Low-field NMR application in the characterization of CO₂ geological storage and utilization related to shale gas reservoirs: a brief review

Abstract

CO₂ geological storage and utilization (CGSU) is considered a far-reaching technique to meet the demand of increasing energy supply and decreasing CO₂ emissions. For CGSUs related to shale gas reservoirs, experimental investigations have attracted variable methodologies, among which low-field NMR (LF-NMR) is a promising method and is playing an increasingly key role in reservoir characterization. Herein, the application of this nondestructive, sensitive, and quick LF-NMR technique in characterizing CGSU behavior in shale gas reservoirs is reviewed. First, the basic principle of LF-NMR for ¹H-fluid detection is introduced, which is the theoretical foundation of the reviewed achievements in this paper. Then, the reviewed works are related to the LF-NMR-based measurements of CH₄ adsorption capacity and the CO₂-CH₄ interaction in shale, as well as the performance on CO₂ sequestration and simultaneous enhanced gas recovery from shale. Basically, the reviewed achievements have exhibited a large potential for LF-NMR application in CGSUs related to shale gas reservoirs, although some limitations and deficiencies still need to be improved. Accordingly, some suggestions are proposed for a more responsible development of the LF-NMR technique. Hopefully, this review is helpful in promoting the expanding application of the LF-NMR technique in CGSU implementation in shale gas reservoirs.

Graphical abstract

Keywords

CO₂/CH₄ competitive adsorption / shale gas reservoir / CO₂ geological storage / gas recovery enhancement / low-field NMR

1 Introduction

Worldwide climate change poses a major challenge to the current situation of energy consumption, and accordingly much attention has been drawn to the development of comprehensive technology as a way of enhancing energy supply and simultaneously reducing carbon emissions (Abdussalam et al., 2021; Qiu et al., 2021). In this context, a technique called CO₂ geological storage and utilization (CGSU) has raised increasing concerns in recent years because it is a win–win strategy and typically promotes synthetic rewards, namely, acquiring energy from geological formations, that is, the economic perspective, and trapping CO₂ in underground strata, namely, the environmental demand (Kirli and Fahrioglu, 2019; Wang et al., 2020; Duguid et al., 2021; Fan et al., 2021; Hu et al., 2021; Pereira et al., 2021). Basically, shale gas reservoirs have been widely accepted and recognized as a suitable geological target to deploy the CGSU technique; however, this method is not mature enough to experience large-scale field promotion and implementation (Streich et al., 2010; Liu et al., 2013; Zhao et al., 2020; Cheng et al., 2021; Liu et al., 2021a; Liu et al., 2021b; Zhao et al., 2021). As a result, the shale-based CGSU is receiving considerable scientific investigation (Xie et al., 2014; Middleton et al., 2015; Sun et al., 2017; Shi et al., 2018; Xie et al., 2021), of which low-field NMR (LF-NMR) plays an increasingly important role among a number of methodologies during related experimental works (Liu et al., 2017, 2019a, 2019 b; Tian et al., 2020).

In the shale-based CGSU process, CO₂ is first captured and purified from exhaust gases of fossil fuel combustion or other industrial production, after which it is injected into underground shale formations, aiming at 1) enhancing gas recovery by CH₄ displacement during CO₂/CH₄ adsorption in shale and, 2) reducing CO₂ emissions by CO₂ sequestration during CO₂/CH₄ interactions in shale (Fig.1) (Godec et al., 2014; Liu et al., 2017; Liu et al., 2019a; Keles et al., 2020; Rani et al., 2020). Essentially, CGSU behavior in shale reservoirs is a result of complicated multiffluid coaction under a thermohydromechano-chemical (THMC) multifield coupling environment (Fatah et al., 2020; Iddphonce et al., 2020; Zhou et al., 2020), where fluids are the conative detection targets for LF-NMR measurements. Regarding CGSU operation in shale gas reservoirs, fluids mainly include H₂O, CH₄, and CO₂ after external CO₂ is injected into shale reservoirs underground (Liu et al., 2016; Luo et al., 2019; Zhou et al., 2020). In other words, the critical point to decode CGSU behavior is to describe the multicomponent and multiphase fluids in shale gas reservoirs. For CH₄ and CO₂, adsorbed and free phases exist, supplemented with few dissolved phases in H₂O, while H₂O is usually distributed as adsorbed and free phases in shale reservoirs (Gensterblum et al., 2014; Li et al., 2019). Moreover, the interaction of multicomponent and multiphase fluids therein is generally a dynamic process, with continuous CO₂ injection into shale gas reservoirs during a CGSU operation (Liu et al., 2019a, 2021b; Fatah et al., 2020). Under these circumstances, LF-NMR is expected to work in such complicated environments, providing new perspectives regarding CGSU studies in shale gas reservoirs.

Meanwhile, to the best of our knowledge, the application of LF-NMR in the CGSU field depending on shale gas reservoirs has not yet been systematically reviewed. Therefore, considering that LF-NMR is an emerging technique for studying the CGSU process in shale gas reservoirs, this brief review mainly concentrates on its current application status and possible future development trends regarding LF-NMR applications in studying CGSU behavior in shale gas reservoirs, mainly for experimental works. Herein, this review first provides an overview and discusses LF-NMR theory and then summarizes the already developed function of LF-NMR in recognizing and monitoring the coupling interaction of multicomponent and multiphase fluids related to CGSU behavior in shale gas reservoirs and accordingly proposes an outlook about this CGSU-related LF-NMR technique. Hopefully, this brief review is helpful in deepening the knowledge on LF-NMR applications for studying CGSU behavior in shale gas formations and has referential significance to other CGSU investigations with other geological strata, such as coal, sandstone and salt rock.

2 Basic principle of LF-NMR

Basically, NMR behavior is induced by NMR-active nuclei (i.e., ¹H and ¹³C) in a magnetic field or exposed to pulsed radiofrequency (RF) irradiation (Hatzakis, 2019; van Beek, 2021). Therein, relaxation comes into being and is characterized as a complicated process whereby nuclei transition from an excited state to equilibrium, owing to the splitting of the nuclear spin levels (Zeeman effect) of an applied magnetic field (Cornillon and Salim, 2000). In general, NMR technology is regularly classified into three types, that is, high-field NMR (≥ 1.0 T), middle-field NMR (0.5–1 T) and LF-NMR (≤ 0.5 T), according to the magnetic field strength (Wang et al., 2021). Therein, compared to high-field NMR and middle-field NMR, the LF-NMR instrument not only requires low costs but also contains shields inside and does not need extra refrigeration; for example, the cost of high-field NMR is ~10 times that of LF-NMR (Wang et al., 2021). Moreover, in geology-related areas, LF-NMR is known for its nondestructive, sensitive, and quick properties in measuring targeted parameters of a certain rock, such as porosity, permeability and wettability (Yao and Liu, 2012; Yao et al., 2015; Yin et al., 2017; Sun et al., 2018; Guo et al., 2020; Liu et al., 2020a). Accordingly, a brief introduction regarding the basis of LF-NMR measurement and analysis is summarized to enhance the understanding of LF-NMR applications in CGSU investigations related to shale gas reservoirs and to promote this LF-NMR technique.

2.1 ¹H-fluid identification using LF-NMR

On the basis of precious achievements (Martin, 1995; Dunn et al., 2002; Levitt, 2015; McPhee et al., 2015), Liu et al. (2020b) summarized the processes of LF-NMR measurement-proton alignment, precession and dephasing, raw data and processed data, during the ¹H-fluid identification in porous media (Fig.2). First, the alignment of ¹H-fluid protons is stimulated by immersion into a constant magnetic field (B₀) (Fig.2(a)), followed by an incline of the aligned protons under an RF pulse and this phenomenon enables an oscillating magnetic field (B₁) orthogonal to the B₀ direction (Fig.2(b)). Basically, LF-NMR belongs to the submicroscopic field (between molecules) via spin-lattice relaxation (i.e., longitudinal relaxation time, T₁) and spin–spin relaxation (i.e., transverse relaxation time, T₂), in which T₂ is capable of identifying the number of ¹H atoms present in the ¹H-fluid (Seevers, 1966) and thus is widely adopted in LF-NMR investigations in geological fields, as well as the object of this review.

Afterward, the RF is stopped and the magnitude of the signal at each echo time (TE) is used to calculate the spin-spin relaxation time constant T₂, where the spins are dispersed in the transverse plane and processed at different rates. During this process, a Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence is regarded as the gold standard for T₂ mapping (Carr and Purcell, 1954; Meiboom and Gill, 1958), where the relaxation curve is sampled at several TE points and then fitted to a single exponential with a relaxation time T₂, as exhibited in Fig.3. This represents all dynamic processes that are not completely reversed by the 180° pulses where the spin-echo is formed and B₀ inhomogeneities are removed. Therein, the CPMG sequence involves taking measurements at different echo times (i.e., TE = 2τ) in an echo train to sample the T₂ decay curve, that is, the raw data (Fig.2(c)). Based on the raw data, inversions are needed to extract relaxation time distributions and reflect the ¹H-fluid information (Fig.2(d)). Accordingly, to seek the best possible solutions, several approaches were proposed, such as the L-curve method (Lawson and Hanson, 1974), generalized cross-validation (GCV) method (Golub et al., 1979), Butler-Reed-Dawson (BRD) method (Butler et al., 1981) and a uniform penalty (UPEN) function (Borgia et al., 1998). By comparison, Testamanti and Rezaee (2019) concluded that the BRD algorithm has good performance and is reliable for shale-based NMR investigation, among the four mentioned methodologies.

2.2 Relaxation phenomena and mechanisms of LF-NMR

In a typical T₂ measurement of ¹H-fluid in shale, the T₂ is codetermined by the bulk, surface and diffusion relaxations (Straley et al., 1994; Brown et al., 2001; Hirasaki et al., 2003; Washburn, 2014):

(1)\frac{1}{T_2}=\frac{1}{T_{2B}}+\frac{1}{T_{2S}}+\frac{1}{T_{2D}},

where the subscripts B, S, and D represent bulk, surface and diffusion relaxation, respectively. Therein, the bulk relaxation (T_2B) is regarded as an intrinsic property of the ¹H-fluid and is affected by the physical properties (e.g., viscosity), temperature and pressure. Surface relaxation (T_2S) occurs at the fluid-solid interface, which is related to the surface area to volume ratio (S/V) of the pores containing ¹H-fluid (Coates et al., 1999; Zhao et al., 2022):

(2)\frac{1}{T_{2S}}=\rho \left(\frac{S}{V}\right),

where ρ is the T₂ surface relaxivity (that is, the T₂ relaxing strength of the grain surfaces, unit: m/s).

In addition, the diffusion relaxation (T_2D) is the pore fluid relaxation induced by the proton spins diffusion across a magnetic field gradient (Coates et al., 1999), and it yields:

(3)\frac{1}{T_{2D}}=\frac{D{\left(\gamma G{T}_E\right)}^2}{12},

where D is the molecular diffusion coefficient (m²/s), γ is the gyromagnetic ratio of a proton (MHz/T), and G is the field-strength gradient (Gs/cm). This also indicates that the influence of T_2D on T₂ could be small enough to be ignored if the involved internal field gradient is homogeneous (G = 0 Gs/cm) because the 1/T_2D tends to be 0 in this situation.

Basically, regarding the CGSU investigations related to shale gas reservoirs, the LF-NMR was introduced to obtain the relaxation phenomena of ¹H fluid in shale reservoirs and thus to acquire the desired information for the CGSU behavior. Accordingly, this review mainly involves the LF-NMR function in the characterization of CH₄ adsorption, CO₂-CH₄ interactions, and the CO₂ storage ability as well as the recovery enhancement of shale gas during the CGSU operation.

3 LF-NMR measuring CH₄ adsorption capacity of shale

Basically, the identification ability of CH₄ adsorption capacity is the precondition that LF-NMR can be used in the CGSU-related investigations regarding shale gas reservoirs, since CO₂/CH₄ competitive adsorption acts as the driving force during this CGSU behavior (Godec et al., 2014; Liu et al., 2019a). The application of LF-NMR in measuring CH₄ adsorption behavior in shale is an emergent reality in recent years as an extension and supplement in addition to conventional approaches, such as the volumetric method and gravimetric method. To some extent, the LF-NMR measurement of CH₄ adsorption in shale is based on the characterization of CH₄ adsorption on coal using the LF-NMR technique (Yao et al., 2014). During the investigation on the CH₄ desorption process of coal measure shale, it is found that the relationship between gas pressure and the integrated T₂ amplitude for the adsorbed CH₄ demonstrates a Langmuir-like property and thus meets a Langmuir-like function exhibited in Fig.4, characterized by the LF-NMR technique (Tang et al., 2017). Afterward, depending on the LF-NMR measurement, Li et al. (2018) distinguished adsorption CH₄ and free CH₄ according to the T₂ spectra and built the ratio of adsorption CH₄ relative to free CH₄, which was verified by the isothermal adsorption experiment using volumetric method. Therein, a shorter T₂ spectra of 0.1–1 ms (peaked at 0.2 ms) was treated as the adsorption CH₄ in micropores, while the longer ones of 1–10 ms (peaked at 3 ms) was regarded as the free CH₄ in macropores (Li et al., 2018). Besides, Liu and Wang (2018) conducted the LF-NMR experiments to explore the absolute CH₄ adsorption of shale, which recognized three types of T₂ relaxation of CH₄ in the shale-filled system, i.e., 0.1–13 ms, 13–280 ms, and 280–2500 ms (Fig.5(a)) and accordingly calculated the integrated amplitudes of the adsorbed CH₄ on the pore surface, the free-state CH₄ in pores, and the bulk CH₄ at different pressures (Fig.5(b)). The comparison between the achievements made by Li et al. (2018) and Liu and Wang (2018) shows that the adsorbed CH₄ and free CH₄ correspond to variable T₂ spectra for different shale samples during the LF-NMR measurement.

In general, the emerging application of LF-NMR in measuring the CH₄ adsorption in shale is admissive and is regarded as reliable, which is guaranteed by the LF-NMR theory and is confirmed by the practical attempts. A systematic LF-NMR-based work organized by Yao et al. (2019) introduced the implementation strategy about the LF-NMR technique in identifying the multiphase CH₄ in shale, where the dynamic variation in adsorbed CH₄ and free CH₄ was performed with respect to different gas pressures. Therein, the accuracy of the LF-NMR-based CH₄ adsorption capacity is approved by the conventional gravimetric method, displayed in Fig.6(a) for Sample SZX and Fig.6(b) for Sample XWX (Yao et al., 2019). In addition, the LF-NMR was also introduced to clarify the dynamic adsorption-desorption process of CH₄ in shale, in which a hysteresis occurred between the adsorption and desorption curves (Fig.7), similar to the low-temperature N₂ adsorption-desorption phenomenon (Zhou et al., 2021). This CH₄ adsorption-desorption hysteresis is due to the surface layer being affected during the CH₄ desorption process, thus inducing the capillary condensation in the micropores; that is, the molecular adsorption path is singular when the pore structure changes (Zhang et al., 2012).

As a whole, the unremitting efforts from precious works enable the LF-NMR application in measuring the CH₄ adsorption capacity in shale to be more reliable and accredited. Simply, the LF-NMR measurement regarding the CH₄ adsorption is dealing with a single component (i.e., CH₄) in shale sample and thus is somehow the low-hanging fruit of promoting LF-NMR methodology. Therefore, it is bound to need a more profound development for the LF-NMR technique for its application in the CGSU process, since this CGSU behavior is a result of the interaction of multicomponent fluids.

4 LF-NMR characterizing CO₂-CH₄ interactions in shale

The CGSU outcome is enabled by the CO₂-CH₄ interplay after the external CO₂ is injected into the shale gas reservoir and destroys the original fluid phase in shale. Therefore, the characterization of CO₂-CH₄ interactions is of significance for CGSU investigations of shale gas reservoirs. By comparison, CH₄ is a ¹H-fluid, while CO₂ is a ¹H-free fluid, which means that the relaxation behavior occurs for CH₄ but not CO₂ when the mixed CO₂-CH₄ is immersed in an LF-NMR field. Accordingly, Liu et al. (2017) self-designed an LF-NMR-based setup and conducted a series of experiments, aiming to explain the CH₄ performance after CO₂ injection into shale reservoirs. In the work made by Liu et al. (2017), the dynamic variation in adsorbed CH₄ and free CH₄ content in shale was monitored after CO₂ was injected into shale samples, where CO₂ injection accelerated the desorption rate of the original adsorbed CH₄ (Fig.8). Therein, compared with the situation without CO₂ involvement, CO₂ injection enables an additional ~25% of residual gas in the adsorbed phase to be recovered at ambient pressure or abandonment pressure, indicating a great potential for CO₂ enhanced shale gas recovery (Liu et al., 2017). Afterward, similar investigations explored the influence of CO₂ on the adsorption of CH₄ on shale (Zhao and Wang, 2019; Huang et al., 2020), where the T₂ measurements on two shale samples indicate that the presence of CO₂ makes the CH₄ in shale samples transform from the adsorbed state to the free-gas state, based on the analysis for the “a”, “b” and “c” regions in Fig.9. The concept of “a”, “b” and “c” regions is also found in the achievements made by Huang et al. (2019), similarly indicating the ability for injected CO₂ displace the adsorbed CH₄ in shale, depending on the LF-NMR T₂ measurements. Moreover, the LF-NMR was also adopted to investigate the CH₄ adsorption behavior on dry and moisture-equilibrated shale under CO₂ “huff-n-puff” (Tian et al., 2020). Therein, for the adsorbed CH₄, its recovery rate increases continuously first and then tends to level off in both the dry and the moisture-equilibrated shale samples, along with the increasing huffing time, and the recovery rate under the moisture-equilibrated condition is slightly smaller than that under dry conditions (Fig.10).

As mentioned above, the ¹H-containing and ¹H-free characteristics ensure that the LF-NMR relaxation can recognize CH₄ from the mixed CO₂-CH₄ environment and thus observe the CH₄ adsorption/desorption behavior with or without CO₂ involvement. These LF-NMR-based achievements confirm the potential and the actionability of CO₂ enhanced shale gas recovery, resulting from the CO₂-CH₄ displacement. However, the CO₂ performance during the CO₂-CH₄ interaction is usually lacking from the LF-NMR measurements because it is apathetic in the magnetic field (CO₂ is ¹H-free). To make up for this deficiency, Liu et al. (2019a) proposed a novel incorporation of LF-NMR approach and volumetric method, simultaneously monitoring CO₂ and CH₄ in the mixed CO₂-CH₄ environment in a dynamic and quantitative manner. By performing this novel methodology on two shale samples, it is observed that an increasing CO₂/CH₄ pressure ratio makes the adsorbed CH₄ content decline, suggesting that CO₂ can reduce the CH₄ adsorption capacity, probably by competing for a finite number of sorption sites in shale (Fig.11). In addition, the combination of LF-NMR approach and volumetric method built the CO₂/CH₄ competitive adsorption ratio, which is defined as the ratio of adsorbed CO₂ content relative to adsorbed CH₄ content under identical conditions in shale (Liu et al., 2019a). Accordingly, the increasing pressures of CO₂ and CH₄ decrease the CO₂/CH₄ competitive adsorption ratio, in spite that the CO₂/CH₄ pressure ratio keeps constant (~1:1) (Fig.12), corresponding with previous molecular simulations of the CO₂/CH₄ competitive adsorption behavior in shale (Wang et al., 2016; Zhou et al., 2019). This phenomenon is due to the increase in adsorbed CO₂ content being larger than that of CH₄ content at low pressure, and this gap diminishes at high pressure and also indicates the differential sensitivity of content variation for adsorbed CO₂ and adsorbed CH₄ under a certain pressure (Liu et al., 2019a).

Herein, the identification proficiency of the LF-NMR technique in detecting CH₄ from the CO₂-CH₄ mixture has been supported, and all related achievements suggest a great potential for enhanced gas recovery from shale by CO₂-CH₄ displacement. Furthermore, a combined approach, based on LF-NMR and the volumetric method, exhibits a novel capacity in recognizing CH₄ and CO₂ simultaneously in a timely, dynamic, and quantitative way. Basically, the LF-NMR performance is able to characterize the CO₂-CH₄ interaction and thus reveals a sanguine expansion in dealing with the CGSU behavior in shale gas reservoirs.

5 LF-NMR investigating CO₂ sequestration and simultaneous shale gas recovery

In general, the purpose of a CGSU project related to a shale gas reservoir includes 1) retrieving shale gas recovered from the reservoir and 2) simultaneously sequestering injected CO₂ in the shale gas reservoir as much as possible. Therefore, from the perspective of energy and the environment, whether a CGSU project is successful mainly depends on the improved shale gas recovery and the CO₂ storage content after CO₂ is involved (Godec et al., 2014; Iddphonce et al., 2020; Zhao et al., 2020). As for the enhanced shale gas recovery, the variable efficiencies like 5%, 25% and 30% exist in previous achievements (Godec et al., 2013; Fathi and Akkutlu, 2014; Liu et al., 2017), indicating that CO₂-CH₄ displacement promotes variable production enhancement levels for different shale gas reservoirs. Aiming to explore the factors affecting the production enhancement caused by CO₂-CH₄ interactions in shale, Liu et al. (2019b) organized a sequence of experiments using LF-NMR setup, where a methodology for measuring the efficiency of CO₂ enhanced shale gas recovery was proposed and demonstrated. Therein, the CO₂ involvement significantly forced more CH₄ to be recovered from shale, as revealed by the comparison of recovery efficiencies with and without CO₂ injection (Fig.13(a)), and an efficiency range from 16.22% to 34.34% of CO₂ enhanced shale gas recovery was obtained, with a mean of 23.54% (Fig.13(b)). Thereafter, with the aid of 1stOpt statistical analysis software with the algorithm of Universal Global Optimization, Liu et al. (2019b) built a model to estimate the CO₂ enhanced shale gas recovery efficiency using the reservoir parameter using LF-NMR measurements, which had also been verified therein. That is

(4)\begin{array}{rl}y=& [0.3986x_1+0.1213x_2+0.002908\mathrm{l}\mathrm{n}\left(x_3\right)\\ & -0.1693x_4]\times 100\mathrm{\%},\end{array}

where y is the CO₂-based EGR efficiency (%) and x₁, x₂, x₃, and x₄ represent the numeric values of the TOC content (%), Langmuir volume (m³/t), permeability (mD) and clay mineral content (%), respectively.

Regarding the CGSU research, as exhibited by the abovementioned achievements, the LF-NMR measurements display a good performance in determining the CO₂ enhanced shale gas recovery efficiency. Unlike the straightforward extraction of diverse desired data from the numerical simulations (Sun et al., 2013; Liu et al., 2016; Cheng et al., 2021), a single experimental work typically offers limited information. Therefore, as far as we are aware, sparing LF-NMR-based work reported the CO₂ sequestration capacity of shale during the CGSU process. By extensive search, we found that Liu et al. (2019a) represented a relationship between the CO₂ adsorption and CH₄ adsorption during the CO₂-CH₄ displacement in shale, relying on the combination of LF-NMR technique and volumetric method (Fig.14). This LF-NMR-related work indicates that the theoretical capacity for CO₂ sequestration (adsorbed phase) is up to ~3.87 cm³/g (Fig.14(a)) for sample YDN-1 and ~5.13 cm³/g (Fig.14(b)) for sample YDN-2, when the adsorbed CH4 tends to be entirely desorbed (namely, CH₄ adsorption content approaches 0 cm³/g), during the CO₂-CH₄ interaction in shale. This work (Liu et al., 2019a) can be considered a useful attempt to measure the CO₂ storage capacity of shale with the participation of the LF-NMR technique, and it is worth popularizing.

Basically, the LF-NMR technique has the capacity of measuring the CO₂ enhanced shale gas recovery with high facility, accomplished by the comparison of CH₄ content with or without CO₂ involvement. That is, the CO₂ appearance or not scarcely affects the detection of CH₄ using LF-NMR theory. Nevertheless, the LF-NME itself feels stuck in measuring the CO₂ (¹H-free) in shale reservoirs, resulting in limited LF-NMR-based works that have acquired CO₂ sequestration information. Accordingly, on the basic function of the LF-NMR technique, composite methodologies are expected and are required for the comprehensive expression of CSGU behavior in shale gas reservoirs, for example, the combinatorial method proposed by Liu et al. (2019a).

6 Perspective and outlook

The LF-NMR application in the petroleum industry dates back to 1956 (Brown and Fatt, 1956), and its function in characterizing the petrophysical properties of oil/gas reservoirs, such as porosity, permeability, and wettability, has relatively matured after years of development (Guo et al., 2020; Liu et al., 2020b; Lin et al., 2021); however, its application in determining CGSU behavior has only emerged in recent years, and is regarded as still in its infancy. For the review of the existing achievements, considered as rapid and non-destructive approach, the LF-NMR technique is gratifyingly developing, in an attractive and irresistible manner, in measuring the CH₄ adsorption capacity of shale and in identifying the CH₄ molecule from CO₂-CH₄ mixture, as well as is budding in investigating the CO₂ sequestration capacity, regarding the experimental works studying CGSU behavior in shale gas reservoir. Herein, the continuously expanding application and the contributed degree of LF-NMR in the CGSU related to shale gas reservoir are guaranteed by the resulting relaxion phenomenon of ¹H-fluid in a magnetic field; however, in this research area, the objective deficiency and ambiguity remain in the current LF-NMR implementation, by taking a panoramic view of the situation. First, the involved LF-NMR parameters were set without a uniform standard recognized by different researchers. For example, the magnetic field strength values of 0.3±0.05 T, 0.5 T and 0.54 T were adopted, while TE values of 0.15 ms and 0.3 ms were applied by different achievements (Liu et al., 2017; Tang et al., 2017; Liu and Wang, 2018; Yao et al., 2019). As another example, for the waiting time, 1.5 s, 3.0 s, 4.0 s, 6.0 s, 8.0 s, and 10.0 s were used under different pressures for CH₄ adsorption measurement using LF-NMR test, where the rules for these settings were insufficiently clear (Liu and Wang, 2018). According to the basic principle of LF-NMR, these parameter settings influence the accuracy of LF-NMR measurements, e.g., TE affects the resolution ratio of LF-NMR. Second, almost all the LF-NMR achievements regarding CO₂-CH₄ in shale gas reservoirs run in the absence of water, which somehow fails to accord to the real situation that moisture content is generally contained in a natural shale gas reservoir (Huang et al., 2018; Wang, 2019; Yang and Liu, 2020). Nevertheless, a few mentioned works involved moisture-equilibrated shale like the one made by Tian et al. (2020) but gave sparing information on the role of moisture in the CGSU behavior in shale gas reservoirs. This deficiency is probably caused by the limitation in detecting and differentiating CH₄ and moisture where ¹H simultaneously exists.

In fact, the rapid development of LF-NMR applications in oil and gas fields meets some other questioned views on the reliability of measurements. For example, with regard to the shale porosity obtained from helium pycnometry and that measured by the LF-NMR method, it is debatable in discussing which porosity is greater (Xu et al., 2015; Adeyilola et al., 2020). Therefore, a responsible application of LF-NMR in investigating CGSU behavior in shale gas reservoirs certainly depends on a constant evolution and improvement in LF-NMR theory and related setups. Herein, we suggest that it is necessary to ferment a widely recognized standard for LF-NMR-based experiments related to the CO₂-CH₄ interplay in shale gas reservoirs, where the key parameter settings, such as the magnetic field strength, waiting time and TE, could be standardized and institutionalized. In addition, the LF-NMR-based experimental design and methodology need to consciously expand the function to differentiate the ¹H-containing multiple fluids, such as CH₄ and water, which is the precondition to conduct a more responsible physical simulation of the CGSU process in shale gas reservoirs (usually containing moisture). That is, LF-NMR will play a more important role in this CGSU study field if the CO₂, CH₄ and water in shale gas reservoirs can be identified in a dynamic, quantitative, and timely manner. In view of this, herein, we suggest that D₂O (deuterium oxide) might be a choice to simulate H₂O during the LF-NMR measurement, since D₂O is insensitive to the magnetic field; however, the specific and detailed operation requires close scrutiny and inspection. In summary, LF-NMR is performing increasingly significantly in CGSU-related studies regarding shale gas reservoirs but also needs to be developed in both theory and experimental setup to acquire more responsible and reliable outcomes.

7 Concluding remarks

Regarded as a rapid and nondestructive method, LF-NMR is developing along an irresistible trend in the application of CGSU areas related to shale gas reservoirs. All the achieved advances are based on the basic theory of LF-NMR, that is, the relaxation phenomena of ¹H-fluid in a magnetic field, which means that LF-NMR is adept at recognizing CH₄ and/or distinguishing CH₄ from the CO₂-CH₄ mixture in shale. Accordingly, examples of higher-level applications indicate that LF-NMR performs well and holds high potential in measuring the CH₄ adsorption capacity of shale, in characterizing CO₂-CH₄ interactions in shale, and in investigating CO₂ sequestration and simultaneous enhanced shale gas recovery. These gratifying achievements also suggest that LF-NMR is capable of detecting the desired information (e.g., CO₂-CH₄ displacement) in a dynamic, quantitative and timely manner and further demonstrate that LF-NMR can substitute, or at least work as a supplement, for conventional approaches in studying CGSU behavior in shale gas reservoirs, such as volumetric and gravimetric methods for isothermal adsorption. Although previous works have increased the ability of LF-NMR to characterize CGSU behavior in shale gas reservoirs, some limitations remain and need to be investigated further. One issue is how to build a widely approved standard or regulation of the settings parameters (e.g., TE and waiting time) for the LF-NMR measurement. Another urgent issue is the requirement for establishing a valid LF-NMR methodology for simultaneously detecting CO₂, CH₄, and water in shale gas reservoirs in a quantitative manner. These two issues are of significance in promoting the development of this LF-NMR technique in experimentally investigating the CGSU behavior related to real natural shale gas reservoirs. In addition, it would be better if the LF-NMR theory goes a step further, enabling more relaxation (especially fast relaxation) to be detected and thus the adsorbed CH₄ behavior in shale to be quantitated more accurately.

References

[1] Abdussalam O, Trochu J, Fello N, Chaabane A (2021). Recent advances and opportunities in planning green petroleum supply chains: a model-oriented review.Int J Sustain Dev World Ecol, 28(6): 524–539

[2] Adeyilola A, Nordeng S, Onwumelu C, Nwachukwu F, Gentzis T (2020). Geochemical, petrographic and petrophysical characterization of the Lower Bakken Shale, Divide County, North Dakota.Int J Coal Geol, 224: 103477

[3] Borgia G C, Brown R J S, Fantazzini P (1998). Uniform-penalty inversion of multiexponential decay data.J Magn Reson, 132(1): 65–77

[4] Brown R J S, Chandler R, Jackson J A, Kleinberg R L, Miller M N, Paltiel Z, Prammer M G (2001). History of NMR well logging.Concepts Magn Reson, 13(6): 335–413

[5] Brown R J S, Fatt I (1956). Measurements of Fractional Wettability of Oil Fields’ Rocks by the Nuclear Magnetic Relaxation Method. Fall Meeting of the Petroleum Branch of AIME, October 14, Los Angeles, California

[6] Butler J P, Reeds J A, Dawson S V (1981). Estimating solutions of first kind integral equations with nonnegative constraints and optimal smoothing.SIAM J Numer Anal, 18(3): 381–397

[7] Carr H Y, Purcell E M (1954). Effects of diffusion on free precession in nuclear magnetic resonance experiments.Phys Rev, 94(3): 630–638

[8] Cheng H L M, Stikov N, Ghugre N R, Wright G A (2012). Practical medical applications of quantitative MR relaxometry.J Magn Reson Imaging, 36(4): 805–824

[9] Cheng L, Li D, Wang W, Liu J (2021). Heterogeneous transport of free CH₄ and free CO₂ in dual-porosity media controlled by anisotropic in situ stress during shale gas production by CO₂ flooding: implications for CO₂ geological storage and utilization.ACS Omega, 6(40): 26756–26765

[10] Coates G R, Xiao L, Prammer M G (1999). NMR Logging Principles and Applications. Houston (Texas): Gulf Publishing Company

[11] Cornillon P, Salim L C (2000). Characterization of water mobility and distribution in low- and intermediate-moisture food systems.Magn Reson Imaging, 18(3): 335–341

[12] Duguid A, Glier J, Heinrichs M, Hawkins J, Peterson R, Mishra S (2021). Practical leakage risk assessment for CO₂ assisted enhanced oil recovery and geologic storage in Ohio’s depleted oil fields.Int J Greenh Gas Control, 109: 103338

[13] Dunn K J, Bergman J D, Latorraca A G (2002). Nuclear Magnetic Resonance: Petrophysical and Logging Applications. In: Handbook of Geophysical Exploration. New York: Pergamon.

[14] Fan C J, Yang L, Wang G, Huang Q M, Fu X, Wen H O (2021). Investigation on coal skeleton deformation in CO₂ injection enhanced CH₄ drainage from underground coal seam.Front Earth Sci (Lausanne), 9: 766011

[15] Fatah A, Bennour Z, Ben Mahmud H, Gholami R, Hossain M M (2020). A review on the influence of CO₂/shale interaction on shale properties: implications of CCS in shales.Energies, 13(12): 3200

[16] Fathi E, Akkutlu I Y (2014). Multi-component gas transport and adsorption effects during CO₂ injection and enhanced shale gas recovery.Int J Coal Geol, 123: 52–61

[17] Gensterblum Y, Busch A, Krooss B M (2014). Molecular concept and experimental evidence of competitive adsorption of H₂O, CO₂ and CH₄ on organic material.Fuel, 115: 581–588

[18] Godec M, Koperna G, Petrusak R, Oudinot A (2013). Potential for enhanced gas recovery and CO₂ storage in the Marcellus Shale in the Eastern United States.Int J Coal Geol, 118: 95–104

[19] Godec M, Koperna G, Petrusak R, Oudinot A (2014). Enhanced gas recovery and CO₂ storage in gas shales: a summary review of its status and potential.Energy Procedia, 63: 5849–5857

[20] Golub G H, Heath M, Wahba G (1979). Generalized cross-validation as a method for choosing a good ridge parameter.Technometrics, 21(2): 215–223

[21] Guo J C, Zhou H Y, Zeng J, Wang K J, Lai J, Liu Y X (2020). Advances in low-feld nuclear magnetic resonance (NMR) technologies applied for characterization of pore space inside rocks: a critical review.Petrol Sci, 17(5): 1281–1297

[22] Hatzakis E (2019). Nuclear magnetic resonance (NMR) spectroscopy in food science: a comprehensive review.Compr Rev Food Sci Food Saf, 18(1): 189–220

[23] Hirasaki G J, Lo S W, Zhang Y (2003). NMR properties of petroleum reservoir fluids.Magn Reson Imaging, 21(3–4): 269–277

[24] Hu T, Xu T F, Tian H L, Zhou B, Yang Y Z (2021). A study of CO₂ injection well selection in the naturally fractured undulating formation in the Jurong Oilfield, China.Int J Greenh Gas Control, 109: 103377

[25] Huang L, Ning Z F, Wang Q, Ye H T, Chen Z L, Sun Z, Sun F R, Qin H B (2018). Enhanced gas recovery by CO₂ sequestration in marine shale: a molecular view based on realistic kerogen model.Arab J Geosci, 11(15): 404

[26] Huang X, Li T T, Gao H, Zhao J S, Wang C (2019). Comparison of SO₂ with CO₂ for recovering shale resources using low-field nuclear magnetic resonance.Fuel, 245: 563–569

[27] Huang X, Xue J J, Li X (2020). Adsorption behavior of CH₄ and C₂H₆ on shale under the influence of CO₂ and flue gas.Energy Fuels, 34(5): 5689–5695

[28] Iddphonce R, Wang J J, Zhao L (2020). Review of CO₂ injection techniques for enhanced shale gas recovery: prospect and challenges.J Nat Gas Sci Eng, 77: 103240

[29] Keles C, Tang X, Schlosser C, Louk A K, Ripepi N S (2020). Sensitivity and history match analysis of a carbon dioxide “huff-and-puff” injection test in a horizontal shale gas well in Tennessee.J Nat Gas Sci Eng, 77: 103226

[30] Kirli M S, Fahrioglu M (2019). Sustainable development of Turkey: deployment of geothermal resources for carbon capture, utilization, and storage.Energy Sources Part A-Recovery Utilization Environmental Effects, 41(14): 1739–1751

[31] Lawson C L, Hanson R J (1974). Solving Least Squares Problems. Prentice-Hall

[32] Li J, Wu Q Z, Lu J, Jin W J (2018). To quantitatively determine the adsorption and free methane volume content in shale gas core based on NMR technique. Well Logging Techn, 42(3): 315–320 (in Chinese)

[33] Li L G, Li C, Kang T H (2019). Adsorption/desorption behavior of CH₄ on shale during the CO₂ Huff-and-Puff process.Energy Fuels, 33(6): 5147–5152

[34] Lin T, Liu X, Zhang J, Bai Y, Liu J, Zhang Y, Zhao Y, Cheng X, Lv J, Yang H (2021). Characterization of multi-component and multi-phase fluids in the Upper Cretaceous oil shale from the Songliao Basin (NE China) using T₁–T₂ NMR correlation maps. Petrol Sci Technol, 39 (23–24): 1060–1070

[35] Liu D Q, Li Y L, Agarwal R K (2016). Numerical simulation of long-term storage of CO₂ in Yanchang shale reservoir of the Ordos Basin in China.Chem Geol, 440: 288–305

[36] Liu D Q, Li Y L, Yang S, Agarwal R K (2021a). CO₂ sequestration with enhanced shale gas recovery.Energy Sources Part A-Recovery Utilization and Environmental Effects, 43(24): 3227–3237

[37] Liu F Y, Ellett K, Xiao Y T, Rupp J A (2013). Assessing the feasibility of CO₂ storage in the New Albany Shale (Devonian-Mississippian) with potential enhanced gas recovery using reservoir simulation.Int J Greenh Gas Control, 17: 111–126

[38] Liu J, Xie L, Elsworth D, Gan Q (2019a). CO₂/CH₄ competitive adsorption in shale: implications for enhancement in gas production and reduction in carbon emissions.Environ Sci Technol, 53(15): 9328–9336

[39] Liu J, Xie L, Yao Y, Gan Q, Zhao P, Du L (2019b). Preliminary study of influence factors and estimation model of the enhanced gas recovery stimulated by carbon dioxide utilization in shale.ACS Sustain Chem & Eng, 7(24): 20114–20125

[40] Liu J, Xie L Z, He B, Gan Q, Zhao P (2021b). Influence of anisotropic and heterogeneous permeability coupled with in-situ stress on CO₂ sequestration with simultaneous enhanced gas recovery in shale: quantitative modeling and case study.Int J Greenh Gas Control, 104: 103208

[41] Liu J, Yao Y, Liu D, Elsworth D (2017). Experimental evaluation of CO₂ enhanced recovery of adsorbed-gas from shale.Int J Coal Geol, 179: 211–218

[42] Liu X, Zhang J, Bai Y, Zhang Y, Zhao Y, Cheng X, Lv J, Yang H, Liu J (2020a). Pore structure petrophysical characterization of the Upper Cretaceous oil shale from the Songliao Basin (NE China) using low-field NMR.J Spectrosc, 2020: 1–11

[43] Liu Y L, Wang C (2018). Determination of the absolute adsorption isotherms of CH₄ on shale with low-field nuclear magnetic resonance.Energy Fuels, 32(2): 1406–1415

[44] Liu Z S, Liu D M, Cai Y D, Yao Y B, Pan Z J, Zhou Y F (2020b). Application of nuclear magnetic resonance (NMR) in coalbed methane and shale reservoirs: a review.Int J Coal Geol, 218: 103261

[45] Luo X R, Ren X J, Wang S Z (2019). Supercritical CO₂-water-shale interactions and their effects on element mobilization and shale pore structure during stimulation.Int J Coal Geol, 202: 109–127

[46] Levitt M H (2015). Nuclear spin relaxation.Resonance, 20(11): 986–994

[47] Martin A (1995). Nuclear-magnetic-resonance imaging — technology for the 21st-century.Oilfield Rev, 7(3): 19–33

[48] McPhee C, Reed J, Zubizarreta I (2015). Nuclear Magnetic Resonance (NMR).Develop Petroleum Sci, 64: 655–669

[49] Meiboom S, Gill D (1958). Modified spin-echo method for measuring nuclear relaxation times.Rev Sci Instrum, 29(8): 688–691

[50] Middleton R S, Carey J W, Currier R P, Hyman J D, Kang Q J, Karra S, Jimenez-Martinez J, Porter M L, Viswanathan H S (2015). Shale gas and non-aqueous fracturing fluids: opportunities and challenges for supercritical CO₂.Appl Energy, 147: 500–509

[51] Pereira P, Ribeiro C, Carneiro J (2021). Identification and characterization of geological formations with CO₂ storage potential in Portugal.Petrol Geosci, 27(3): petgeo2020–123

[52] Qiu S, Lei T, Wu J T, Bi S S (2021). Energy demand and supply planning of China through 2060.Energy, 234: 121193

[53] Rani S, Prusty B K, Pal S K (2020). Characterization of shales from Damodar valley coalfields for CH₄ recovery and CO₂ sequestration.Environ Techn & Innov, 18: 100739

[54] Seevers D O (1966). A Nuclear Magnetic Method for Determining The Permeability Of Sandstones. SPWLA 7th Annual Logging Symposium. Society of Professional Well Log Analysts Transactions, Tulsa, Oklahoma, 1–14

[55] Shi J L, Shen G F, Zhao H Y, Sun N N, Song X H, Guo Y T, Wei W, Sun Y H (2018). Porosity at the interface of organic matter and mineral components contribute significantly to gas adsorption on shales.J CO₂ Utilization, 28: 73–82

[56] Straley C, Vinegar H, Morriss C (1994). Core analysis by low field NMR.1994 International Symposium of the Society of Core Analysts, Proceedings: 43–56

[57] Streich R, Becken M, Ritter O (2010). Imaging of CO₂ storage sites, geothermal reservoirs, and gas shales using controlled-source magnetotellurics: Modeling studies.Geochemistry, 70: 63–75

[58] Sun H, Yao J, Gao S H, Fan D Y, Wang C C, Sun Z X (2013). Numerical study of CO₂ enhanced natural gas recovery and sequestration in shale gas reservoirs.Int J Greenh Gas Control, 19: 406–419

[59] Sun H Y, Zhao H, Qi N, Li Y (2017). Molecular insights into the enhanced shale gas recovery by carbon dioxide in Kerogen Slit Nanopores.J Phys Chem C, 121(18): 10233–10241

[60] Sun X X, Yao Y B, Liu D M, Zhou Y F (2018). Investigations of CO₂-water wettability of coal: NMR relaxation method.Int J Coal Geol, 188: 38–50

[61] Tang J P, Tian H N, Ma Y, Sun S J, Li W J (2017). Experimental study on desorption characteristics of gas in coal shale based on NMR technology. J Liaoning Technical U (Natural Science Edition), 36(3): 282–287 (in Chinese)

[62] Testamanti M N, Rezaee R (2019). Considerations for the acquisition and inversion of NMR T₂ data in shales.J Petrol Sci Eng, 174: 177–188

[63] Tian F, Li T T, Huang X, Dang H L (2020). Adsorption behavior of CH₄, C₂H₆, and CO₂ on moisture-equilibrated shale.Energy Fuels, 34(8): 9492–9497

[64] van Beek T A (2021). Low-field benchtop NMR spectroscopy: status and prospects in natural product analysis.Phytochem Anal, 32(1): 24–37

[65] Wang J, Zhang Y, Xie J (2020). Influencing factors and application prospects of CO₂ flooding in heterogeneous glutenite reservoirs.Sci Rep, 10(1): 1839

[66] Wang Q B (2019). Effects of clay mineral characteristics on shale adsorbed gas:based on the experimental analysis of shale samples in Longmaxi Formation of Dingshan Area, Southeast Sichuan. J Chongqing U Sci Techn (Natural Sciences Edition), 21(3): 20–24 (in Chinese)

[67] Wang X Q, Zhai Z Q, Jin X, Wu S T, Li J M, Sun L, Liu X D (2016). Molecular simulation of CO₂/CH₄ competitive adsorption in organic matter pores in shale under certain geological conditions.Pet Explor Dev, 43(5): 841–848

[68] Wang X Y, Xie J, Chen X J (2021). Applications of non-invasive and novel methods of low-field nuclear magnetic resonance and magnetic resonance imaging in aquatic products.Front Nutr, 8: 651804

[69] Washburn K E (2014). Relaxation mechanisms and shales.Concepts Magn Reson Part A Bridg Educ Res, 43A(3): 57–78

[70] Xie H P, Li X C, Fang Z M, Wang Y F, Li Q, Shi L, Bai B, Wei N, Hou Z M (2014). Carbon geological utilization and storage in China: current status and perspectives.Acta Geotech, 9(1): 7–27

[71] Xie Y C, Hou Z M, Liu H J, Cao C, Qi J G (2021). The sustainability assessment of CO₂ capture, utilization and storage (CCUS) and the conversion of cropland to forestland program (CCFP) in the Water-Energy-Food (WEF) framework towards China’s carbon neutrality by 2060.Environ Earth Sci, 80(14): 468

[72] Xu H, Tang D Z, Zhao J L, Li S (2015). A precise measurement method for shale porosity with low-field nuclear magnetic resonance: a case study of the Carboniferous-Permian strata in the Linxing area, eastern Ordos Basin, China.Fuel, 143: 47–54

[73] Yang Y, Liu S M (2020). Review of shale gas sorption and its models.Energy Fuels, 34(12): 15502–15524

[74] Yao Y B, Liu D M (2012). Comparison of low-field NMR and mercury intrusion porosimetry in characterizing pore size distributions of coals.Fuel, 95(1): 152–158

[75] Yao Y B, Liu D M, Liu J G, Xie S B (2015). Assessing the water migration and permeability of large intact bituminous and anthracite coals using NMR relaxation spectrometry.Transp Porous Media, 107(2): 527–542

[76] Yao Y B, Liu D M, Xie S B (2014). Quantitative characterization of methane adsorption on coal using a low-field NMR relaxation method.Int J Coal Geol, 131: 32–40

[77] Yao Y B, Liu J, Liu D M, Chen J Y, Pan Z J (2019). A new application of NMR in characterization of multiphase methane and adsorption capacity of shale.Int J Coal Geol, 201: 76–85

[78] Yin T T, Liu D M, Cai Y D, Zhou Y F, Yao Y B (2017). Size distribution and fractal characteristics of coal pores through nuclear magnetic resonance cryoporometry.Energy Fuels, 31(8): 7746–7757

[79] Zhang T, Ellis G S, Ruppel S C, Milliken K, Yang R (2012). Effect of organic-matter type and thermal maturity on methane adsoption in shale-gas systems.Org Geochem, 47(6): 120–131

[80] Zhao G, Wang C (2019). Influence of CO₂ on the adsorption of CH₄ on shale using low-field nuclear magnetic resonance technique.Fuel, 238: 51–58

[81] Zhao P, He B, Zhang B, Liu J (2022). Porosity of gas shale: is the NMR-based measurement reliable?.Petrol Sci, 19(2): 509–517

[82] Zhao P, Xie L, He B, Liu J (2020). Strategy optimization on industrial CO₂ sequestration in the depleted Wufeng-Longmaxi Formation Shale in the northeastern Sichuan Basin, SW China: from the perspective of environment and energy.ACS Sustain Chem & Eng, 8(30): 11435–11445

[83] Zhao P, Xie L Z, He B, Liu J (2021). Anisotropic permeability influencing the performance of free CH₄ and free CO₂ during the process of CO₂ sequestration and enhanced gas recovery (CS-EGR) from Shale.ACS Sustain Chem & Eng, 9(2): 914–926

[84] Zhou G Z, Hu Z M, Gu Z B, Chang J, Duan X G, Liu X G, Zhan H M (2021). Low-field NMR investigation of the dynamic adsorption–desorption process of shale gas.Energy Fuels, 35(6): 4762–4774

[85] Zhou J P, Yang K, Tian S F, Zhou L, Xian X F, Jiang Y D, Liu M H, Cai J C (2020). CO₂-water-shale interaction induced shale microstructural alteration.Fuel, 263: 116642

[86] Zhou W N, Wang H B, Yang Y Y, Liu X L (2019). Adsorption Mechanism of CO₂/CH₄ in Kaolinite Clay: insight from molecular simulation.Energy Fuels, 33(7): 6542–6551

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