sorption potential of shale mineral surfaces, especially that of the organic matter, indicating that at high pressure, the Langmuir model may not be appropriate. The ratio of sorbed methane to bulk methane decreases nonlinearly with pressure, which is also associated with the distribution of mineral surfaces. The hexagonal close packing provides a more reasonable explanation for the experimental measurements.