Caprock self-sealing effect due to CO2 leakage from geologic carbon sequestration reservoirs: a case study at Ping’an, China

As a bottom technology for CO₂ reduction, geological CO₂ storage has attracted great attention from geologists, but there are few reports on the research of the caprock self-sealing effect due to CO₂ leakage. Ping’an is a natural CO₂ leakage site, which can be compared to the leakage scenarios of geological CO₂ storage. Based on the water quality test results and geological observation data, the numerical simulation of geochemistry is carried out. The results show that: First, gypsum dissolves and calcite precipitates during the migration of CO₂-rich water to the surface. This process presents a self-sealing effect, and the closer to the surface, the more obvious the self-sealing; Second, the self-sealing effect is formed rapidly. For a 30 cm wide fissure, it only takes a few hundred days to achieve self-sealing; Third, the CO2 leakage was estimated, about 140,813.3 m³, or about 251.28 tons for 1 m long fissure.

Introduction

Geological CO₂ storage (GCS) is considered to be an effective solution to alleviate global warming and environmental degradation because it can absorb greenhouse gases on a large scale (Jefferson, 2015). However, due to the complex deep geological conditions, unexpected geological events and human engineering activities, the risk of CO₂ leakage still exists (Xu et al., 2008). Potential leakage pathways for sequestered CO₂ include wells, faults, and “weak zones” in the caprock (Xie et al., 2017).

CO₂ leakage can cause serious geological environmental problems, such as the pollution of freshwater aquifers, and the enrichment of CO₂, which endangers nearby human health and local ecosystems (Cui et al., 2011; Jonathan et al., 2014; Ren et al., 2014; Taylor and Stahl, 2015; Vialle et al., 2016; Zhao et al., 2017; Zhai et al., 2018; Ma et al., 2020; Xiao et al., 2022). Many scholars have used a variety of methods to study the mechanism of CO₂ leakage and migration, the environmental impact and prevention measures. For example, Roberts has studied the relationship between the lithology, structure and leak location of a natural CO₂ spring in Daylesford, Australia (Roberts et al., 2019). Ma D explored the identification method of geological CO₂ storage leakage based on neural network (Ma et al., 2020). Gal et al. studied CO₂ leakage and migration paths through controlled release experiments in shallow formations (Gal et al., 2013; Do et al., 2022; Melis et al., 2015; Schroder et al., 2017; Jones et al., 2014). Erik uses the method of analog gas well leakage to study the leakage path and leakage mechanism under the worst-case scenario of CO₂ geological storage (Erik et al., 2017). Mark L et al. studied the interception and accumulation effects of heterogeneous caprocks on CO₂ gas through laboratory experiments and numerical simulations (Mark et al., 2014; Ha et al., 2017; Saleem et al., 2021). Taking a natural CO₂ gas reservoir leaking site in South Africa as an object, Johnson used an analogy method to study the leakage of CO₂ geological storage along wells and faults (Johnson et al., 2017). Anelia et al. studied the effect of CO₂ leakage on the physical and chemical indicators of shallow aquifers (Susan et al., 2014; Qafoku et al., 2017; Choi, 2019; Delkhahi et al., 2020; Anelia et al., 2021). Li Xiaochun et al. used geomechanical modeling and numerical simulation to evaluate the sealing ability of caprock and the risk of fault activation in CO₂ storage sites (Li et al., 2003; Gale, 2004; Cui et al., 2011). Karas et al. introduced the repair measures and plugging effect of natural CO₂ gas reservoir leakage caused by blowout in Serbia (Karas et al., 2016). Zahasky C analyzes the feasibility of controlling reservoir pressure through injection-producer wells to stop CO₂ leakage (Zahasky and Benson, 2016).

Although many important achievements have been made in the migration path, environmental impact, and response measures of CO₂ geological storage leakage, the research on the self-sealing effect of CO₂ geological storage leakage are rarely reported. There are many natural CO₂ springs in Ping’an, which can be used to study the leakage scenarios of CO₂ geological storage by analogy. And there are many travertine platforms associated with the springs. The veins filled in the platform fissures provide a good research object for studying the self-sealing effect of CO₂ geological storage leakage.

Geological background

The study area is located in Ping’an County, Qinghai Province, China. The landform are mainly low mountains and hills, with an altitude of about 2400 m. The terrain is high in the south and low in the north. The tectonic structure is in the transition zone from the Loess Plateau to the Qinghai-Tibet Plateau. Affected by regional structure, it mainly develops two sets of faults. The NNE trending faults are extensional and torsional, and the NWW are compressional and torsional (Figure 1). Secondary tectonic units divided by faults have different hydrogeological conditions.

FIGURE 1

FIGURE 1. Geological scheme of the study area.

The structure of the study area is located in the Ping’an Sag of the Xining Basin. As a secondary structural unit of the Xining Basin, the Ping’an Sag has a deep basement and well-developed strata. The Proterozoic strata constitute the crystalline basement. The upper strata are dominated by Mesozoic Cretaceous sandstone, glutenite and Cenozoic mudstone and siltstone. Quaternary aeolian, alluvial, ice-water deposits and a small amount of travertine deposits are widely distributed (Fu et al., 2019; Gupta and Yadav, 2020).

The Minhe Formation is mainly exposed in the study area, which is a lakeside sedimentary facies, and the lithology is mainly gypsum-bearing clastic rock. The lower part is dominated by conglomerate, which together with the underlying Early Cretaceous Estuary Group constitute the main reservoir in the region, with a thickness of 320.4–1,374.3 m. The upper part of the Minhe Formation is dominated by mudstone and silty mudstone, interspersed with fine sandstone and gypsum layers, with a thickness of 85.4–201.4 m, forming a regional caprock.

There are many natural CO₂ leakage points in the Ping’an area, including the Bing Lingshan spring, Shang Yaozhuang spring, and the ZK10 hole (Figure 1). Bing Lingshan spring is about 4.2 km northeast of Sanhe Town, with a water temperature of 19.5°C. There are sand gushing and bubbling in the spring, and a large thickness of travertine is developed. Shang Yaozhuang spring is about 6.4 km southwest of Sanhe Town, with a water temperature of 17.0°C. The spring is gushing sand and bubbling, and there is a large thickness of travertine around. ZK10 hole is located in the terrace of Qijiachuan Valley, 0.5 km southwest of Sanhe Town. It was the first time that high-pressure artesian water containing CO₂ gas was exposed in the Ping’an Sag. The depth of the well is 212.43 m, and the thickness of the aquifer containing CO₂ gas is 35.09 m. At the end of drilling in 2002, the water head was 73.00 m above the surface, and the artesian flow was 156.82 m³/d. At present, the artesian flow has been greatly reduced, and it is speculated that the wellbore may be self-sealed by sediment. The well water temperature is 17.0°C, the salinity is 2.94 g/L, and the water chemical type is HCO₃·SO₄-Ca.

The ZK10 borehole is 212.43 m deep and mainly encounters the Minhe Formation (K2m). The 0–132.18 m stratum is dominated by brown-red mudstone, intercalated with thin layers of siltstone, which is a closed caprock for CO₂ sequestration; below 132.18 m is an interbed of gray-white fine sandstone and argillaceous sandstone, which contains water and gas, and is a CO₂ reservoir. When the borehole was drilled to 51 m, artesian water associated gas were found. After the drilling is completed, the still water level is 73 m above the surface. Under the premise that the reservoir is not exposed, water and gas are encountered in the closed caprock, indicating that the upper strata of the Minhe Formation have poor sealing performance. Under these conditions, deep CO₂ leaks to the surface in some way, as confirmed by a nearby natural CO₂ spring.

Methods and results

CO₂ source

The sources of CO₂ in groundwater can be roughly divided into organic and inorganic origins. Organic CO₂ mainly refers to the CO₂ produced by the decomposition of organic matter in the near-surface soil or by the respiration of plant roots. Inorganic CO₂ mainly related to deep metamorphism or magmatic activity, including the CO₂ produced by the pyrolysis of carbonate minerals, and the CO₂ carried in the hydrothermal fluid after the magmatic stage.

The origin of CO₂ can be identified according to δ¹³C, ³He/⁴He and R/Ra (³He/⁴He ratio of sample to standard air). The δ¹³C value of organic CO₂ is generally less than -10‰, and the R/Ra is less than 1. The δ¹³C value of inorganic origin CO₂ is greater than 8‰. According to ³He/⁴He and R/Ra, inorganic CO₂ can be further divided into crust-derived type, mantle-derived type and mixed crust-mantle type. The ³He/⁴He of crust-derived type is low, and the R/Ra value is less than 1. The ³He/⁴He of mantle-derived type is high, and R/Ra is greater than 2. And the mixed crust-mantle type, due to the combined effect of the two factors, has an R/Ra value in the range of 1–2.

The gas composition and isotope analysis of typical CO₂ leakage sites in Ping’an are carried out, and the results are shown in Table 1. According to the results, the components in the gas sample are relatively simple, the main component is CO₂, and the content is more than 87.63%. Others are nitrogen, oxygen and trace argon, while hydrogen sulfide, hydrogen, methane and other alkanes were not detected. The δ¹³C value of CO₂ gas reservoirs in the study area is -1.5‰ to -2.5‰, indicating that the CO₂ gas is inorganic origin. The lower values of ³He/⁴He and R/Ra indicate that the CO₂ gas lacks the participation of mantle fluids, which is a typical crust-derived type.

TABLE 1

TABLE 1. Gas composition and isotope analysis of CO₂ leakage points in the study area.

The study area is geothermally abnormal, and the distribution of numerous hot springs and high terrestrial heat flow also confirm the existence of deep heat sources. Therefore, it is speculated that the main source of CO₂ in the study area is the pyrolysis of deep carbonate in the crust.

Water chemistry

Water samples from natural CO₂ leak sites in the study area were tested for hydrochemistry. The water sample numbers of Bing Lingshan spring, Shang Yaozhuang spring and ZK10 boreholes are BLS, SYZ and ZK10 respectively. The water quality was analyzed by AT-510 automatic titration analyzer, and the results are shown in Table 2. The results show that SYZ and ZK10 have the same water chemical type, which is HCO₃·SO₄-Ca; and the water chemical type of BLS is HCO₃·SO₄-Na·Ca. Different hydrochemical types indicate that Bing Lingshan spring is in different hydrogeological units with ShangnYaozhuang spring and ZK10 borehole.

TABLE 2

TABLE 2. Hydrochemical analysis of CO₂ leakage site.

Plot the Na-K-Mg giggenbach diagram with the water chemistry data of the three samples to judge the equilibrium degree of the water-rock reaction (Figure 2). The results show that the three samples all fall in the unbalanced water area of the giggenbach diagram, indicating that during the migration of water along the fractures of the caprock to the surface, the dissolution and precipitation reaction will occur, which may promote the self-sealing of the caprock.

FIGURE 2

FIGURE 2. Na-K- Mg giggenbach plot with samples in the study area.

Microscopic morphology of veins

There are three forms of travertine. The first is the travertine veins developed in the bedrock strata; the second is the travertine platform cemented by the springs; the third is the chemical accumulation formed at the mouth of the hot spring and the wellhead of the geothermal well. The travertine veins filled in the fractures of the bedrock have the deepest influence, and have the most significant influence on GCS.

The area of travertine in Bingling Mountain reaches 0.5 km². Affected by the uplift of neotectonic movement, the springs moved down to form three travertine platforms with a height difference of less than 30 m. The travertine platform is distributed in the SW-NE direction. The third-level platform in the SW direction is high and ancient, and the first-level platform in the NE direction is low and new. The third-level travertine platform is about 10 m thick, covered with Malan loess, and the lower stratum is Minhe Formation calcareous siltstone. The profile shows a number of vertical travertine veins, which are formed by the mineral precipitation in the original water-conducting fissures. It affirms the existence of the self-sealing effect of the caprock, and provides a good research target for studying the travertine self-sealing.

A typical travertine vein is selected for research (Figure 3). The thick of the vein is 24–30 cm, and can be divided into four layers from the center to the edge, representing four self-sealing stages. Described in order of formation time from late to early. The first layer is located in the middle of the vein, 6–8 cm thick, massive, uniform in texture, and earthy gray; the second layer is about 2 cm thick, in the shape of thin strips, including seven small laminae, alternating light and dark, with a single thin layer about 2 mm; the third layer is about 2–3 cm thick, massive, uniform in texture, light brown, with a longitudinal bright crystal calcite vein inside; the fourth layer is about 1–4 cm thick, in the shape of thin strip, including six small laminae, with a single layer thickness of 1–2 mm.

FIGURE 3

FIGURE 3. Photo of travertine vein in Bing Lingshan.

The vein The SEM photos of BLS01 and BLS02 are shown in Figure 4. BLS01 has a uniform structure, mainly composed of calcite and quartz. The sediment has a large particle size and a low clay content. It is speculated that the provenance was abundant and the deposition was rapid. BLS02 shows the contact relationship of laminae. The two sides are mainly calcite. The contact zone is composed of gypsum, biotite and illite. It is speculated that the two sides of the contact surface were formed in different stages. The small particle size of the sediment and the high content of clay minerals indicate that the sedimentation process was slow, the provenance was few and unstable.

FIGURE 4

FIGURE 4. The SEM photos of BLS01(A) and BLS02 (B).

REE in travertine

Travertine veins are the product of the interaction between fluid and deep fractured rock mass. It reflects the chemical composition of groundwater and is used to analyze the source of water supply. It can also reflect the temperature and pressure conditions and surrounding rock information of the fracture system.

As a special element, rare earth elements (REE) occupy an important position in geochemical research. Because of their similar chemical properties, they coexist in nature, but there are slight differences in atomic structure, resulting in certain differences in the chemical properties of each element. Therefore, REE can undergo a certain fractionation in different geological processes, resulting in their distribution patterns with different characteristics. Based on this, according to the content data of REE in rocks and minerals, the chemical composition of the ancient fluid medium, the characteristics of the source area and the regional evolution history can be discussed.

REE analysis was carried out on the four travertine samples (Tables 3, 4), and the chondrite standard spider diagram and NASC normalized patterns were drawn as shown in Figures 5, 6. According to the distribution pattern of REE, the samples can be divided into two groups, BLS01 and BLS03 as a group, and BLS02 and BLS04 as a group. This result is consistent with the morphology of travertine, indicating that BLS01 and BLS03, BLS02 and BLS04 samples have different material sources and formation processes.

TABLE 3

TABLE 3. Chemical analysis results of travertine samples.

TABLE 4

TABLE 4. Trace elements analysis of travertine samples.

FIGURE 5

FIGURE 5. Travertine REE chondrite standard spider diagram.

FIGURE 6

FIGURE 6. NASC normalized patterns of REE in travertine.

Numerical simulation

PHREEQC software was used for simulation. It is a powerful hydrogeochemical simulation software developed by the US Geological Survey and widely used in the field of hydrogeology. It is based on the water chemistry model of ionic connection, which can simulate reversible reactions such as mineral dissolution and precipitation, surface complexation, ion exchange, and irreversible reactions such as mixing with solution and redox. It can also do reverse modeling and find that minerals and gases transfer in water with indeterminate compositions. This software can simulate the mineral dissolution and precipitation reaction caused by pressure reduction and CO₂ overflow during the migration of CO₂-rich fluid to the surface, and is suitable for the study of self-sealing of CO₂ geological storage.

A conceptual model was established based on the vein body shape of the travertine platform in Bing Lingshan and the strata of the ZK10 borehole (Figure 7). The caprock is 200 m thick, and the lower part is a confined aquifer containing CO₂ gas. The fissure is 0.3 m wide and runs through the caprock longitudinally. The pressure at the bottom of the fissure is 28 atm, gradually reaching 1 atm to the surface, and the CO₂-rich fluid migrates from the reservoir to the surface along the fissure, and finally emerges into a spring. The fluid in the reservoir is in the supersaturated state of CO₂, and the main water quality ion indicators are set according to the ZK10 borehole water quality analysis results. The main ionic components are shown in Table 5.

FIGURE 7

FIGURE 7. Caprock Self-sealing model of the CO₂ leakage site.

TABLE 5

TABLE 5. Main ionic components in groundwater.

CO₂ partial pressure and temperature have no effect on the reaction of feldspar and clay minerals, but these factors have a great influence on the reaction of calcite and gypsum. In this study, only calcite and gypsum were considered. The temperature of the fluid from the reservoir to the surface changes little, and only the effect of pressure on the solubility of CO₂ is considered. In the process of fluid migration to the surface, the single-width flow rate of fluid is set to 2 L/s, and the fluid is in equilibrium with the calcite and gypsum in the caprock.

The simulation results are shown in Figure 8. In the figure, it can be seen that gypsum dissolves and calcite precipitates during the migration of CO₂-rich water to the surface, and the closer to the surface, the greater the rate of dissolution and precipitation. During this process, the volume of precipitated calcite is larger than that of dissolved gypsum, and the overall performance is self-sealing of fissures, and the closer to the surface, the more obvious the self-sealing phenomenon.

FIGURE 8

FIGURE 8. Mineral dissolution/precipitation and void filling rate variation curve with depth.

Discussion

CO₂ leakage mechanism in Qinghai Ping’an

There are two sets of faults in the study area. The NNE-trending extensional faults have water-conducting properties, and the NWW-trending faults have water-blocking properties. NNE-trending extensional faults are developed along Qijiachuan (F1), and the outcropping lithology of the mountains on both sides of the fault is different. On the west side, the upper part of the Minhe Formation is exposed with fine-grained mudstone mixed with gypsum, which is a relatively descending depression; and on the east side, the coarse-grained strata in the lower part of the Minhe Formation is exposed, which is a relatively rising bulge. A south-dipping compressional fault (F2) is developed near the east-west direction on the south side of Sanhe Town. The study area is divided into checkerboard-like structural units by two sets of faults. In addition, on the north side of the F2 fault, there is an open syncline structure with an axial distribution in a near-north-south direction and the hub dipping to the north. The ZK10 borehole is located in good structural sealing conditions, with sufficient groundwater recharge source and good fault water storage structure. At the same time, the syncline structure and groundwater runoff also easily promote the migration and accumulation of CO₂ to the two flanks of the syncline structure, and then leak into spring along the fault.

The ZK10 borehole has intermittent CO₂ self-spraying phenomenon during construction. The eruption period is about 10 min, and the self-spraying duration is 210 s. The maximum instantaneous self-spraying water volume is 5835 m³/d. The reason for this phenomenon is presumed that deep CO₂ migrates upwards into the reservoir through deep and large faults, and is blocked by the caprock. When the position is affected by the pressure reduction and volume expansion, the self-spray phenomenon occurs. The reason for this phenomenon is speculated to be that deep CO₂ migrated upward into the reservoir through deep and large faults. Prevented by the caprock, CO₂ accumulates on top of the reservoir. CO₂ migrates with the groundwater in the reservoir, and when it migrates to the wellbore position, a self-spray phenomenon occurs, which is affected by pressure reduction and volume expansion.

Based on the above, there are three leakage channels in natural CO₂ gas reservoirs in the study area, including drilling holes, faults, and the “weak caprock zone” verified by hole ZK10. The natural CO₂ leakage mechanism in the study area is as follows: the deep carbonate minerals in the crust are decomposed at high temperature to generate CO₂, and the generated CO₂ rises into the reservoir along with the deep and large fault. Blocked by the caprock, CO₂ accumulates at the top of the reservoir, and then migrates to the above leakage channel with the groundwater flow, and finally leaks to the surface (Figure 9).

FIGURE 9

FIGURE 9. CO₂ leakage pattern diagram of Qinghai Ping’an.

Self-sealing effect due to CO₂ leakage

Travertine self-sealing refers to the hydrothermal alteration or precipitation of minerals contained in underground hot water in some previously uncapped reservoirs, which can transform the overlying high-permeability geological structures into low-permeability ones. The formed travertine cemented rock layer can act as a caprock due to its low permeability, isolating the deep fluid from the surface.

The formation of travertine is mainly because the partial pressure of CO₂ in the water is higher than surrounding atmosphere, so that CO₂ escapes from the water, and the carbonate in the water is supersaturated and precipitated. The reaction formula of travertine formation is shown in Eq. 1.

${\text{Ca}}^{2+}+{2\text{HCO}}_{3^{-}}\iff {\text{ CaCO}}_3\downarrow {+\text{CO}}_2\uparrow +{\text{H}}_2\text{O}(1)$

The chemical precipitation in Bing Lingshan spring is obvious, indicating that Ca²⁺ and HCO₃- in groundwater are in a supersaturated state. Due to the change of pressure, the leakage channel of CO₂ meets the conditions of travertine self-sealing.

Numerical simulation results show that the CO₂-rich fluid migrates from the reservoir to the surface along the fracture, and the CO₂ partial pressure decreases in the process, resulting in the dissolution of gypsum and the precipitation of calcite in the fracture channel. During the process of migration to the surface, the dissolution and precipitation rates gradually increase. At the surface, 0.5644 g of gypsum can be dissolved per kilogram of fluid, and 0.9030 g of calcite can be precipitated. Set the density of gypsum to be 2.31 g/cm³ and the density of calcite to be 2.71 g/cm³. The volume of calcite precipitated during this process is larger than the volume of dissolved gypsum.

In the surface position, each liter of fluid can fill the gap up to 0.088881 cm³, and in the 200 m deep crack, the gap can be filled by 0.04261 cm³ per liter of fluid. The single-width flow rate of the fissure is set to be 2 L/s, and the top of the 0.3 m wide fissure is the first to complete self-sealing, which takes 390.66 days; and it took 814.89 days for the bottom of the fissure to finally complete self-sealing.

According to the flow monitoring of Bing Lingshan spring, the gas-liquid ratio of the fluid is about 0.8. It is speculated that when the 0.3 m-wide crack is completely self-sealing, the leakage of CO₂ from a single-width crack is about 140,813.3 m³. Under the condition of 1 atm and a temperature of 25°C, the CO₂ density is 1.7845 kg/m³, and the mass of CO₂ leaked from a single-width crack is about 251.28 tons.

Conclusion

1) The natural CO₂ leakage mechanism in Qinghai Ping’an is as follows: the deep carbonate minerals in the crust are decomposed at high temperature to generate CO₂, and the generated CO₂ rises into the reservoir along with the deep and large fault. Blocked by the caprock, CO₂ accumulates at the top of the reservoir, and then migrates to the leakage channel with the groundwater flow, and finally leaks to the surface.

2) Several vertical travertine veins can be seen in the section of the ancient travertine platform of Bing Lingshan in Ping’an County, Qinghai Province. They are formed by mineral precipitation from the CO₂-rich water in the water-conducting fissures, which confirms the existence of the self-sealing effect of the caprock. The microscopic morphology and geochemical characteristics show that the fractures have experienced 4 times of self-sealing, corresponding to four tectonic events.

3) Based on the hydrochemical analysis and geological observation data, the hydrogeochemistry numerical simulation was carried out. The results show that: first, gypsum dissolves and calcite precipitates during the migration of CO₂-rich water to the surface, and the closer to the surface, the faster the dissolution and precipitation rate; 2nd, self-sealing occurs during the migration of CO₂-rich water to the surface, and the closer to the surface, the more significant the self-sealing effect is. The single-width flow rate of the fracture is set to 2 L/s. The top of the fracture is the first to complete self-sealing, which takes 390.66 days; and the bottom of the fracture is finally self-sealing, which takes 814.89 days; 3rd, according to the flow monitoring of Bing Lingshan spring, the gas-liquid ratio of the fluid is about 0.8. It is speculated that when the 0.3 m wide fissure achieves complete self-sealing, the leakage of CO₂ from a single wide fissure is about 140,813.3 m³, or about 251.28 tons.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

LF: Conceptualization, Methodology, Investigation, Funding acquisition. YD: Investigation, Writing—Original Draft. CZ: Resources, Methodology, Funding acquisition. XM: Software, Formal analysis, Visualization. TL: Supervision. XJ: Project administration. WS: Data Curation.

Funding

This research was funded by the, 2020 Qinghai Province “Kunlun Talents” Action Plan (青人才字[2020]18号), 2021 “High-level Talent Project” of Bureau of Geological Exploration and Development of Qinghai Province (青地矿科[2021]16号), Qinghai Province Science and Technology Application Basic Research Project under Grant No. 2022-ZJ-735, and China Geological Survey Project under Grant No. DD20221818.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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