Super-critical carbon dioxide flow behaviour in low rank coal: A meso-scale experimental study

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Abstract

The carbon dioxide (CO2) adsorbed in coal seams during CO2-enhanced coal bed methane recovery (CO2-ECBM) causes substantial coal matrix alterations, resulting in significantly reduced flow performance. Many studies have been conducted to date on the effect of CO2 phase on coal mass permeability. However, the effect of coal rank on these permeability changes with CO2 phase has not yet been studied. Therefore, the main aim of this study is to investigate how the influence of CO2 phase condition on coal flow performance varies with rank. A series of tri–axial permeability tests was conducted using Australian brown coal samples for both CO2 and N2 under various confinements and injections at 35 °C. The results were then compared with those for high-rank coal reported in the literature. According to the test results, greater coal macro-pore-structure rearrangement occurs with super-critical CO2 adsorption, resulting in lower permeability in coal, regardless of rank. However, this CO2 phase influence is much greater for high-rank coal. Although coal permeability reduces with depth for any rank of coal, this depth effect reduces with increasing rank. Furthermore, although N2 has the ability to recover CO2 adsorption-induced swelled areas in coal regardless of rank, that capability is much greater for high-rank coal.

Section snippets

Introduction and background

Of the various CO2 mitigation options, CO2-enhanced coal bed methane (ECBM) recovery is being implemented and tested as a viable option to reduce the amount of CO2 in the earth’s atmosphere, while recovering a valuable energy product: coal bed methane (CH4) [1], [2], [3], [4], [5]. The CO2-ECBM process involves introducing CO2 through an injecting well into deep coal seams, in which CO2 acts as a displacing gas that allows the already adsorbed CH4 in the coal seam to be desorbed, and eventually

Sample preparation

Permeability tests were conducted using natural brown coal samples obtained from the Hazelwood coal mine located at Morwell in South Gippsland, Victoria. The coring, cutting and grinding machines available in the Monash University Deep Earth Energy Research Laboratory (DEERL) were used to produce samples 38 mm in diameter and 76 mm high from large coal blocks. These brown coal samples had around 57% natural moisture content, and to avoid moisture loss from the samples, they were wrapped in

Experimental results and discussion

The following sections discuss (i) the effect of CO2 phase and pressure on permeability in brown coal; (ii) the potential of reversing CO2 adsorption-induced coal matrix alterations using N2 in brown coal; and (iii) the effect of CO2-induced coal matrix alterations in the pore structure of brown coal.

Implications for CO2 sequestration in coal seams

Generally, super-critical CO2 is preferred for CO2-ECBM recovery due to its higher adsorption capacity to the coal matrix [13] and the ability for stable storage of CO2 due its higher densities [59]. However, reduction of CO2 flow ability through the coal matrix during super-critical CO2 sequestration makes the CO2-ECBM process less productive, since the targeted amount of CO2 cannot be stored in the coal mass, leading to unpredictable CH4 recovery. The field-scale CO2-ECBM projects which

Conclusions

The following major conclusions can be drawn from the study:

  • Although the reduction of permeability with increasing depth due to associated effective stress variation, is common for any type of coal, the amount of permeability reduction with increasing depth is much higher for low-rank coals compared to high-rank coal, due to their partially mature, soft and highly compressible nature.

  • The uniform cellular-like pore structure with relatively large pores in natural brown coal significantly alters

Acknowledgements

The authors acknowledge the use of facilities in the Monash Centre of Electron Microscopy (MCEM) and the funding provided by the Australian Research Council (DE130100124) and the Postgraduate Publication Award (PPA) of Monash University.

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