Adsorptive-mechanical properties of reconstituted granular coal: Experimental characterization and poromechanical modeling

The understanding of transport and mechanical processes in rocks with a tight matrix, such as coal, is important to assess unconventional hydrocarbon-bearing geological formations. Whenever coal cores cannot be recovered to be studied in the laboratory, reconstituted coal samples from crushed granular coal can help estimate the coal matrix transport and adsorptive-mechanical properties. In fact, the use of crushed granular coal is advantageous to measure coal adsorption isotherms quickly. Yet, reconstituted coal specimens have an intergranular porosity system and a mechanical behavior that significantly differ from the in-situ coal seam. We apply a double porosity poromechanical model that captures the influence of the coal matrix adsorptive-mechanical properties on the overall response of reconstituted granular coal specimens. Two laboratory examples provide evidence for (1) adsorption-induced swelling stress during constrained CO2 adsorption and (2) desorption upon mechanical loading at constant CO2 pressure. Results show that the magnitude of adsorption-mechanical couplings in reconstituted granular coal specimens is extremely sensitive to the compaction of the granular specimen. High initial compaction, high effective stress during testing, and relatively low fluid pressure facilitate identifying the impact of the coal matrix adsorption and swelling on the overall specimen response. Yet, creep and elastic nonlinearities may hinder sorption-induced strains and stresses. Well designed experiments and application of a double porosity model are critical to obtain a robust interpretation and understanding of the adsorptive mechanical properties of reconstituted granular coal. Conclusions from our findings, however, discourage the use of reconstituted granular coal to quantify swelling strains and stresses and their potential impact on reservoir permeability. (C) 2016 Elsevier B.V. All rights reserved.