Full Length ArticleChanges in the microstructure of low-rank coal after supercritical CO2 and water treatment
Introduction
The effect of the increasing concentration of greenhouse gases (primarily CO2) in the atmosphere on the global environment is considered to be one of the most important problems facing society at present. Thus, it is extremely urgent to investigate how to reduce CO2 emission in the atmosphere. To solve this problem, CO2 capture and sequestration (CCS) technology has been utilized considerably in recent years. This is because CCS technology is thought to exert a significant effect, and it is regarded as an effective approach to immediately influence the CO2 concentration level in the atmosphere [1], [2]. The available CCS technologies presently include geological sequestration (e.g., petroleum and natural gas fields, unworkable coal fields, and deep salt marshes), ocean sequestration (directly released into marine waters or seabed), and solidification of CO2 into inorganic carbonates.
The geological sequestration of CO2 in unworkable coal seams is considered to be one of the most promising methods for realizing long-term sequestration [3]. In recent years, the geological sequestration technology of CO2 in coal seams has been widely considered [4]. Unworkable deep coal seams (>800 m) are regarded as one of the most common treatment sites in the world and are usually located in the vicinity of large-scale point sources for CO2 emission. To understand more clearly the process of CO2 sequestration in coal and predict the reliability of long-term CO2 sequestration in coal, it is necessary to evaluate the effect on coal seams after the interaction between the coal seams and CO2. Research has shown that the injection of CO2 into deep unworkable coal seams for sequestration can effectively decrease the CO2 emission in the atmosphere and relieve the greenhouse effect [5]. Additionally, CO2 can be adsorbed by coal in significant quantities, showing strong adsorption capacity and long-term stability. Moreover, natural gas stored in coal seams is a gaseous fossil fuel that is cleaner than other fossil fuels (e.g., coal, oil) [6]. CO2 shows a significant displacement effect on coalbed methane (CBM) after being injected into coal seams to realize enhanced coalbed methane (ECBM) recovery [7], [8]. Therefore, the geological sequestration technology of CO2 in unworkable coal seams shows multiple benefits.
However, CO2 exists in a supercritical state on temperature and pressure conditions of deep coal seams, showing complex properties totally different from those in the subcritical state [9]. Thus, before CO2 is geologically stored in deep coal seams, it is necessary to explore the effect on coal seams after supercritical CO2 (ScCO2) is injected therein. At present, the adsorption and desorption mechanisms, mechanical properties, and changes in the pore structures of coal seams due to the injection of ScCO2 have gradually received an increasing amount of attention from researchers across the world. After the injection of CO2 into coal seams, some minerals and organics in coal are possibly extracted to trigger the expansion and shrinkage of coal masses, even resulting in the softening and plasticization effects of coal masses [10], [11], [12], [13]. Moreover, owing to the water contained in the coal seams, injected CO2 is transformed into carbonic acid and, thus, minerals are dissolved, leading to changes in the porosity, connectivity of pores, and permeability [14]. All these changes alter the microstructures of coal seams, ultimately influencing the sequestration capacity of coal seams, injection process of CO2, and output process of CBM [15]. Therefore, it is still necessary to further explore the change in the microstructures of coal seams after the injection of CO2. Through the acoustic emission (AE) test and triaxial compression test, Meng et al. [16] found that after ScCO2 treatment, various mechanical parameters (e.g., dynamic Young’s modulus, static Young’s modulus of coal, cohesion, peak strength of rocks) are significantly reduced. Therefore, it is thought that ScCO2 can decrease the mechanical strength of coal. By investigating the changes in the morphologies of pores in high-, middle-, and low-rank coal seams under the effect of ScCO2, Zhang et al. [2] found that ScCO2 promotes the formation of effective pore spaces within coal of all ranks and, thus, improves the connectivity of the pore system. Additionally, ScCO2 strengthens the desorption performance of the coal samples for gas, validating the result of the morphologies of pores.
Although ScCO2 is crucial for the change in the coal structures, another fluid, H2O, also appears in coal seams injected with CO2. The synchronous existence of the two fluids (ScCO2 and H2O) exerts a coupling effect on coal. By conducting a series of experiments on coal samples exposed to CO2 under 3 MPa, Vishal et al. [17] suggested that, compared with single-fluid (CO2 or H2O) treatment, the samples are damaged earlier in the loading stage and present the largest reduction in the strength under the CO2–H2O interaction. Liu et al. [18] performed simulation experiments on the coal samples with four different ranks separately at the temperatures of 32–80 °C and under the pressures of 9–20 MPa to explore the changes in the morphologies of pores in coal. The result revealed that the positive changes in the pore size, shape, and roughness contribute to improving the specific surface area, connectivity, storage volume, etc. However, Vishal only considered the effect of subcritical CO2 (under 3 MPa) and H2O while the influence of ScCO2–H2O interaction on CO2–ECBM technology still remains to be explored further. Liu C.J. ignored the effect of water. However, natural coal seams generally contain water, whereas the existence of water possibly strengthens the effect of CO2 injection on the stability of coal seams [19]. Busch et al. [20] once reported that the existence of water hinders the adsorption of coal for CO2, owing to the competition of H2O molecules with CO2 molecules for the adsorption sites. Therefore, research into the effect of the existence of water on the microstructures of coal is crucial. However, the existing research related to the field is scant, and not enough attention has been given to this field. Numerous experiments reported so far are all performed under the treatment of a single fluid, CO2, and the CO2–H2O coupling effect on coal needs to be explored further [21], [22], [23]. By exploring the effect of the ScCO2–H2O interaction on the mechanical strength of coal samples with different particle sizes, Liu et al. [21] found that some carbonate minerals in the coal samples are dissolved and scoured by water. This leads to an increase in the true density within the micropore range and the enlargement of the pore volume, increasing the porosity. Du et al. [22] surveyed the effect of the CO2–H2O–Coal interaction on mineral compositions and pore structures under different pressures, temperatures, and times. The result showed that the CO2–H2O–Coal interaction can greatly increase the solubilities of Ca, S, Mg, K, Si, Al, Fe, and Na and the contents of quartz and kaolinite, as main secondary minerals, both slightly rise in various coal samples. Zhang et al. [23] observed the effects of CO2, H2O, and CO2–H2O interaction on bituminous coal. The result indicated that a single CO2 or H2O fluid weakens the strength of coal while the CO2–H2O coupling effect possibly triggers the greater change in the strength, which significantly influences the overall stability of the system. In practical field operation, coal seams contain water. Although researchers have identified water as an influencing factor in recent years, the change in the microstructures of coal samples under the ScCO2–H2O–Coal interaction still has not been comprehensively and systematically explored. Therefore, it is quite necessary to analyze the changes in the microstructures (pore structure and mineral composition) of the coal samples before and after treatment by comparing CO2 treatments on coal samples with and without water in the supercritical and subcritical states.
The change in the microstructures of the coal samples induced by the ScCO2–H2O coupling effect on the coal samples was experimentally investigated. To more favorably explore the interaction mechanism between ScCO2 and H2O on the microscopic pore structures of low-rank coal samples and further evaluate the CO2 sequestration capacity in deep coal seams, the low-rank coal samples were partitioned into dry and water-saturated groups, which were subjected to CO2 treatment under 3, 6, and 9 MPa, respectively. As a typical porous medium, coal has received considerable attention, as nuclear magnetic resonance (NMR) is applied to porous media. The transverse relaxation (T2) distribution based on NMR is closely related to the pore structures of coal, which allows the qualitative analysis of the change in the pore structures of coal samples before and after treatment under different pressures of CO2. Additionally, differences in the changes in the pore structures of coal samples in dry and water-saturated states can be attained. X-ray powder diffraction (XRD) can be used to analyze the change in the minerals in coal samples. Different from the dry coal samples, more carbonic acid is formed in the water-saturated coal samples, owing to the injection of CO2. Furthermore, through testing and analysis by using XRD technology before and after CO2 treatment, the change in minerals in the coal samples can be obtained.
Section snippets
Coal sample preparation
Bituminous coal (low-rank coal) taken from Liaoning Province, China was used in the experiment. The bulk coal samples were taken from the fresh working face and immediately sealed, packaged, and sent to the laboratory. The coal blocks were processed into cylindrical samples with the dimensions of Φ50 mm × 100 mm and fine particles in the laboratory. Based on the intuitive observation of the development degrees of the pores and fractures, six similar coal samples were selected from the
XRD
XRD semi-quantitative analysis confirmed that mineral compositions of the samples included clay minerals, quartz, calcite, siderite, pyrite, and other minerals. In the test results, the overall mineral compositions amounted to 100%; that is, the percentage of each type of minerals depended on the contents of the other minerals. The mineral compositions of all the tested coal samples obtained by the XRD analysis are shown in Fig. 3, Fig. 4. In the untreated low-rank coal samples, calcite
Mechanism of the interactions of CO2 and H2O with low-rank coal
The change in the microstructures of unworkable coal seams, as the site for long-term CO2 sequestration, plays an important role in the gas adsorption, desorption, diffusion, and seepage in the coal seams [2], [36]. According to the test results, the porosity of coal samples increases, and some minerals are dissolved after CO2 treatment. It can be inferred that ScCO2 triggers small pores to be transformed into large pores, and some new pores and fractures are initiated, which are favorable for
Conclusions
In this paper, low-rank bituminous coal samples were divided into dry and water-saturated groups, which were separately subjected to CO2 treatment of 3 MPa, 6 MPa, and 9 MPa (supercritical state). By applying NMR and XRD testing methods, the pore microscopic structures and minerals in the coal samples were separately analyzed. The interaction mechanism between CO2 and H2O with low-rank coal samples and its influence on CO2 geological sequestration were also discussed, as summarized below:
- (1).
After
CRediT authorship contribution statement
Yao Song: Data curation, Investigation, Writing - original draft. Quanle Zou: Writing - review & editing. Erlei Su: Conceptualization, Writing - review & editing. Yongjiang Zhang: Project administration, Resources. Yingjun Sun: Resources.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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