Measurement of CO₂ adsorption capacity with respect to different pressure and temperature in sub-bituminous: implication for CO₂ geological sequestration

Abstract

CCUS (carbon capture, utilization, and storage) technology is regarded as a bottom method to achieve carbon neutrality globally. CO₂ storage in deep coal reservoirs serves as a feasible selection for CCUS, and its storage potential can be attributed to the CO₂ adsorption capacity of the coal. In this paper, a series of CO₂ adsorption isotherm experiments were performed at different pressures and temperatures in sub-bituminous coal from the southern Junggar Basin (reservoir temperature ~25.9°C and pressure ~3.91 MPa). In addition, the high-pressure CO₂ adsorption characteristics of the southern Junggar Basin coal were characterized using a supercritical D-R adsorption model. Finally, the CO₂ storage capacities in sub-bituminous coal under the in situ reservoir temperature and pressure were analyzed. Results indicated that the excess adsorption capacities increase gradually with increasing injection pressure before reaching an asymptotic maximum magnitude of ~34.55 cm³/g. The supercritical D-R adsorption model is suitable for characterizing the excess/absolute CO₂ adsorption capacity, as shown by the high correlation coefficients > 0.99. The CO₂ adsorption capacity increases with declining temperature, indicating a negative effect of temperature on CO₂ geological sequestration. By analyzing the statistical relationships of the D-R adsorption fitting parameters with the reservoir temperature, a CO₂ adsorption capacity evolution model was established, which can be further used for predicting CO₂ sequestration potential at in situ reservoir conditions. CO₂ adsorption capacity slowly increases before reaching the critical CO₂ density, following a rapid decrease at depths greater than ~800 m in the southern Junngar Basin. The research results presented in this paper can provide guidance for evaluating CO₂ storage potential in deep coal seams.

Graphical abstract

Keywords

CCUS / CO₂ adsorption capacity / pore structure / CO₂ geological sequestration / Junggar Basin

1 Introduction

CCUS (carbon capture, utilization, and storage) refers to the process of separating CO₂ from industrial processes or the atmosphere and then directly utilizing or injecting it into the stratum to achieve permanent CO₂ emission reduction (Andersen et al., 2020; Leflay et al., 2021; Zheng et al., 2022a). According to the International Energy Agency (IEA) sustainable development scenario goals, CCUS will achieve a 15% cumulative carbon emission reduction by 2070. The key technical links of CCUS include CO₂ geological storage potential evaluation, CO₂ emission sources investigation, and source-sink matching optimization (Fig.1). Potential geological storage sites include depleted oil and gas reservoirs, deep brine layers, deep unrecoverable coal seams, shale reservoirs and so on (Huo et al., 2017; Zheng et al., 2020).

CO₂ storage in deep coal beds has a desirable dual benefit both in natural gas efficiency development and excess carbon reduction (Zhang et al., 2013; Sun et al., 2018; Liu et al., 2019a). There are three main controlling factors in the CO₂ storage potential of coals (Han et al., 2019; Zhou et al., 2019; Zhou et al., 2021a). The first is the coal seam geological features, such as sedimentary environment, burial history, and tectonic development. The second is the in situ coal reservoir characteristics, such as porosity, permeability, and adsorption capacity. The third is on-site engineering construction, such as injection pressure and rate.

CO₂ adsorption capacity is essential in controlling the coal seam storage potential. The isothermal adsorption experiments are usually considered an essential tool for describing the adsorption behavior characteristics of coals (Liu et al., 2017; Zheng et al., 2018; Luo et al., 2019; Sun et al., 2021; Zhou et al., 2021b). With in-depth study of the six types of isothermal adsorption curves, adsorption theories to explain the curves have been successively developed, such as the Langmuir equation, BET equation, FHH equation, Kelvin equation, Polanyi theory, D-R model, and so on (Du et al., 2021; Shi et al., 2022; Liu et al., 2022).

Recently, the supercritical phase effect on the CO₂ adsorption behavior in coals continues to be widely studied (Perera et al., 2011; Wang et al., 2015; Dutka, 2019; Ma et al., 2022). The volumetric isotherm adsorption experiment results indicate that methane adsorption volume decreases supercritical CO₂ pretreatment (Zhou et al., 2018). Hui et al. (2019) performed gravimetric isotherm adsorption experiments to analyze the influence of supercritical CO₂ exposure on adsorption capacity. The experimental results indicated that the maximum CO₂ adsorption capacity declined as the supercritical phase gas treatment continued. Additionally, some literature has shown that after supercritical CO₂ treatment, the CO₂ adsorption capacity of coal and the overall adsorption heat decrease. Furthermore, the degree of CO₂ adsorption capacity decrease appears to be related to coal rank (Liu et al., 2019b; Zhang et al., 2019). It should be noted that few researchers have considered the effect of supercritical CO₂ adsorption volume on CO₂ storage capacity in coals. The inadequacy in CO₂ adsorption curves with respect to temperatures may contribute to an inaccurate evaluation of CO₂ storage capacity in coals.

In this paper, four temperatures (30°C, 45°C, 60°C, and 75°C) were chosen to characterize the CO₂ adsorption capacity of sub-bituminous coal taken from the southern Junggar Basin. Supercritical D-R adsorption theory was introduced for adsorption data fitting, and the temperature effect on adsorption capacity was estimated successively. Finally, an absolute/excess CO₂ adsorption capacity prediction model was established, which was used to estimate the CO₂ geological storage potential under in situ conditions – from the perspective of adsorbed phase.

2 Sampling and experiments

2.1 Samples

In this study, coal was obtained from a coal mine (depth ~400 m) in the southern Junggar Basin. The petro-physical parameters are listed in Tab.1. R_o and proximate analysis experiments were performed under the Chinese standard GB/T 6948-2008. The R_o of the selected coal values as ~0.53%, classifying it as sub-bituminous coal. The porosity (Φ) of the coal was measured using helium porosity experiments, following the Chinese standard GB/T 34533-2017. The helium-measured porosity of the coal was 6.43%, as shown in Tab.1.

2.2 Pore structure characterization experiments

Low-temperature N₂ gas adsorption (LT-N₂GA) measurements were performed using a Micromeritics ASAP-2020 instrument, following the Chinese standard of SY/T6154-1995. For sample preparation, large-sized coals were crushed to 60−80 mesh, then dried at 110°C for 12 h in an oven to remove impurity gas and moisture. After sample drying, all coal powders were evacuated for complete outgassing in a high vacuum system for 5 h. Using N₂ gas with a purity of 99.99%, adsorption/desorption isotherms of coal powders were measured successively with the relative pressures (P/P₀) of 0.009–0.998 at a temperature of 77.4 K. Based on the N₂ adsorption branch, the Barrett-Joyner-Halenda (BJH) model and multi-point Brunauer-Emmett-Teller (BET) model were applied to calculate the pore volume and specific surface area, respectively (Yao et al., 2008).

NMR is a quick, non-destructive assessment of pore structure characterization (Liu et al., 2020; Zhou et al., 2020, 2022; Lin et al., 2021; Zhao et al., 2022). The workflow of the NMR experiments for pore structure characterization was as follows.

a) A standard-size core (radius ~12.5 mm, length ~ 50 mm) was drilled from the large lump of coal.

b) 72-h vacuum drying occurred and was sufficient to remove all the moisture in the coal cores.

c) Full water saturation of coal cores was obtained by using pressurized water extraction.

d) The NMR T₂ relaxation of coal cores was obtained using a Niumag-60 NMR apparatus. Note that the NMR parameters set in this study were the same as the previous study (Zheng et al., 2022b), with 0.3 ms echo spacing, 10000 echo numbers, and 64 repetition times.

2.3 High-pressure CO₂ adsorption measurements

In this study, high-pressure CO₂ adsorption isotherm measurements were performed by the volumetric method. The experimental CO₂ excess adsorption capacities were estimated based on the ideal gas equation of state. The CO₂ adsorption amount of the coal sample is the difference between the total injected gas amount and the free gas amount in the cells. First, the coal powder (with a size of 60−80 mesh) underwent dry treatment for 72-h at a temperature of 110°C before the experiment to remove moisture. Then, the high-pressure CO₂ adsorption isotherm measurements were performed by following the Chinese standard GB/T 19560-2008. Four temperatures of 30°C, 45°C, 60°C, and 75°C were chosen to characterize the CO₂ adsorption capacity of sub-bituminous coal.

The CO₂ adsorption capacities direct estimates from adsorption isotherm measurements were defined as the Gibbs adsorption equation. The D-R adsorption model is often used for supercritical CO₂ adsorption capacity characterization (Sakurovs et al., 2007). The D-R model is expressed follows:

(1)n_{\mathrm{e}\mathrm{x}}=n_0\left(1-\frac{{\rho }_{\mathrm{g}}}{{\rho }_{\mathrm{a}}}\right)\exp \left[-D{\left(\ln \frac{{\rho }_{\mathrm{a}}}{{\rho }_{\mathrm{g}}}\right)}^2\right]+ k{\rho }_{\mathrm{g}},

where n_ex is the excess adsorption capacity, cm³/g; n₀ is the maximum adsorption capacity, cm³/g; D is the affinity coefficient of coal-CO₂; ρ_g is the density of the free phase CO₂, g/cm³; ρ_a is the density of the adsorbed phase CO₂, set as 1.0 g/cm³ (Day et al., 2008); k is the correction coefficient due to the CO₂ swelling-adsorption. The density of free phase CO₂ under specific conditions was directly estimated from the Physical Property Calculation Query platform. In addition, the absolute adsorption capacity of CO₂ (n_ab) for typical coal was calculated as follows:

(2)n_{\mathrm{a}\mathrm{b}}=\frac{n_{\mathrm{e}\mathrm{x}}}{1-{\displaystyle \frac{{\rho }_{\mathrm{g}}}{{\rho }_{\mathrm{a}}}}}.

3 Results and discussion

3.1 Pore structure characterization of coal

Fig.2(a) presents the LT-N₂GA experimental curves of the coal. Based on the isotherm adsorption curve classification described in IUPAC, the adsorption/desorption curves in the studied coal as classified as H3. In the curves, the adsorption branch slowly rose when the relative pressure < 0.9 but rapidly increased in the relative pressure range of 0.9−1.0, with a weak hysteresis loop at approximately P/P₀ ~0.5. The typical adsorption/desorption curves of the coal indicate a pore morphology of open plate-like pores. The LT-N₂GA BET surface area and BJH pore volume calculated values were 1.428 m²/g, and 2.274 × 10⁻³ cm³/g, respectively.

As shown in Fig.2(b), the surface area distribution of the coal exhibited a significant peak at approximately 1−2 nm, indicating the primary development of pores ranging in size from 1 to 2 nm. LT-N2GA experimental results suggest a significant contribution to the surface area from the micro-pores, as obtained from N₂ vapor adsorption at 273 K. The T₂ spectrum of the selected coal under full water saturation is shown in Fig.3, exhibiting three distinct spectra peaks. For left-to-right, the peaks located at < 5 ms, 20−200 ms, and > 200 ms correspond to adsorption pores, seepage pores, and fractures, respectively – based upon the NMR pore size classification in Zheng et al. (2019).

3.2 CO₂ isothermal adsorption of coal

Fig.4 represents the high-pressure CO₂ adsorption data for different temperatures. Results show that the excess CO₂ adsorption capacity was at its maximum values in the pressure range of 6−8 MPa under specified temperature conditions. After that, the excess CO₂ adsorption capacity decreased with pressure increase. Note that the maximum excess CO₂ adsorption capacity gradually decreased with temperature increase.

Taking 30°C high-pressure CO₂ adsorption results as an example, the maximum excess adsorption capacity was ~34.55 cm³/g. The CO₂ excess adsorption amount increased gradually with the injection pressure before reaching an asymptotic maximum magnitude of ~34.55 cm³/g. After that, the curves present irregular descents: the first represents a rapid drop from 6 to 10 MPa and then follows a long adsorption tail. Note that the descents had different pressures where the tail formed at different temperatures. In addition, the excess adsorption capacity decreased with temperature at a relatively low-pressure range. Conversely, the higher temperature indicates a greater excess adsorption capacity within a high pressure scenario.

This paper used the free phase density instead of the equilibrium pressure to fit the supercritical CO₂ adsorption data, creating D-R adsorption model. An essential first step in D-R adsorption model fitting is determining the free-phase CO₂ density under specific pressures and temperatures. The estimated free CO₂ densities in this paper are listed in Tab.2. By introducing Eq. (1), the fitting relationship between excess adsorption capacity and free CO₂ densities was displayed is Fig.5.

The curves exhibit two distinct trends: the first represents a rapid increase in the CO₂ density of 0−0.2 g/cm³, following a quick decline. Compared with the excess adsorption curves in Fig.4, the excess adsorption capacity under every experimental point declined with increasing temperature. In addition, the excess adsorption curves at different temperatures show no intersections.

The fitting results of the D-R adsorption model are listed in Tab.3. Fitting results show that all correlation coefficients are greater than 0.99, indicating that the D-R adsorption model can accurately describe the gaseous to supercritical CO₂ adsorption characteristics in sub-bituminous coal. In the 30°C–75°C temperature range, the maximum adsorption capacity n₀ ranged from 51.725 to 27.651 cm³/g, D ranged from 0.048 to 0.068, and k ranged from −2.096–−10.902.

The relationships between the D-R adsorption model fitting parameter vs. temperature are displayed in Fig.6. n₀ shows a statistically significant positive linear relationship with the reciprocal of temperature (R² ~0.9798). A linear fit to the relationship of D to temperature to the 2 photons yielded the relationship between temperature and D. In addition, k shows a statistically significant positive linear relationship with experimental temperature, with a high correlation coefficient ~0.9810. The exceptional linear relationships between the experimental temperature and the supercritical D-R adsorption model fitting parameters indicate the reliability of the modified Eq. (1).

3.3 CO₂ adsorption capacity of the southern Junggar Basin

As discussed in Section 3.2, the supercritical D-R adsorption model fitting parameters exhibit highly significant linear relationships with reservoir temperature. Thus, it is acceptable to use the temperature parameter instead of other parameters such as maximum adsorption capacity n₀, D, and k, to describe Eq. (1). By integrating the expressions in Fig.6 with the supercritical D-R adsorption model, Eq. (1) can be rewritten as

(3)\begin{array}{rl}{n}_{\mathrm{e}\mathrm{x}}=& \left(54091{\displaystyle \frac{1}{T}}-127.57\right)\left(1-{\displaystyle \frac{{\rho }_{\mathrm{g}}}{{\rho }_{\mathrm{a}}}}\right)\\ \times & \exp \left[-\left({\displaystyle \frac{0.0013T^2}{1000}}-0.073\right){\left(\ln {\displaystyle \frac{{\rho }_{\mathrm{a}}}{{\rho }_{\mathrm{g}}}}\right)}^2\right]\\ +& (-0.1977T+58.326){\rho }_{\mathrm{g}}.\end{array}

To estimate the CO₂ adsorption capacity in the southern Junggar Basin with respect to reservoir depth, the first essential step is to estimate the free phase CO₂ densities under reservoir conditions. In this study, the geological conditions were assumed as 1) a geothermal gradient of 20°C/km and reservoir pressure gradient of 9.78 MPa/km (Liu et al., 2022); 2) sub-bituminous coal whose adsorption characteristics can be represented by the experimental coal sample in this paper; 3) water saturation was not considered. First, the temperatures and pressures at different depths were calculated based on the geothermal and pressure gradients, respectively. In addition, CO₂ density curves at depth were obtained from the Physical Property Calculation Query. Finally, the absolute adsorption capacity changes of sub-bituminous coal under in situ reservoir temperature and pressure were estimated based on Eq. (3).

The changing characteristics of CO₂ density over reservoir depth are shown in Fig.7. The CO₂ density increased monotonically before reaching a critical point, and then increased slowly until reaching an asymptotic maximum magnitude. After calculating the CO₂ density curve, Eq. (1) and Eq. (3) were combined to estimate the absolute adsorption capacity of sub-bituminous coal under reservoir conditions in the southern Junggar Basin, which is shown in Fig.8. Simulation results indicate the absolute CO₂ adsorption capacity first increased then declined with burial depth. Han et al. (2019) explained the changes in the absolute CO₂ adsorption capacity with burial depth mainly because of the CO₂ phase change. CO₂ adsorption capacity has a strong link with CO₂ density, suggesting that supercritical adsorption behavior on coals may be related to density variation.

4 Conclusions

In this study, four temperatures of 30°C, 45°C, 60°C, and 75°C were chosen to investigate the CO₂ adsorption capacity of sub-bituminous and CO₂ sequestration potential of the southern Junggar Basin. The main conclusions are as follows.

1) The pore structure of the sub-bituminous is characterized by adsorption pores, with the T₂ relaxation ranging from 0.1 to 2.0 ms. Low-temperature nitrogen adsorption/desorption curves indicate a pore morphology of open plate-like pores.

2) The excess adsorption capacity increases gradually with the injection pressure before reaching an asymptotic maximum magnitude of ~34.55 cm³/g. The supercritical D-R adsorption model is suitable for characterizing the excess/absolute CO₂ adsorption capacity, as evident by the high correlation coefficients > 0.99.

3) The CO₂ adsorption capacity of the coal decreases with increasing temperature. CO₂ adsorption capacity increases at a lower rate before reaching the critical CO₂ density, following a decreasing trend at depths greater than ~800 m in the southern Junngar Basin.

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