Anisotropic Adsorption Swelling and Permeability Characteristics with Injecting CO₂ in Coal

Abstract

The changes of anisotropic adsorption–swelling and permeability with injecting CO2 in coal influence the CO2 injectivity during CO2-ECBM or CGS (ECBM = enhancing coal bed methane; CGS = CO2 geological sequestration). To strengthen the understanding of this issue, two special-made cubic coal samples were adopted to test the porosity, swelling, and permeability in parallel face cleat and bedding plane direction, parallel butt cleat and bedding plane direction, and vertical bedding plane direction. To quantitatively characterize the anisotropic porosity, anisotropic swelling, and anisotropic permeability, an anisotropy index was introduced in this work. The results show that porosity anisotropy reflects the pore connectivity in different directions, which fall in the order of parallel face cleat and bedding plane direction > parallel butt cleat and bedding plane direction > vertical bedding plane direction. The porosity varieties can be owed to the compaction effect, thermal evolution effect, banded structure, and cleat distribution in coal seams. The maximum swelling ratios of the vertical bedding plane direction to the parallel bedding plane direction are 2.30 in sample 1 and 1.89 in sample 2. However, the ratios of parallel face cleat to parallel butt cleat are 1.28 in sample 1 and 1.30 in sample 2. The inhomogeneity of matter composition in the vertical bedding direction and the difference of cleat distribution in various coal bands mainly cause the anisotropic swelling. Both injecting CO2 in coal and raising its temperature increase the anisotropy swelling index, but the effect of thermal swelling is quite weak. Adsorbing CO2 especially for supercritical CO2 will enhance the permeability anisotropy of coal. This is because the low-permeability cleat possesses higher permeability adsorption sensitivity and the bedding plane fracture with higher permeability instead does not produce a pronounced permeability drop because of its lower permeability adsorption sensitivity. Cleats that are easily affected by adsorption–swelling always serve as throats between fractures and the coal matrix in a high-anisotropic coal, which will restrain CO2 flow in coal pores. Accordingly, cleat seepage and corresponding potential enhanced permeability measures deserve being paid enough attention to in future research. This work clarifies the understanding and offers some implications for CO2 injecting into coal seams from the perspective of anistropic properties of coal.

Experimental Study on the Brittle-Ductile Response of a Heterogeneous Soft Coal Rock Mass under Multifactor Coupling

“…Coal Specimen [51,52] have done a lot of researches on natural and reconstituted anthracite coals, and the results show that the swelling strain of reconstituted coal is similar to the homogeneous isotropic variety. Therefore, this method is used to produce heterogeneous soft coal specimens.…”

The Influence of Temperature on the Expansion of a Hard Coal-Gas System

“…The sample expands more in the direction perpendicular to the layering. Expansion anisotropy can be derived from the anisotropy of coal porosity and permeability [37]. The measured volume change in the vacuum, in the presence of helium, is high, which confirms that, in the case of non-isothermal processes in the coal mine gas system, the expansion of coal is the result of the coal's physical properties.…”

“…With isothermal measurements, the changes in the perpendicular direction are greater. This lack of similarity can be explained by the anisotropy of porosity changes as a result of coal shrinkage, which changes with temperature [37]. To determine the thermal expansion of the sample itself, measurement of the linear changes in the sample was carried out in a vacuum-degassed system (Figure 6a).…”

An experimental study on the permeability changes of anthracite reservoirs in different depths of Qinshui Basin induced by supercritical CO2 injection

“…Carbon dioxide (CO 2 ) is a kind of greenhouse gas affects atmospheric temperature, and geological sequestration of CO 2 has been considered as one of the most promising options for reducing greenhouse gas emission . The permeability of a coal seam, a key parameter reflecting the ability of fluids to flow inside the coal seam and a critical parameter estimating the CO 2 injectivity, determines the success of CO 2 storage in coal seam . The deep coal seam and coal bed methane (CBM) exploration and development are difficult, but the deep coal seam is one of the geological media to potentially store huge amounts of CO 2 .…”

“…[1][2][3] The permeability of a coal seam, a key parameter reflecting the ability of fluids to flow inside the coal seam and a critical parameter estimating the CO 2 injectivity, determines the success of CO 2 storage in coal seam. [4][5][6] The deep coal seam and coal bed methane (CBM) exploration and development are difficult, but the deep coal seam is one of the geological media to potentially store huge amounts of CO 2 . 7-10 However, deep coal seams with a low in situ permeability and CO 2 injection to the deep coal seam will be in the supercritical state, that is, ScCO 2 , which affect the permeability of coal seam induced by ScCO 2 adsorption-swelling.…”