International Journal of Greenhouse Gas Control 5 (2011) 1284–1293 Contents lists available at ScienceDirect International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc Effect of the effective stress coefficient and sorption-induced strain on the evolution of coal permeability: Experimental observations Zhongwei Chen a,b , Zhejun Pan b,∗ , Jishan Liu a , Luke D. Connell b , Derek Elsworth c a School of Mechanical and Chemical Engineering, The University of Western Australia, WA 6009, Australia b CSIRO Earth Science and Resource Engineering, Private Bag 10, Clayton South, Victoria 3169, Australia c Department of Energy and Mineral Engineering, Penn State University, PA 16802-5000, USA article info Article history: Received 22 October 2010 Received in revised form 4 July 2011 Accepted 6 July 2011 Available online 9 August 2011 Keywords: Effective stress coefficient Swelling and shrinking Adsorption CO2 storage abstract Permeability is one of the most important parameters for CO 2 injection in coal to enhance coalbed methane recovery. Laboratory characterization of coal permeability provides useful information for in situ permeability behavior of coal seams when adsorbing gases such as CO 2 are injected. In this study, a series of experiments have been conducted for coal samples using both non-adsorbing and adsorbing gases at various confining stresses and pore pressures. Our observations have showed that even under con- trolled stress conditions, coal permeability decreases with respect to pore pressure during the injection of adsorbing gases. In order to find out the causes of permeability decrease for adsorbing gases, a non- adsorbing gas (helium) is used to determine the effective stress coefficient. In these experiments using helium, the impact of gas sorption can be neglected and any permeability reduction is considered as due to the variation in the effective stress, which is controlled by the effective stress coefficient. The results show that the effective stress coefficient is pore pressure dependent and less than unity for the coal samples studied. The permeability reduction from helium experiments is then used to calibrate the sub- sequent flow-through experiments using adsorbing gases, CH 4 and CO 2 . Through this calibration, the sole effect of sorption-induced strain on permeability change is obtained for these adsorbing gas flow-through experiments. In this paper, experimental results and analyses are reported including how the impact of effective stress coefficient is separated from that of the sorption-induced strain on the evolution of coal permeability. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Knowledge of changes in coal permeability due to coal matrix swelling/shrinkage strain is crucial for the evaluation of both pri- mary gas production from coal reservoirs and for CO 2 -enhanced coalbed methane recovery (ECBM) (van Bergen et al., 2009a). For primary gas production, as the gas pressure decreases below the desorption pressure, methane is released from the coal matrix to the fracture network and the coal matrix shrinks. As a direct consequence of this matrix shrinkage, the fractures dilate and frac- ture permeability correspondingly increases. Thus, a rapid initial reduction of fracture permeability (due to decrease of pore pres- sure) is supplanted by a slow increase in permeability at later production stage (due to matrix shrinkage). Whether the ultimate, long-term permeability is greater or less than the initial permeabil- ity depends on the net results of these two competing mechanisms. ECBM involves the injection of CO 2 into coal seams to displace ∗ Corresponding author. Tel.: +61 3 9545 8394; fax: +61 3 9545 8380. E-mail address: Zhejun.Pan@csiro.au (Z. Pan). methane recovered as an energy source, while providing the addi- tional benefit of reducing greenhouse gas emissions by storing the CO 2 underground (White et al., 2005; Liu et al., 2010b). 1.1. Experiments on coal swelling/shrinkage and permeability change Coal swelling/shrinkage due to gas adsorption/desorption is a well-known phenomenon and is regarded as a key component for coal reservoir permeability behavior during primary and enhanced coalbed methane recovery (e.g. Palmer, 2009; Shi and Durucan, 2004). Measurements of the effects of gas desorption on coal vol- umetric strain have been performed for the injection of different gases. The implications for changes in cleat permeability have been evaluated using the matchstick geometry model (Harpalani and Schraufnagel, 1990; Palmer and Mansoori, 1996; Seidle et al., 1992; Seidle and Huitt, 1995; Shi and Durucan, 2004; St. George and Barakat, 2001). Laboratory measurements of coal swelling with gas sorption and the causes of swelling have been investigated by various researchers (Bustin et al., 2008; Chikatamarla et al., 2004; Cui et al., 2007; Day et al., 2008; Levine, 1996; Moffat and 1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2011.07.005