Application of Microbial Technology for Enhancing Carbon Dioxide Geosequestration in Shallow Seabed Caprock

Abstract

The utilization of fossil fuels releases a large amount of carbon dioxide (CO₂) gas, leading to global temperature changes and climate warming. Carbon dioxide geological sequestration (CCS) is an effective solution, including the use of shallow seabed hydrate reservoirs as a geological sink. However, the sealing and strength of the caprock affect the sequestration effectiveness. Therefore, this study assessed the strength and sealing properties of a shallow seabed layer reinforced with Microbial-induced Carbonate Precipitation (MICP) technology through a combination of triaxial tests and X-ray CT. In addition, carbon dioxide sequestration experiments were conducted to investigate the factors influencing the ability of MICP technology to accelerate the mineralization and sequestration of carbon dioxide. The results demonstrate that MICP technology can enhance the sealing capacity of caprock by increasing its strength, reducing its porosity, and accelerating CO₂ mineralization. After 120 h of treatment, the CO₂ concentration in the air decreased from 887 ppm to 310 ppm, showing a significant mineralization effect. The bacteria used, Bacillus megaterium, can simultaneously secrete urease and carbonic anhydrase (CA). During the urease hydrolysis of urea, this not only increases the rate of calcium carbonate formation and improves the sealing performance but also accelerates the catalytic mineralization of CO₂ by carbonic anhydrase by creating an alkaline environment.

1. Introduction

The rapid development of human society has led to an increase in energy consumption, inevitably resulting in large-scale emissions of carbon dioxide. As a primary component of greenhouse gases, excessive emissions of CO₂ can cause global temperature changes and climate warming [1,2]. Reducing carbon dioxide emissions through human intervention is one effective way to tackle this challenge, with another approach being carbon dioxide capture and sequestration from point sources [3]. According to the International Energy Agency (IEA) estimation, by 2050, CCS could reduce global carbon dioxide emissions by around 19% and effectively lower costs [4].

The increased rate of consumption of fossil fuels is the primary driver of carbon dioxide emissions [5,6]. Cement production, steelmaking, coal-fired power generation, and other point sources of emissions have become the main targets for the development and application of carbon capture measurements [7,8]. There have been a series of reports on carbon capture technologies that are applied before, during, and after the combustion of fossil fuels [9,10]. In addition, carbon dioxide sequestration, as another important technical route for reducing CO₂ emissions, focuses on terrestrial sequestration, geologic sequestration (including ocean and subsurface), and mineralization [11]. Among these, geologic sequestration refers to the transport of large quantities of supercritical CO₂ to geological formations through pipelines (wells) for long-term storage, which is considered to be the most feasible long-term sequestration method [12,13]. Hydrates, abandoned oil and gas reservoirs, and coal seams can serve as geological storage areas for carbon dioxide [14,15,16].

Sequestering CO₂ in hydrate reservoirs is currently a hot research topic, which is manifested by the use of carbon dioxide displacement methods to extract hydrates [17,18]. However, the hydrate sedimentary environments on shallow sea bottoms are weakly consolidated and less stable, posing a risk of CO₂ leakage into the seawater [19]. This could lead to the re-entry of CO₂ into the global carbon cycle and contribute to ocean acidification [20]. The sealability of the caprock is one of the important indicators for evaluating the grade of the storage areas, and a good sealability can limit CO₂ leakage, thus enabling long-term sequestration [11]. Therefore, it is necessary to enhance the strength and sealing performance for the CO₂ storage operations implemented in submarine hydrate reservoirs. MICP technology has seen a wealth of research and applications in fields such as soil and rock reinforcement [21,22], crack prevention, and water seepage control [23]. Urease-producing bacteria induce the hydrolysis of urea to produce carbonate ions, which then react with some free metal cations to form carbonate precipitates, thereby cementing the soil and filling in the fractures [24,25,26]. In addition, there are bacteria that can secrete CA while catalyzing the hydrolysis of urea [27]. The CA can be used to catalyze the hydration reaction of CO₂ to form bicarbonate (HCO₃⁻) and hydrogen ions (H⁺) as shown in Equation (1). Then, calcium carbonate precipitate is formed by the reaction of HCO₃⁻ with Ca²⁺ in an alkaline environment [28,29]. This biochemical reaction can effectively remove CO₂ gas from the environment and has promising application prospects in the field of CCS [30].

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}$$

(1)

In summary, the application of MICP technology to treat shallow seabed caprock can not only effectively enhance the sealing and strength but also accelerate the mineralization process, thereby achieving a better CO₂ sequestration effect. On this basis, this study tested the ability of MICP technology to accelerate mineralization through CO₂ sequestration experiments, and it evaluated the strength and sealing performance obtained using MICP technology to reinforce the shallow seabed caprock by combining triaxial tests and X-ray CT. Furthermore, the mechanisms of carbon sequestration were investigated using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). This study helps to advance the research progress on CO₂ geological sequestration in hydrate-bearing formations and provides theoretical guidance for the application of MICP technology in CCS.

2. Materials and Methods

2.1. Materials

2.1.1. Bacteria

The bacteria utilized in this experiment were Bacillus megaterium (CCTCC AB 92075), due to their ability to simultaneously secrete urease and CA [31]. To meet the concentration specifications for applications, the bacteria must be cultivated. The composition of the culture medium employed for this consists of 10 g/L of peptone, 3 g/L of beef extract, and 3 g/L of NaCl. The specific cultivation process is as follows: Firstly, the culture medium is sterilized by exposure to a high temperature of 121 °C. Subsequently, 1% of the bacteria are inoculated into the culture medium under sterile conditions, and the culture is incubated with shaking at 37 °C for 24 h. The specific rate of ureolysis, K_u (mol/L.s.OD), is an important index for evaluating the mineralization capacity of bacteria. Testing the K_u value of each group of bacteria after cultivation helps to minimize the experimental error, and its value can be defined as follows:

$$K_u=K/S_u$$

(2)

where $K$ is the bacterial urease activity (mol/L·s), which can be tested using the conductivity method proposed by Whiffin [32]; $S_u$ is the bacterial concentration (OD₆₀₀), which is represented by the absorbance of the bacterial solution at 600 nm⁻¹.

2.1.2. Carbon Dioxide

CO₂ was prepared by mixing anhydrous sodium carbonate with dilute hydrochloric acid. Specifically, 0.57 g of anhydrous sodium carbonate with a purity of 99% was weighed, placed in a sealed box, and mixed with 50mL of dilute hydrochloric acid (1 mL/L) before the start of the experiment. After the reaction had completely taken place, a constant initial concentration of carbon dioxide (887 ppm) was obtained.

2.1.3. Sediment

The simulation sediment was mixed in a proportionate manner using quartz sand with mesh sizes of 70–110 and 110–160. The sand samples, uniformly mixed, displayed a particle size distribution that was nearly identical to that of the hydrate-bearing strata in the Nankai Trough of Japan [33] and were thus suitable for simulating actual sediment, as shown in Figure 1.

2.2. Test Procedures

2.2.1. Carbon Sequestration Test

In the practical application of MICP technology, it is necessary to supply substrates for the cementation reaction, including urea and a calcium source (usually calcium chloride, CaCl₂). The experimental scheme for the carbon sequestration test is shown in

Table 1. Carbon sequestration experiment scheme in an enclosed environment.

Column	CaCl2	Urea	Bacteria
N	−	−	−
CUB *	+	+	+
CB *	+	−	+
UB *	−	+	+

* CUB, CaCl₂ + urea + bacteria; CB, CaCl₂ + bacteria; UB, urea + bacteria.

, with the aim of investigating the roles that each component plays in the reaction. Specifically, a “+” denotes the addition of a component, whereas a “−” indicates that the component is not added. The initial concentrations of urea and CaCl₂ were both set to 1 mol/L.

As shown in Figure 2, a GDA incubator is used to provide a sterile and sealed environment. Before the reaction is initiated, carbon dioxide gas is prepared through the utilization of anhydrous calcium chloride. Once the internal carbon dioxide concentration within the incubator has reached a stable level, the initial CO₂ concentration is meticulously documented, and the reaction reagents are mixed. Subsequently, the carbon dioxide concentration should be recorded every 6 h. The CO₂ concentration detector used in this experiment had a measurement range of 0 to 5000 ppm, and a fan was installed inside the incubator to promote a uniform distribution of the gas. In addition, the reaction reagents were mixed with sand and the above experiments were repeated to conduct a microscopic analysis.

2.2.2. Triaxial Test

Enhancing the strength of sediment is beneficial for improving the stability of caprock, thereby achieving a superior sealing effect. This article investigated the changes in sediment strength before and after MICP reinforcement using consolidated drained (CD) triaxial tests. The test procedure is as follows.

(1) Sample preparation: To 126 g mixed quartz sand, 15 mL deionized water is added, and the mixture is stirred together. Subsequently, using a hydraulic jack and a cylindrical sample mold, the quartz sand is pressed into cylindrical sand samples of size φ38 × 76 mm, in three stages, and then placed in the freezing room for freezing. After the samples have frozen and molded, they are removed and set aside for later use. The porosity of the obtained sand samples is 0.42.

(2) Injection and reaction: A sand sample is placed inside a reaction kettle and wrapped with a 0.5 mm thick rubber membrane to separate it from the annular pressure water. Subsequently, annular pressure is applied to the sample separately at 0.6, 0.8, and 1.0 MPa. In accordance with the actual conditions of the hydrate-bearing formation [34], the water injection temperature is set to 12 °C. Subsequently, the sand sample is allowed to be statically placed for 2 h to ensure that the sand sample has consolidated and the water within the pores has completely dissolved. Deionized water, bacterial solution, and cementation reaction liquid are injected in turn at a flow rate of 5 mL/min, each in an amount equal to one times the pore volume of the sample. The injection of the cementation reaction liquid is repeated in cycles, three times in total, with an interval of 8 h between cycles.

(3) Consolidated drained triaxial test: After the reaction is complete, deionized water is introduced to rinse the sample in order to exchange the pore water. Subsequently, a strength test is conducted.

2.2.3. XRD, SEM, and CT

An XRD analysis was performed on the precipitated substances generated in the carbon sequestration experiment to obtain the mineral composition and crystallographic structure of the samples. Firstly, the collected samples were dried multiple times (until the mass no longer changed) and ground. Then, each sample was tested within the scanning range of 5° to 90° at a scanning speed of 0.02° per step. Additionally, the micro-morphology of some sand samples from the carbon sequestration experiment was analyzed through SEM testing.

This paper used X-ray CT to test the change in porosity of the sand after treatment with the MICP technique, in order to evaluate its impact on the sealing property of sediments. The test employed a model Nano-3000X-ray CT device, manufactured by Tianjin Sanying Precision Instrument Co., Ltd. (Tianjin, China). The experimental parameters included a test voltage of 130 kV, a current of 120 mA, an exposure time of 0.60 s, a rotational step of 0.25 degrees per revolution, a total of 1440 frames captured, and a resolution of 24.49 um. Moreover, each specimen was a syringe-shaped sample with a diameter of 3 mm. The bacterial suspension and the cementing reaction liquid were sequentially injected into the interior of the syringe. This study separately evaluated the impact of the cement slurry concentration, bacterial suspension concentration, and treatment duration on the porosity of the sand samples.

3. Results

3.1. Evolution Law of CO₂ Concentration

Figure 3 shows the evolution law of the carbon dioxide concentration in the different experimental groups. All groups show a trend of first increasing and then decreasing. In the early stage, carbon dioxide gradually dissolves in water, forming free HCO₃⁻. Later on, due to the biological activity and the reversibility of hydration reactions, CO₂ is regenerated and released back into the air. Finally, the CO₂ concentrations show that that for UB is the highest, followed by those for N and CB, with that for CUB being the lowest (310 ppm). The reason for this can be explained as follows: The Bacillus megaterium in the CUB group secrete urease, which promotes the decomposition of urea, producing a large amount of NH₄⁺ and creating an alkaline environment favorable for the hydration reaction. The hydrolysis of urease and hydration of CO₂ result in the formation of CO₃²⁻ and HCO₃⁻. These ions react with Ca²⁺ and precipitate, thus promoting the hydration process and allowing for more CO₂ to dissolve into the water. Furthermore, the secreted CA is also conducive to the consumption of carbon dioxide in the air. In contrast, in the UB group, due to the lack of Ca²⁺, the CO₃²⁻ and HCO₃⁻ in the solution cannot be consumed, thereby inhibiting the hydration process; this leads to an increase in the concentration of CO₂. In addition, the CB group lacks urea, and from this combined with the effect of Ca²⁺, the concentration of CO₃²⁻ and HCO₃⁻ in the solution is lower, which is conducive to hydration. In summary, MICP technology utilizing Bacillus megaterium as the producer of urease effectively immobilizes carbon dioxide. This is achieved by increasing the pH of the solution and promoting the precipitation of bicarbonate and bicarbonate ions to fix the carbon dioxide.

3.2. Microstructure of Particles Formed by Carbon Sequestration

XRD and SEM tests were performed on the precipitates generated by the CUB and CB groups to obtain their microstructure of calcium carbonate. Figure 4 shows the components of the precipitates in the CUB and CB groups. The results show that the precipitates in both groups consisted of calcium carbonate, mainly in the form of calcite, with a small amount of aragonite present. Obviously, the presence of urea did not alter the crystallographic phases in the system.

Figure 5 shows SEM images of the sand samples from the CUB and CB groups, magnified 500 times. In comparison, the CB group generated a very small amount of calcium carbonate. In addition, the calcium carbonate crystals formed by the CUB group were mainly of the stable rhombohedral calcite type, with 2–5 µm sizes, and the majority of the calcium carbonate crystals were distributed in clusters within the pores and on the surface of sand particles. The growth of calcium carbonate crystals through “bridging” connects the sand grains together. The roughness of the sand grain surfaces also continually increases with the duration of the reinforcement. Due to the attraction of the negatively charged groups on the cell surface, Ca²⁺ in the material medium is adsorbed onto the cell surface and combined with CO₃²⁻ and HCO₃⁻ formed from the hydrolysis of urea and the hydrolysis of CO₂ in the air to produce a large amount of calcium carbonate, thereby fixing atmospheric CO₂. In addition, calcium carbonate crystals are mainly distributed in the vicinity of where soil particles are in contact with each other. The reasons for this include the following: on one hand, microorganisms are more likely to adsorb on the smaller surfaces near the contact points of the particles; on the other hand, as the solution flows through the pores, deposited calcium carbonate adsorbs near the points where particles are in contact with each other.

3.3. Reinforcement Effect

Figure 6a illustrates the stress–strain curves of the untreated sand samples and the sand samples after three reinforcement cycles under confining pressures of 0.6 MPa, 0.8 MPa, and 1 MPa. The sand samples treated with the MICP technology showed an increase in their maximum principal stress. Both untreated and treated specimens displayed strain hardening under different confining pressures.

The strength envelope was utilized to evaluate the impact of MICP cementation on the cohesion and friction angle of the samples. Here, $({\delta }_1+{\delta }_2)/2$ and (${\delta }_1-{\delta }_2)/2$ are, respectively, the abscissa and ordinate of the strength envelope line [35]. Furthermore, a strength envelope line was created using 15% strain points, taking into account the strain-hardening properties inherent to the specimen.

The strength envelope lines for the untreated and treated samples are illustrated in Figure 6b. It is apparent that the cohesion and friction angle of the MICP-reinforced samples both increased. Specifically, the cohesion (c) and friction angle (φ) of the untreated samples were 1.2 kPa and 31.5°, respectively, while the corresponding values for the untreated samples were 29.4 kPa and 31.9°. This can be attributed to the fact that, in the process of MICP acting to form calcium carbonate bridges between the sand particles, the calcium carbonate also adheres to the surface of the sand particles, thereby effectively enhancing the surface roughness of the sand [36].

3.4. Sealing Property

The sealing performance is an important indicator for evaluating the rank of hydrate reservoirs as geological sinks for carbon dioxide storage. MICP technology can effectively reduce the porosity and permeability of the caprock, thereby enhancing the effectiveness of carbon dioxide storage using hydrate reservoirs. In particular, calcium carbonate is the main factor controlling the changes in porosity and permeability, and its content and distribution are influenced by the concentration of the cementation solution, the concentration of the bacterial solution, and the duration of the reaction. Therefore, this section presents a sensitivity analysis of the aforementioned parameters conducted to obtain regular findings that can guide practical engineering applications.

3.4.1. Cementation Reaction Fluid Concentration

In order to study the effect of the cementing fluid concentration on the reinforcement effect, only the cementing fluid concentration was adjusted. The cementing fluid concentrations were set at 0.60 mol/L, 0.90 mol/L, 1.20 mol/L, and 1.50 mol/L. Figure 7a shows the relationship between the overall porosity, the connected porosity of the samples, and the cementing fluid concentration. It is evident that the porosity of the samples shows a decreasing trend with an increase in the cementing fluid concentration. Specifically, when the cementing fluid concentration was 1.50 mol/L, the porosity reached its minimum with a porosity of 0.16, which was about 53% lower than that of the control group. Furthermore, it is noted that the connected porosity was always lower than the overall porosity, indicating that calcium carbonate cementation effectively sealed off the pores. To further investigate the effect of the cementing fluid concentration on the porosity of the samples, the layer-by-layer porosity of the samples was analyzed, as shown in Figure 7b. Within a small range, the variation in porosity is relatively small, and the distribution is more uniform. In addition, it is noted that the change in porosity is not uniform, which is caused by differences in the permeation paths and pore structures. The higher the concentration of the cementing fluid, the more likely it is to cause pore blockage. Therefore, in practical applications, the cementing fluid concentration should be reasonably adjusted according to the required range of consolidation for the caprock to achieve a better sealing effect.

3.4.2. Bacterial Suspension Concentration

The bacterial concentration is an important factor controlling the activity of urease [28]. Different concentrations of bacterial suspension lead to varying levels of bacterial activity, which, in turn, result in different efficiencies of urease in promoting the decomposition of urea. A particular bacterial concentration can be achieved by controlling the incubation time. Figure 8 shows the urease activity under different bacterial concentrations, and the results indicate that an increase in bacterial concentration is beneficial to enhancing urease activity. An increase in bacteria provides more urease and nucleation sites for calcium carbonate. Additionally, enhanced urease activity leads to the production of more ammonium ions and carbonate ions from the urea decomposition reaction per unit time. While raising the environmental pH, it also facilitates the formation of more calcium carbonate particles.

The cementing fluid concentration was maintained at 1.5 mol/L, with the bacterial solution concentration set at 0.45, 0.90, 1.35, and 1.80. Figure 9 shows the relationship between the porosity of the samples and the bacterial solution concentration. As shown in Figure 9a, the porosity of the samples decreased with an increase in the bacterial solution concentration. At lower bacterial solution concentrations (OD₆₀₀ = 0.45), the porosity decreased significantly. After the bacterial solution concentration was further increased, the rate of decrease changed little. When the bacterial solution concentration reached 1.80, the porosity was the lowest, at 0.16, which is about a 46.7% decrease compared to the control group. At this point, the connected porosity of the samples also decreased by nearly 50% compared to the untreated state. Additionally, the porosity distribution was uniform across the depths, and the distribution of porosity did not vary significantly with changes in the bacterial solution concentration, as shown in Figure 9b.

3.4.3. Treatment Duration

The reaction time is an important factor in controlling the calcium carbonate content, and extending the reaction time is beneficial for the precipitation of calcium carbonate. This study set the reaction time at gradients to investigate its impact on the permeability performance of the caprock. The optimal concentrations of the cementation reaction solution and the bacterial solution were selected as the remaining conditions. Compared to the untreated samples (Figure 10a), the porosity decreased from the original 0.338 to 0.12 after 8 days of treatment, a reduction of approximately 64.7%. Additionally, it is noted that the reduction in porosity was most pronounced within the first 2 days of treatment, with an approximate decrease of 33.4%. As the reaction time increased, the rate of decrease in the porosity gradually slowed down. The porosity of the samples after 6 days of reaction and after 8 days of reaction was almost at the same level. The reason for this can be explained as follows: The rate of precipitation of calcium carbonate is determined by both the activity of urease and the concentration of the reaction substrate. As the reaction progresses, the activity of the bacteria decreases and is unable to provide sufficient urease to support the rapid progression of the reaction [37]. When the activity of urease is low, the concentration of the reaction substrate becomes the primary factor controlling the rate of the reaction. As the reaction progresses, the concentration of the reaction substrate decreases gradually, which accelerates the trend of the decreasing reaction rate.

Figure 10b shows the distribution of porosity along the layers for different treatment durations. Obviously, the porosity in the samples gradually decreased with an increase in the processing time. It is noteworthy that the test involved a single injection of the cementation reaction fluid. However, in actual engineering applications, there is a procedure for multiple fluid replenishments. Extending the duration of a single cycle reaction may affect the secondary grouting. A lower permeability would increase the difficulty of the cementation reaction fluid’s flow, thereby causing local blockages. Moreover, a longer duration for a single cycle of reaction is not conducive to the effective expression of bacterial activity. Therefore, in practical applications, the measure of “reducing the duration of each cycle and increasing the number of cycles” should be adopted, thereby achieving a better reinforcement effect and sealing capability while shortening the reinforcement time.

4. Discussion

4.1. The Mechanism to Improve the Sequestration Capability

Utilizing MICP technology to reinforce the shallow marine sediment layer can effectively enhance the ability to store carbon dioxide in hydrate reservoirs. Its mechanism of action primarily manifests in three aspects: improving strength, reducing porosity, and accelerating CO₂ mineralization. Specifically, Bacillus megaterium can secrete urease and CA. Urease hydrolysis of urea and CA catalyze the hydration of carbon dioxide, forming a large number of calcium carbonate crystals. As shown in Figure 6b, calcium carbonate crystals adhere to the surfaces of sediment particles and form a “bridge” structure in the particle gaps, effectively enhancing the strength of the sediment (Figure 6). Moreover, the generated calcium carbonate crystals effectively fill the pores and reduce the permeability, which increases the difficulty of carbon dioxide gas passage. Thus, more of the gas is forced to stay and participate in the synthesis reaction of CO₂ hydrate.

4.2. The Role of Urea and Ca²⁺ in Accelerated Mineralization

The presence of urea and Ca²⁺ plays a dual role. On one hand, as shown in Figure 4 and Figure 5, urea hydrolysis generates CO₃²⁻ that reacts with Ca²⁺, significantly accelerating the formation rate of calcium carbonate without altering its crystal structure. On the other hand, the presence of urea and Ca²⁺ greatly enhances the rate of CO₂ mineralization, which is beneficial for absorbing free CO₂ gas at a faster rate in practical applications. Additionally, large amounts of produced calcium carbonate rapidly fill the sediment pores, which helps slow down the migration rate of CO₂ gas through the cap layer, thereby extending the hydration reaction time.

4.3. Significance and Limitations

This study demonstrates that the use of MICP technology to reinforce shallow marine sediments has significant potential for accelerating mineralization and enhancing the CO₂ storage capacity of hydrate reservoirs. However, there are challenges that need to be addressed. Firstly, further laboratory experiments, numerical simulations, and field tests are required to verify the feasibility of this technology in real marine environments. Additionally, the aforementioned methods should be employed to optimize the process (including determining the required thickness of the cap layer and construction parameters), with a focus on ensuring sealing performance while reducing costs. Furthermore, after the optimal construction process is established, an economic analysis should be conducted in comparison with other carbon sequestration methods to demonstrate its practical value. Secondly, the low enzymatic activity of Bacillus megaterium under low-temperature conditions limits the application potential of MICP technology. In the future, exploring other microbial strains or using mixed microbial strains could be considered to address this issue. Additionally, the impact of reaction materials and microbial strains on the original marine ecological environment requires further analysis. Finally, the stability of MICP-reinforced sediments under different application conditions should be evaluated to verify whether this technology can be used for long-term CO₂ sequestration.

5. Conclusions

In this study, the reinforcement of shallow seabed caprock using MICP technology to accelerate carbon mineralization and improve the CO₂ storage capacity of hydrate reservoirs was investigated. Based on this, a sensitivity analysis was conducted for the potential factors affecting the enhanced sealing performance of MICP technology. The conclusions and recommendations are as follows:

(1) MICP technology can effectively accelerate the mineralization rate of carbon dioxide. The carbon sequestration effect was optimal when Bacillus megaterium was used in combination with urea and calcium chloride. After 120 h of treatment, the concentration of carbon dioxide in the air decreased from 887 ppm to 310 ppm. The decomposition of urea enhanced the rate of carbon dioxide storage.

(2) The strength of shallow seabed caprock sediment reinforced by MICP technology was improved. The formed calcium carbonate crystals bridged the particles and adhered to the particle surface, thereby increasing the cohesion and friction angle of the sample.

(3) The sealability of the caprock reinforced by MICP technology was enhanced. The concentration of the cementation reaction solution, the concentration of the bacterial solution, and the duration of the reaction were the key factors affecting the sealing effect, primarily controlling the amount of calcium carbonate formed.