Competition between adsorption-induced swelling and elastic compression of coal at CO2 pressures up to 100 MPa

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Abstract

Enhanced Coalbed Methane production (ECBM) by CO2 injection frequently proves ineffective due to rapidly decreasing injectivity. Adsorption-induced swelling of the coal matrix has been identified as the principal factor controlling this reduction. To improve understanding of coal swelling in response to exposure to CO2 at high pressures, numerous laboratory studies have been performed in the past decades. These studies consistently reveal an increase in swelling with CO2 pressure. However, it remains unclear what the relative contributions are of adsorption-induced swelling versus elastic compression of the coal framework, and hence what is the true relationship between adsorption-induced swelling and CO2 uptake.

Here, we report the results of dilatometry experiments conducted on unconfined, cylindrical coal matrix samples (∼4 mm long and 4 mm in diameter) of high volatile bituminous coal, where we aim to measure the effective volumetric effect of CO2 and to separate this into a component caused by adsorption-induced swelling and a component caused by elastic compression. The experiments were performed using a high pressure eddy current dilatometer that was used to measure one-dimensional sample expansion or contraction (resolution <50 nm). The tests were conducted at a constant temperature of 40 °C, and CO2 pressures up to 100 MPa. Our results show that the matrix samples reveal anisotropic expansion over the full range of CO2 pressures used. Expansion perpendicular to the bedding was about 1.4 times the average expansion measured in the bedding plane. Net volumetric strains, which were computed from the net linear strain in all directions measured, reveal that the response of coal is characterised by an expansion-dominated stage below 10–20 MPa of CO2 pressure and a contraction-dominated stage at higher CO2 pressures. Our data demonstrate direct competition between adsorption-induced swelling and elastic compression in the coal matrix. We propose a model for coal swelling, which expresses the net volumetric strain as the sum of the adsorption-induced swelling strain and the elastic compression with the adsorption-induced swelling being taken as linearly related to adsorbed CO2 concentration. A comparison of experimentally determined adsorption-induced swelling strain with the adsorbed concentration of CO2 (data Gensterblum et al., 2010) confirms the assumed linear dependence. We go on to compare our experimentally determined adsorption-induced swelling strains to those calculated from an adsorbed concentration model. Good agreement was found over the full range of CO2 pressures up to 100 MPa. This shows that combining this thermodynamically based model for adsorbed concentration with the elastic compression of our samples, obtained from their bulk modulus, provides a good description of the measured volumetric behaviour of our samples, and suggests that the physical basis for the model is also valid.

The implications of our results for ECBM operations are that compliant coals (low K), which exhibit little adsorption-induced swelling (hence low dependence C), will show relatively small reductions or even increases in permeability due to competition between swelling and compression when CO2 pressure increases during ECBM operations. These coals will tend to be more suitable for ECBM operations. Coals exhibiting high stiffness (K) and high adsorption capacity are less suitable for ECBM.

Highlights

► Eddy-current dilatometer used to measured expansion of coal matrix in scCO2 up to 100 MPa. ► Coal samples show anisotropic expansion. ► Competition between adsorption-induced swelling and elastic compression observed. ► Model for coal swelling agrees with data for CO2 pressures up to 100 MPa. ► Compliant, low-swelling coals are most suitable for Enhanced Coalbed Methane production.

Introduction

Despite vast global reserves of coalbed methane, Enhance Coalbed Methane production (ECBM) by CO2 injection frequently proves ineffective due to rapidly decreasing injectivity. Adsorption-induced swelling of the coal matrix has been identified as the principal factor controlling this reduction in injectivity of CO2 during ECBM (e.g. van Bergen et al., 2006). To determine the magnitude of coal swelling, numerous laboratory studies have been performed in which unconfined coal samples are exposed to CO2 at pressure and temperature conditions relevant to ECBM operations (e.g. Day et al., 2008, Day et al., 2010, Durucan et al., 2009, Majewska et al., 2010, van Bergen et al., 2009). These studies have shown that the observed swelling of coal is directly related to the CO2 pressure and amount of CO2 adsorbed, generally in a non-linear manner (Astashov et al., 2008, Day et al., 2008, Day et al., 2010, Kelemen and Kwiatek, 2009, Pini et al., 2009). The relations obtained form key input for interpreting and modelling the progress of adsorption, swelling and permeability/productivity evolution during ECBM operations (Liu and Rutqvist, 2010, Palmer and Mansoori, 1998, Shi and Durucan, 2003).

However, two fundamental problems are associated with the application of the relations found between experimentally determined swelling, CO2 pressure and absolute adsorption. First, recent thermodynamic/thermo-mechanical models demonstrate that the volumetric response of unconfined coal matrix material with increasing CO2 pressure is a net effect caused by adsorption-induced swelling combined with elastic compression of the solid matrix framework (Pan and Connell, 2007, Vandamme et al., 2010). Although the latter effect has been demonstrated experimentally by Moffat and Weale (1955) for CH4-equilibrated coal, it is generally neglected in formulating pressure–swelling–adsorption relations that can be applied to in situ conditions. A simple, order-of-magnitude calculation using typical bulk moduli for coal shows that this can result in significant inaccuracies in describing swelling–adsorption data, especially at higher CO2 pressures or in cases where the sample material exhibits a low or strongly pressure-dependent bulk modulus, K. Second, swelling/adsorption experiments are often carried out on cm-scale coal cores or blocks, which inevitably contain cleats, cracks, and preparation-induced ‘core damage’ (Harpalani, 1988, Pini et al., 2009). Because of these features, coal cores and blocks display non-linear elastic behaviour at low applied stress, as demonstrated by conventional elastic stiffness tests (Deisman et al., 2008, Gentzis et al., 2007, Ko and Gerstle, 1976). Accordingly, bulk swelling measurements may yield highly variable results that differ significantly from the “true” volumetric response of intact (cleat/damage-free) coal matrix material. To test recent theoretical models linking matrix swelling to CO2 pressure and adsorbed content, and to obtain reliable swelling–sorption relations that can be coupled with poro-elastic constitutive models, it is therefore necessary for experiments to focus on “true” (intact, cleat-free) matrix swelling. This requires the use of matrix-scale samples, plus independent investigation of both adsorption-induced swelling and elastic compression effects.

Here, we report the results of detailed dilatometry experiments conducted on three unconfined, cylindrical coal matrix cores (approximately 4 mm in diameter by 4 mm in length) of high volatile bituminous coal (Brzeszcze, Poland). Our aim was to measure the volumetric response of unconfined, cleat-free coal matrix material to exposure to high pressure CO2, separating this into adsorption-induced swelling and elastic compression components. In each experiment, the sample was equilibrated with CO2 at a constant temperature of 40 °C, using CO2 pressures up to 100 MPa. Volumetric strains were computed from the linear strains measured parallel and perpendicular to the bedding of the coal samples. Since we did not subject the samples to an externally applied effective stress, poro-elastic effects in this study reduce to purely elastic volumetric effects of changing fluid pressure. These were measured independently by exposing the samples to high pressure He, yielding the bulk modulus of the samples (cf. Wang et al., 2011). Using the volumetric strain and bulk modulus data obtained, we calculate the adsorption-induced swelling for each matrix sample. Together with independent, high-precision determinations of absolute CO2 adsorption for crushed Brzeszcze coal (<2 mm), reported by Gensterblum et al. (2010), our analysis yields a swelling–adsorption relationship for our coal matrix samples.

Section snippets

Approach

In this study, we used a specially developed, high-pressure eddy-current dilatometer to measure the unconfined dimensional changes of matrix samples of high volatile bituminous coal exposed to CO2, at pressures up to 100 MPa at a fixed temperature of 40 °C. We aimed to separate the measured strain response into a component caused by adsorption-induced swelling plus a component caused by elastic compression. Millimetre-scale coal samples were used to exclude effects of cleats or cracks on the

Results

Dilatometry data obtained for samples treated with CO2 are presented in Fig. 4, Fig. 5. Fig. 4 shows typical data illustrating how the strain evolved with time during individual experiments. Fig. 5 shows the dependence of equilibrium strains eieq on CO2 pressure for all samples tested. The development of equilibrium strain with He pressure for all samples is shown in Fig. 6. All equilibrium data obtained during upwards pressurisation are tabulated in Table 2 (CO2 experiments) and Table 3 (He

Discussion

The present dilatometry experiments have shown that significant net swelling or expansion of matrix-scale coal samples occurs due to interaction with CO2 at pressures in the range 0–100 MPa at a constant temperature of 40.0 °C (Fig. 5a–c). Our data demonstrate a transition from an expansion-dominated to a contraction-dominated strain response that occurs in all samples above ∼15 MPa applied CO2 pressure (Fig. 5a–c). The same samples exhibit reversible linear compression when pressurised with

Conclusion

This study has addressed the volumetric response of coal matrix material to the application of CO2 pressures up to 100 MPa, at a constant temperature of 40.0 °C. Our aim was to measure the net volumetric effect of CO2 on coal and to assess if this can be separated into a component caused by adsorption-induced swelling and a component caused by elastic compression. To do this, we performed dilatometry experiments on three unconfined, cylindrical coal matrix samples (4 mm in diameter and ∼4 mm long)

Acknowledgements

The authors acknowledge the Shell International for funding the research presented in this paper (Contract no. 4600003671). Rick Wentinck and Claus Otto are thanked for their support and valuable suggestions during the course of this study. We also acknowledge the Central Mining Institute (Poland) and the Netherlands Organization for Applied Scientific Research (TNO) for the availability of the coal samples, that taken from the coal seam into which CO2 was injected as a part of the RECOPOL ECBM

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