Permeability evolution of coal subjected to triaxial compression based on in-situ nuclear magnetic resonance

Abstract

A knowledge of permeability evolution with stress is important for the enhancement of coalbed methane production . In this study, a series of compaction-permeability tests with in-situ nuclear magnetic resonance (NMR) measurement were conducted by using a designed NMR testing system equipped with a loading device. Based on the NMR results, the permeability variation mechanism is investigated during the complete stress-strain process. The stress damage and its influence on permeability are analyzed. Experimental results illustrate that the permeability curve shows a slow decrease first, followed by a stable change, then a slow increase, and finally a sharp rise during the complete stress-strain process. Permeability evolution is closely related to coal pores. Adsorption pores have little contribution to the coal permeability while seepage pores contribute over 99% to the permeability. The permeability growth is found to lag behind the coal pores growth due to the poor connection of new pores. With the increase of damage, coal permeability increases nonlinearly. A power function can be used to describe the relationship for the tested samples.