Effects of cleat performance on strength reduction of coal in CO2 sequestration

The natural cleat system in coal is highly important in the CO2 sequestration process as injected CO2 first moves through the cleat system, which eventually changes the coal's permeability and strength. The main objective of this study is to investigate the effects of cleat density and direction on the strength reduction of coal with CO2 adsorption. A series of strength experiments was conducted on non-CO2-saturated and CO2-saturated (1, 2, 3 and 16 MPa) coal samples with two different cleat densities (low-rank lignite and high-rank bituminous) and two different cleat angles (around 20 and 70 to the loading directions). According to the experimental results, CO2 saturation, at up to 3 MPa saturation pressure, causes up to 4.5 times higher strength reduction in bituminous coal (43%) compared to lignite (9.6%). The compressive strength reduction percentage in coal shows a linearly increasing trend with CO2 saturation pressure (1-3 MPa), where the slope is significantly higher for bituminous coal (15.3) compared to lignite (3.2). When the cleat direction reduces from 70 to 20, the CO2 adsorption (at 16 MPa) induced UCS strength reduction in bituminous coal reduces by around 20%. It is interesting to report that cleat density and direction do not exhibit a significant influence on elastic modulus reduction in coal compared to the strength reduction. (C) 2012 Elsevier Ltd. All rights reserved.