Sorption and changes in bulk modulus of coal — experimental evidence and governing mechanisms for CBM and ECBM applications

Abstract

Recent studies report that the stiffness of coal changes, when exposed to sorbing gases such as CH 4 and CO 2 . Although this effect might be potentially important for predicting the in situ mechanical behavior of coal seams during CH 4 production or CO 2 sequestration, very little understanding exists on the underlying mechanisms. In this paper, we report a single, long-duration mechanical test on a large, cube-shaped coal sample (Ruhr Basin, Germany). The sample was subjected to isostatic loading and unloading; first in the evacuated state (as received), and then each time after exposing it to pressurized, non-adsorbing He, or sorbing N 2 , CH 4 , and CO 2 . We find that exposure to sorbing gases led to swelling, which introduced a hysteresis in the stress-strain trajectory followed during mechanical loading. This hysteresis was almost absent in the evacuated state, and its magnitude depended strongly on the amount of adsorption-induced swelling exhibited by the sample. The apparent bulk modulus determined here for the CO 2 -equilibrated state was approximately 25% lower compared to the evacuated state. We qualitatively consider mechanisms that affect the elastic behavior of coal at the matrix and bulk scale, and argue that the observed effects are a combined effect of 1) changes in surface roughness at cleat interfaces and microfractures, and 2) a thermodynamic coupling between stress state, sorption capacity and swelling strain. The changes in stiffness relate to fluid-rock interaction effects, as well as to the presence of fractures, and hence not to the inherent elastic properties of the solid phase itself. We discuss the characteristics of each mechanism and the connection of the proposed mechanisms with formation-scale geomechanical effects of coal layers under conditions relevant to CBM/ECBM.

Permeability dynamic variation under the action of stress in the medium and high rank coal reservoir

“…(Li et al, 2014a). Meanwhile, Hol et al (2014) recently reported that the interaction of fluid and rock and the roughness of the fracture surface will also lead to the change of the coal hardness and reservoir conditions, which make it very difficult to describe a single factor quantitatively.…”

Correlation between coal and gas outburst risk and adsorption properties of coal seams

“…It is becoming increasingly serious as the mining depth extends. Brenner et al [13][14][15] found that the adsorption of coal was the result of the solid-gas coupling under certain temperature and pressure conditions. Most of them hold that gas plays a vital role in the development of coal and gas outburst.…”

“…[7][8][9][10][11][12] Numerous achievements regarding the adsorption property of coal have been obtained. Brenner et al [13][14][15] found that the adsorption of coal was the result of the solid-gas coupling under certain temperature and pressure conditions. Hol et al 16,17 studied the thermodynamic effects of CO 2 and CH 4 competitive adsorption on the surface of coal and pointed out that the adsorption capacity of CO 2 was about 2~10 times to that of CH 4 .…”

Strength Reduction of Coal Pillar after CO2 Sequestration in Abandoned Coal Mines

“…Gas adsorption mainly affects the cohesion, and the decrease of cohesion leads to obvious plastic deformation of coal, but, for the internal friction angle, it decreases to a certain extent and the hold over no longer changes. Hol et al [40] verified that gas sorption can lead to a decrease in bulk modulus, while an increase in swelling caused the strain hysteresis to be oversized during the process of loading-unloading. Hagin and Zoback [50] compared the adsorption characteristics of CO 2 with that of helium, and found that the Young's modulus decreased after the CO 2 saturation adsorption.…”