Qualitative and quantitative evaluation of the alteration of micro-fracture characteristics of supercritical CO2-interacted coal
Graphical abstract
Introduction
The enhanced coal bed methane (ECBM) recovery through CO2 geo-sequestration has become popular, as it provides a viable solution for the present energy crisis. Energy consumption statistics confirm that the world is moving towards utilizing the untapped ECBM resources, rather than burning traditional fossil fuels. In fact, the world natural gas consumption is estimated to be increased by 43% from 2015 to 2040, whereas the world petroleum and other liquid fuel consumption will grow only by 18% [1].
As the CO2 geo-sequestration in coal reservoirs becomes increasingly apparent, it is important to evaluate the CO2-induced coal micro-structural alterations under high pressure/temperature conditions. When the overburden pressure and typical geothermal gradient are considered, CO2 at reservoir conditions is mostly above its critical point and therefore, the injected CO2 turns into its supercritical state at in-situ conditions. Besides, coal seams are unique formations, made with partially decomposed vegetation and consequently, they consist of complex micro-structures with large heterogeneities and anisotropies. Hence, the supercritical CO2 (S−CO2) – coal interaction causes complex chemical and physical alterations in coal reservoirs, inferring the necessity of investigating the resultant micro-structural alterations.
The principal interaction of S−CO2 with coal is the physical adsorption to the coal matrix [2], which causes coal matrix swelling. The S−CO2-induced swelling alters 3D anisotropic features of coal, including porosity, fracture characteristics, mineral/maceral compositions and the resultant permeability and strength characteristics. Perera et al. [3] reported that S−CO2 interaction significantly reduces the coal permeability, due to associated higher swelling potential of S−CO2. Meng and Qiu [4] revealed that S−CO2 treatment reduces the mechanical properties of coal, including rock cohesion, peak strength, static and dynamic Young’s moduli. Zhang et al. [5] observed partial dissolution of calcite and increment of absolute porosity and connectivity in S−CO2-interacted, water-saturated coal. Moreover, researchers have observed the irreversible changes in coal micro-structure due to formation of micro-fractures, as a result of S−CO2 adsorption-induced drying and shrinkage of coal, and differential swelling of maceral and mineral phases [6,7].
However, since there is no standard period to treat the coal specimens with S−CO2 prior to analyses, researchers have used different treatment periods varying from hours to months, inferring difficulties of interpreting and comparing the results found in literature. Hence, it is important to evaluate the significance of temporal effect on coal-S−CO2 interaction. Even though in-situ stress applied on the coal matrix plays a significant role in controlling the coal-S−CO2 interaction and the resultant micro-structural alterations, most of the non-visual and visual characterization techniques such as mechanical characterization experiments like uni-axial compression tests [[8], [9], [10]], S−CO2 adsorption experiments [[11], [12], [13]], mineral dissolution/precipitation experiments [14] and some micro-fracture characterizing experiments [15,16] were conducted under unconstrained conditions, due to technical difficulties caused by unique experimental methods. Coal-S−CO2 interaction under unconstrained condition can alter the S−CO2-induced swelling mechanisms in coal, thus the outcome of those studies would be different from the actual confined conditions. Therefore, it is essential to observe and quantify what is really happening in S−CO2-interacted coal micro-structure under unconstrained condition to interpret such results and to correlate them to actual confined conditions.
In this study, heterogeneous coal specimens were interacted with S−CO2 for different time periods, under unconstrained conditions and imaged to characterize the S−CO2-induced temporal alterations in coal micro-structure. A detailed chemical analysis, including a SEM-EDS and an XRD analysis has been carried out to identify the elemental composition and the mineral structure. The fracture evolution caused by coal-S−CO2 interaction and the influence of mineral/maceral spatial distribution on the preferable induced-fracture orientations have been visualized with micro computed tomography (micro-CT), high resolution scanning image microscopy (SEM) and optical microscopy. Fracture characteristics, including fracture fraction (i.e. the ratio between total volume and the segmented fracture volume), fracture-matrix interface area and fracture geometry have been analysed, based on a rigorous micro-CT image-based quantification. Finally, the micro-structural alterations under unconstrained conditions are compared with that of confined condition and the causative factors for the observed differences, including swelling mechanisms are comprehensively discussed.
Section snippets
Coal samples
A high volatile bituminous coal sample was collected from Tashan coal mine, Shanxi province of China. Results from a proximate analysis are given in Table 1. Based on gravimetric methods, the average dry density and the porosity of the samples are 1550 kg m−3 and 5.1%, respectively. The samples used for chemical analyses and different visual characterization techniques have been obtained from adjacent locations of the same coal block to minimize the difference of sample properties. A core
Chemical characterization
As identified from SEM-EDS analysis, the selected coal sample is highly heterogeneous and contains several chemical constituents with varying percentages (see Fig. 2). Except carbon and oxygen that form the coal maceral phase, Al (8.48% by weight) and Si (8.38% by weight) are the other two significant inorganic elements found in the analysed coal sample. It is inferred that these two elements contribute to the coal composition at similar proportions, as the atomic percentages are also
Conclusions
High resolution visualization techniques were used to characterize the S−CO2 interaction induced temporal and spatial evolution of coal micro-fracture network, under unconstrained conditions. Based on the experimental results, the following conclusions are made:
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High-resolution imaging techniques like SEM imaging are essential for the precise characterization of S−CO2-induced complex coal structural alterations, because full scale scanning of coal samples limits the micro-CT image resolution,
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