An Experimental Study on the Influence of CO₂ Real-Time Contact on the Mechanical Properties of Shale

Abstract

The influence of CO₂ on the mechanical properties of shale is one of the key factors to consider for enhancing shale oil and gas exploitation and realizing CO₂ geological storage. In this paper, triaxial mechanical experiments of rock under real-time contact with CO₂ under different conditions were carried out for the Chang 7 shale of the Yanchang Formation in the Ordos Basin. The results show that under the influence of real-time contact with CO₂, the triaxial compressive strength of shale decreases with an average decrease of 3.77% and a maximum decrease of 6.58% under the experimental conditions. The elastic modulus increased with an average increase of 8.54% and a maximum increase of 11.95%. The core compression failure presents a small degree of multi-fracture complex failure. With an increase in CO₂ exposure time, temperature, and pressure, the triaxial compressive strength gradually decreases, the elastic modulus gradually increases, and the compression failure of shale core is gradually complicated. The variation in mechanical parameters with time, temperature, and pressure under the influence of CO₂ real-time contact was quantitatively described. The effect of gaseous CO₂ on shale mechanical parameters and core compression failure is significantly weaker than that of supercritical CO₂. This research provides theoretical and data support for supercritical CO₂-enhanced shale oil and gas recovery and carbon geological storage from a rock mechanics perspective.

1. Introduction

With the growing demand for fossil energy around the world [1], the exploration and development of unconventional oil and gas reservoirs [2], such as shale oil and gas reservoirs, have become a research hot spot [3]. Methods of extracting shale oil and gas efficiently and in an environmentally friendly manner while achieving geological burial have become the focus of research [4]. Shale is less effective in exploration and development due to its low porosity and low permeability [5], high clay content, and difficulties in oil and gas flow extraction, making it difficult to undertake sustainable, large-scale exploitation [6]. Currently, horizontal well volume fracturing technology is the primary development method for shale reservoirs, but it confronts a series of problems and challenges, such as high water consumption and reservoir contamination [7]. CO₂ can enhance the recovery of shale oil and gas while enabling effective geological storage, making it a highly promising and valuable carbon storage approach in the unconventional oil and gas industry [8].

Supercritical CO₂ is characterized by high density, low viscosity, high diffusivity [9], etc. Studies have shown that supercritical CO₂ can efficiently fracture rocks [10,11,12,13], reduce reservoir fracture pressure, readily form complex fracture networks, and effectively displace methane. Carbon dioxide, as a greenhouse gas that does not pollute the Earth’s strata, has been widely researched and exhibited in the fields of shale oil and gas intensification, high-efficiency development, and geological storage, especially in China’s “two-carbon” environment [14]. China’s Changqing Oilfield and Yanchang Petroleum have conducted numerous pilot tests and demonstration applications of CO₂ fracturing, CO₂ flooding, and CO₂ geological storage, achieving favorable results [15,16].

The interaction between supercritical CO₂ and shale, which affects shale’s mechanical properties, is a key issue for CO₂-enhanced shale oil and gas recovery and geological storage, and existing research in this field still has certain limitations [17,18]. An experimental study [19,20] by Yin Hong et al. was performed under the condition that the shale samples were fully immersed in CO₂, after which the samples were retrieved for mechanical testing. Results indicated that CO₂ exposure reduces the triaxial compressive strength and elastic modulus of shale. While it focuses on the dissolution effect of CO₂ on shale under longer time conditions [21], it fails to capture the real-time influence of CO₂ on rock behavior. Specifically, it does not account for factors such as dynamic changes in CO₂ temperature and pressure, the adsorption of CO₂, and the pressure extension effects of CO₂ on the pores, microcracks and weak surfaces of shale laminae. Several scholars [22,23] have carried out uniaxial compression tests under CO₂ soaking conditions. Their experimental results indicate that CO₂ soaking induces a decrease in the uniaxial compressive strength of shale while simultaneously increasing its elastic modulus [24]. Although these studies account for the real-time interaction between CO₂ and shale [25], the derived uniaxial compression mechanics parameters exhibit limited reference value, making it challenging to directly apply the data to practical engineering scenarios. Both of these experimental research methods exhibit a certain discrepancy from the actual field engineering practices of CO₂ fracturing and sequestration. To address this gap, this paper presents a self-developed triaxial mechanical experimental testing system for rocks under CO₂ real-time contact conditions [26]. Experimental research and analysis were conducted on the changes in shale mechanical properties under CO₂ real-time contact with different time, temperature and pressure conditions. The variation laws of triaxial mechanical parameters of rocks affected by CO₂ real-time action were obtained, making the indoor experimental test conditions closer to the site of the actual working conditions. This provides strong support for the development of key technologies in supercritical CO₂-enhanced shale oil and gas extraction and CO₂ geological storage.

2. Experimental Methodology

2.1. Experimental Apparatus

The schematic diagram of the experimental testing system for rock triaxial mechanical parameters under CO₂ real-time contact (Changchun Chaoyang Experimental Instrument Co., Ltd, Changchun, Jilin, China) [26] is shown in Figure 1. This system mainly comprises a triaxial mechanical experimental testing device for rock cores under CO₂ real-time contact (see Figure 2), a computer data recording and control system, an axial pressure-loading system (maximum axial force: 1000 kN), a confining pressure-loading system (maximum confining pressure: 80 MPa), a heating system (electric heating, room temperature to 120 °C), a CO₂ pressurization system (maximum pressurization: 30 MPa), CO₂ cylinders, a CO₂ recycling, a vacuum device, valves and pipelines.

2.2. Sample Preparation

The samples were collected from full-size underground shale cores (depth interval: 2129.79~2135.42 m) of the Chang 7 shale member, Yanchang Formation, obtained from Well (W-209) in the western region of the Yanchang oilfield in the Ordos Basin. In accordance with the recommendations of the International Society of Rock Mechanics (ISRM), the rock core specimens have a height of 50 mm and a diameter of 25 mm. The two end surfaces of the cores are required to be parallel to each other and perpendicular to the axial direction, as illustrated in Figure 3.

The Chang 7 shale samples used in this experiment exhibit the characteristics of low porosity and low permeability with an average porosity of 0.887% and an average permeability of 0.06 × 10⁻³ μm². The average total organic carbon (TOC) content of the Chang 7 shale is approximately 0.96%. X-ray diffraction (XRD) analysis was performed on the prepared shale samples, and their mineral compositions are presented in Figure 4. X-ray diffraction (XRD) analysis of the mineral composition indicates that shale samples have a high clay content of 27.7%, which is second only to quartz (37.9%). The clay minerals assemblage of the shale consists of an illite–smectite mixed-layer (35%), illite (23%), kaolinite (19.1%), and chlorite (15.9%). The contents of plagioclase and potassium feldspar are 16.8% and 12.4%, respectively. Research has demonstrated that feldspar, clay minerals, and other minerals in shale can undergo specific physical and chemical reactions with CO₂. These reactions induce changes in the rock’s pore-permeability structure, which subsequently alter its mechanical properties [22].

2.3. Experimental Procedure

2.3.1. Basic Process

(1) The core sample is placed into the core holder of the test apparatus. The core is wrapped with heat-shrinkable tubing with the upper and lower ends of the tubing sealed using self-adhesive tape. A deformation transducer is mounted externally on the core and secured in place, following which the confining pressure chamber is closed.

(2) The confining pressure chamber is filled with high-temperature silicone oil. First, an axial loading of 2 KN is applied, which is followed by the gradual application of confining pressure to 15 MPa (the confining pressure must exceed the experimental CO₂ injection pressure by at least 2 MPa to prevent CO₂ from escaping into the hydraulic oil chamber). Subsequently, the heating system is activated and the temperature is raised to the predetermined experimental value.

(3) The CO₂ pressurization system is activated, and the buffer tank is filled with CO₂ to the predetermined pressure and temperature values (the buffer tank is equipped with an external electric heating system).

(4) The core was evacuated first, after which CO₂ from the buffer tank was introduced into the test setup. The experimental temperature and pressure were maintained until the predetermined duration was reached. CO₂ was injected into the injection end through the lower core gripper to keep the injection pressure constant, while the upper core gripper was connected to the discharge end; this end remained closed prior to and during the experiment.

(5) With the core maintained in real-time contact with CO₂ under the preset experimental conditions, triaxial compression testing was performed on the core. Throughout the entire loading process, the injection end was kept at a constant CO₂ pressure. The data-logging system recorded the load and strain during rock compression to facilitate the calculation of mechanical parameters.

(6) At the end of the test, CO₂ injection at the injection end was stopped, and the discharge end was opened to release internal CO₂; then, the CO₂ was recovered from the upper core gripper of the testing device. Subsequently, the confining pressure was unloaded, the damaged core was removed, and the failure mode of the core was recorded and described in detail.

2.3.2. Experimental Program

(1) Triaxial compression tests were conducted on shale under controlled conditions: (a) without the influence of CO₂ real-time contact and with varying CO₂ real-time contact times (b1, b2, b3), temperatures (c1, c2, c3) and pressures (d1, d2, d3). The experimental protocols are presented in

Table 1. Experimental scheme for the interaction between CO₂ and shale.

Number	Time (min)	Temperature (°C)	Pressure (MPa)	CO2 Phase
a	0	35	0	/
b1	30	35	7.5	supercritical
b2	60	35	7.5	supercritical
b3	120	35	7.5	supercritical
c1	60	45	7.5	supercritical
c2	60	55	7.5	supercritical
c3	60	65	7.5	supercritical
d1	60	35	2	vapor
d2	60	35	5	vapor
d3	60	35	8	supercritical

.

(2) Each group of tests was conducted using adjacent core samples from the same core to minimize the experimental errors induced by core heterogeneity. After eliminating outliers from the experimental data, the final mechanical parameters for each group were obtained by averaging the results of three replicate tests.

2.3.3. Calculation of the Rock Mechanical Parameters

(1) Axial Stress (${\sigma }_1$)

$${\sigma }_1={\displaystyle \frac{F}{A}}$$

(1)

where $F$ is the axial load (N) and $A$ is the cross-sectional area of the specimen (mm²).

(2) Axial Strain (${\epsilon }_1$)

Axial strain was measured using two sets of linear displacement sensors for axial displacement ($\Delta L$), which was calculated as

$${\epsilon }_1={\displaystyle \frac{\Delta L}{L_0}}$$

(2)

where $L_0$ represents the original height (mm) of the specimen.

(3) Compressive Strength (${\sigma }_p$)

Compressive strength is defined as the maximum axial stress that the specimen can withstand prior to failure corresponding to the peak point on the stress–strain curve.

(4) Modulus of elasticity ($E$)

The modulus of elasticity (the elastic modulus, the tensile modulus, or Young’s modulus) is defined as the slope of the stress–strain curve in the elastic deformation region. The elastic modulus is expressed as follows:

$$\lambda \stackrel{\det }{=}{\displaystyle \frac{stress}{strain}}$$

(3)

Young’s modulus ($E$), often simply referred to as the elastic modulus, is defined as the ratio of tensile stress to tensile strain. In the data-processing stage, the elastic modulus is calculated using stress–strain data from the elastic phase with the middle 1/3 segment of the elastic range.

3. Results and Discussion

3.1. Rock Mechanics Parameter Results

3.1.1. Effect of CO₂ Real-Time Contact Time

The compressive strength of shale in the control group (a) was 173.29 MPa, and its elastic modulus was 17.81 GPa. The variations in the compressive strength and elastic modulus of shale with a CO₂ contact time of 30 min (b1), 60 min (b2), and 120 min (b3), respectively, are presented in Figure 5.

As can be seen from Figure 5, under specific temperature and pressure conditions, with the increase in CO₂ real-time contact time, the compressive strength decreases gradually with an accelerating rate of decline. In contrast, the elastic modulus increases gradually with an accelerating rate of increase. The rates of change in compressive strength under the influence of different CO₂ real-time contact times (b1, b2, b3) are −3.4%, −4.16% and −5.68%, respectively, with an average rate of change of −4.41%, as shown in Figure 5A. Meanwhile, the rates of change in the elastic modulus are 4.8%, 9.1%, and 11.95%, respectively, with an average rate of change of 8.62%, as shown in Figure 5B. Owing to the low surface tension and high diffusivity of supercritical CO₂ [27], it can exert an influence on shale within a relatively short period [28], and the effect of CO₂ on the shale is enhanced with the extension of real-time contact time.

3.1.2. Effect of CO₂ Real-Time Contact Temperature

The compressive strength of shale in the control group (a) was 173.29 MPa, and its elastic modulus was 17.81 GPa. For shale specimens contacted with CO₂ at temperatures of 45 °C (c1), 55 °C (c2), and 65 °C (c3), the variations in their compressive strength and elastic modulus are presented in Figure 6.

As can be seen from Figure 6, under specific time and pressure conditions, with the increase in CO₂ real-time contact temperature, the compressive strength decreases gradually with an accelerating rate of decline. In contrast, the elastic modulus increases gradually with an accelerating rate of increase. Figure 6A illustrates the rate of change in compressive strength under the influence of different CO₂ real-time contact temperatures (c1, c2, c3), which are −3.58%, −4.10% and −6.58%, respectively, with an average rate of change of −4.75%. Figure 6B shows the rates of change in the elastic modulus, which are 8.93%, 11.78% and 13.57%, respectively, with an average rate of change of 11.43%. Within a specific range, an increase in temperature can enhance CO₂ adsorption on shale [29] and promote the pore pressure effect of CO₂ on rocks as well as the expansion of natural fractures and weak lamina surfaces [30]. This strengthens CO₂’s influence on shale, while temperature elevation also exerts a certain impact on the rock structure.

3.1.3. Effect of CO₂ Real-Time Contact Pressure

The compressive strength of shale in the control group (a) was 173.29 MPa, and its elastic modulus was 17.81 GPa. For shale specimens exposed to CO₂ at pressures of 2 MPa (d1), 5 MPa (d2), and 8 MPa (d3), the variations in their compressive strength and elastic modulus are presented in Figure 7.

As can be seen from Figure 7, under specific contact time and temperature conditions, the compressive strength decreased gradually with an accelerating rate of decline as the CO₂ real-time contact pressure increases. In contrast, the elastic modulus increases gradually with an accelerating rate of increase. Figure 7A shows the rates of change in compressive strength under the influence of different CO₂ real-time pressures (d1, d2, d3), which are −0.1%, −1.18%, and −5.1%, respectively, with an average rate of change of −2.13%. Figure 7B presents the rates of change in elastic modulus under the influence of different CO₂ real-time contact pressures (d1, d2, d3), which are 0.22%, 4.75%, and 11.56%, respectively, with an average rate of change of 5.51%. It is particularly noted that the changes in the compressive strength and elastic modulus of shale are slight under CO₂ pressure of 2 MPa (d1, vapor state) and 5 MPa (d2, vapor state). In particular, at 2 MPa (d1), the rate of change of mechanical parameters is less than 1%, which is attributed to CO₂ not being in the supercritical state. This indicates that supercritical CO₂ has a much more significant impact on shale than gaseous CO₂. Within a specific range, an increase in pressure can enhance CO₂ adsorption on shale [22]. Meanwhile, it directly strengthens the pore pressure effect of CO₂ on shale and promotes the propagation of natural fractures and weak lamina surfaces [29], thereby enhancing CO₂’s influence on shale.

3.1.4. Discussion

Compressive strength is one of the most important indicators for evaluating the difficulty of core failure, while elastic modulus is a key elastic parameter of rocks that exerts a significant influence on rock brittleness [31]. A decrease in compressive strength indicates that CO₂ facilitates rock fracturing, and an increase in elastic modulus implies that CO₂ enhances shale brittleness. This to a certain extent demonstrates that CO₂ fracturing can reduce the fracturing difficulty of shale, increase reservoir compressibility, and promote the generation of multiple fractures [32]. Given that most shales contain numerous natural fractures and laminated weak surfaces, the CO₂ fracturing of shale reservoirs is more conducive to the formation of complex fracture networks.

Supercritical CO₂ can rapidly penetrate the core owing to its high density, low viscosity and high diffusivity, inducing a certain degree of core swelling [33]. CO₂ exhibits a strong adsorption capacity within shale, and studies [34] have demonstrated that the adsorption capacity of CO₂ on shale is more than ten times that of CH₄. On the one hand, CO₂ exerts an additional pressure effect on rock pore space, leading to a reduction in the effective stress of the rock skeleton [35] and a decrease in the cohesion between rock skeleton particles [36]. On the other hand, CO₂ imposes a certain degree of pressure expansion on the natural fractures and weak surfaces of shale [21], which results in a decrease in the surface energy of shale and makes it more prone to damage [37]. Changes in the internal intrinsic structure of shale result in a decrease in the compressive strength of the core and an increase in the magnitude of axial stress change under the same amount of axial deformation, i.e., an increase in elastic modulus.

In particular, it should be emphasized that the two experimental conditions—rock compression under direct CO₂ contact and immersion and rock compression after CO₂ contact and immersion—are significantly different, leading to distinct CO₂ effects on the rock. Distinct interactions between CO₂ and rock induce different changes in the rock mechanics properties. The influence of CO₂ on the rock mechanical properties arises from the combined effect of multiple factors. Under different interaction conditions, varying dominant factors lead to different or even opposite effects of CO₂ on the rock mechanics parameters. Considering the real-time interaction between the CO₂ and rock, the uniaxial compressive strength of rock decreases while the elastic modulus increases under supercritical CO₂ soaking conditions, which is primarily attributed to CO₂ adsorption and its pressure expansion on rock micropores, microcracks, and weak planes within the bedding structure [20]. Although this study considered the real-time impact of CO₂, only uniaxial compression tests were conducted. Considering the dissolution effect of rock upon prolonged soaking in CO₂, both the triaxial compressive strength and elastic modulus of rock decrease; this is mainly attributed to the dissolution of rocks after soaking in supercritical CO₂ [19]. Although this study conducted triaxial compression tests, it only focused on the dissolution effect of CO₂ on rock.

In this paper, the triaxial compression tests under the influence of CO₂ real-time contact are closer to the actual field working conditions, and their combined effects are more convincing. The results of this study are consistent with the conclusions of Ding et al. [23] and Bai et al. [22]; i.e., in the uniaxial and triaxial compression tests of rocks under CO₂ contact and soaking conditions, the compressive strength of shale decreases while the elastic modulus increases.

At longer time scales, such as CO₂ huff-n-puff, CO₂ oil flooding, and CO₂ geological storage, the CO₂ dissolution effect is significant. A study [34] showed that after CO₂ dissolution, the proportion of large pores in shale increases while the proportion of small pores decreases. At shorter time scales, such as CO₂ fracturing, CO₂ rock breaking, and the initial stage of CO₂ geologic storage, the dominant effects are CO₂ adsorption, CO₂-induced pore pressure, and pressure extension effects of CO₂ on microfractures, natural fractures, and bedding weak planes [22].

3.2. Rock Mechanics Parameter Fitting

In order to quantitatively describe the influence law of CO₂ on the triaxial mechanical parameters of shale, the Levenberg–Marquardt optimization iterative algorithm was adopted for nonlinear curve fitting using a first-order exponential fitting function, as shown in Equation (4).

$$y=y_0+A_1{e}^{-x/t_1}$$

(4)

3.2.1. Effect of CO₂ Real-Time Contact Time

In order to describe the variation law of compressive strength with CO₂ real-time contact time, first-order exponential decay fitting was performed using the Levenberg–Marquardt optimization algorithm, as shown in Figure 8A. The fitting converged after 8 iterations, with a fitting coefficient R² (COD) of 0.985, indicating a good fitting effect. For the variation law of elastic modulus with CO₂ real-time contact time, first-order exponential growth fitting was conducted using the same Levenberg–Marquardt optimization algorithm, as shown in Figure 8B. The fitting converged after 4 iterations, and the fitting coefficient R² (COD) was 0.994, demonstrating an excellent fitting effect.

3.2.2. Effect of CO₂ Real-Time Contact Temperature

For the variation law of compressive strength with CO₂ real-time contact temperature, first-order exponential decay fitting was performed using the Levenberg–Marquardt optimization algorithm, as shown in Figure 9A. The fitting converged after 37 iterations, with a fitting coefficient R² (COD) of 0.948, indicating a good fitting effect. For the variation law of elastic modulus with CO₂ real-time contact temperature, first-order exponential growth fitting was conducted using the same Levenberg–Marquardt optimization algorithm, as shown in Figure 9B. The fitting converged after 12 iterations, and the fitting coefficient R² (COD) was 0.998, demonstrating an excellent fitting effect.

3.2.3. Effect of CO₂ Real-Time Contact Pressure

For the variation law of compressive strength with CO₂ real-time contact pressure, first-order exponential decay fitting was performed using Levenberg–Marquardt optimization algorithm, as shown in Figure 10A. The fitting converged after 11 iterations, with a fitting coefficient R² (COD) of 0.999, indicating an excellent fitting effect. For the variation law of elastic modulus with CO₂ real-time contact pressure, first-order exponential growth fitting was conducted using the same Levenberg–Marquardt optimization algorithm, as shown in Figure 10B. The fitting converged after 11 iterations, and the fitting coefficient R² (COD) was 0.992, demonstrating a good fitting effect.

3.3. Rock Damage Form Results

3.3.1. Comparison of Failure Mode

The failure modes of cores after triaxial compression tests under different CO₂ real-time contact conditions are shown in Figure 11. In the control group (Figure 11a), the shale core failure is mainly characterized by oblique shear failure at 30–60° with a single and relatively regular fracture surface. Figure 11(b1–b3,c1–c3,d1–d3) illustrates the core failure modes under triaxial compression as influenced by different CO₂ real-time contact conditions. With increases in CO₂ real-time contact time, temperature, and pressure, the core failure modes generally exhibit a trend of increasing complexity. In this experiment, although the effect of CO₂ real-time contact on core failure is relatively minor under conditions of shorter action time, lower temperature, and lower pressure, it is still not negligible.

Figure 11(b1–b3) show the triaxial compression failure modes under constant temperature and pressure conditions with CO₂ real-time contact times of 30 min, 60 min, and 120 min. With the increase in real-time contact time, the tortuosity of failure fractures in the cores increases, and the failure modes of the cores in Figure 11(b2,b3) become particularly more complex. Figure 11(c1–c3) illustrate the triaxial compression failure modes under constant time and pressure conditions with CO₂ real-time contact temperatures of 45 °C, 55 °C and 65 °C, respectively. As the CO₂ real-time contacting temperature increases, the tortuosity of failure fractures increases, and multi-fracture failure occurs in Figure 11(c3). Figure 11(d1–d3) show the triaxial compression failure modes under contact time and temperature conditions with CO₂ real-time contact pressures of 2 MPa, 5 MPa, and 8 MPa on shale cores. With the increase in CO₂ pressure, the rock failure gradually becomes more complex, and multi-fracture failure occurs in Figure 11(d3). The influence of gaseous CO₂ on the shale core failure mode (Figure 11(d1,d2)) is significantly weaker than that of supercritical CO₂ (Figure 11(d3)), which is associated with the unique properties of supercritical CO₂ such as low surface tension, high density and high diffusivity. The failure modes in Figure 11(c3,d3) reflect the influence of longitudinal bedding weak planes in the cores.

3.3.2. Discussion

Under CO₂ contact and immersion conditions, supercritical CO₂ can rapidly penetrate into the interior of shale, owing to its unique properties (e.g., high density, low viscosity, low surface tension, and high diffusivity). Its adsorption and the pressure extension effects on microcracks and bedding weak planes include changes in the internal rock structure, which in turn affect the rock pore pressure and the effective stress on the rock skeleton. This makes the rock more prone to damage and multi-fracture formation [36], which serves as the primary mechanism governing rock failure. The real-time contact between CO₂ and rock induces certain physicochemical reactions (e.g., dissolution damage to bedding surfaces and microcracks), and its influence is relatively minor in a shorter time range [38,39,40]. The combined effects of CO₂ on rock weaken the inherent structure of the rock, leading to the generation of more microcracks and microfractures during shale failure [22]; consequently, the failure mode becomes more complex [41], which is highly conducive to the formation of complex fracture networks in shale reservoirs during fracturing. Within a certain range, the effect of CO₂ on shale is intensified with the increases in time, temperature, and pressure [29].

If the core is free of natural fractures and weak facies, the failure mode of the compression test is mainly oblique shear failure; if the core contains natural fractures and weak facies, its failure mode will be affected by these natural fractures and weak facies, resulting in a more complex failure mode, and rock may even preferentially fail along the natural fractures and weak facies [42]. This phenomenon is more pronounced in uniaxial compression tests, while it is hardly reflected in triaxial compression tests due to the presence of higher confining pressure, which is consistent with the actual conditions during shale reservoir fracturing and stimulation. In the absence of special interference, reservoir fractures during conventional hydraulic fracturing are mainly controlled by the direction of the minimum principal stress, and the influence of the natural fractures and bedding weak planes on the reservoir fractures is extremely limited. In contrast, during supercritical CO₂ fracturing, CO₂ can rapidly penetrate into the microfractures, natural fractures, and weak planes of shale reservoirs. The pressure extension effect it induces can promote the opening of multiple fractures during shale reservoir fracturing [43], increase fracture tortuosity and complexity, and facilitate the formation of complex fracture networks.

During real-time interaction with supercritical CO₂, shale strength decreases while elastic modulus increase, which enhances shale brittleness and thus is conducive to shale reservoir fracturing. Compared with hydraulic fracturing, CO₂ fracturing can reduce reservoir fracture pressure and generate a more complex fracture network. Additionally, compared to conventional hydraulic fracturing, a better fracturing stimulation effect can be achieved with a lower CO₂ injection rate, which helps reduce the cost and construction difficulty of CO₂ fracturing. For CO₂-enhanced oil recovery (CO₂-EOR) or CO₂ displacement of shale oil and gas, this enhanced brittleness may facilitate fracture formation, leading to CO₂ breakthrough (uncontrolled CO₂ flow through fractures). Compared with waterflooding, the maximum injection pressure for CO₂ flooding should be lower. Therefore, in the design of CO₂ displacement for shale oil and gas, the maximum injection pressure must fully account for this influencing factor. The shale reservoirs’ reduced strength and increased brittleness may exert a negative impact on the stability of CO₂ storage within the reservoir, but it does not necessarily render them unsuitable for CO₂ geological storage. Nevertheless, the modification of shale reservoirs into a complex fracture network via CO₂ fracturing also facilitates CO₂ adsorption and storage. From this perspective, CO₂ fracturing can also serve the purpose of geological sequestration. As a core key technology in the CCUS field, the integrated technology of CO₂-enhanced oil recovery (CO₂-EOR) and CO₂ geological storage has been extensively researched and field-implemented. As a newly proposed concept, the integrated technology of CO₂ fracturing and CO₂ storage is also a crucial research direction for the future development of CCUS. The evaluation of CO₂ storage capacity and safety primarily relies on the stability and sealing capacity of the caprock, which warrants further investigation.

4. Conclusions

In this study, triaxial compression experiments were conducted on the Chang 7 shale from the Yanchang Formation of the Ordos Basin under different CO₂ contact conditions. The influences of CO₂ contact time, temperature, and pressure on the mechanical parameters and compression failure modes of the shale were analyzed with the main findings summarized as follows:

(1) Under the influence of CO₂ real-time contact, the triaxial compressive strength of shale decreased, the elastic modulus increased, and the triaxial compression failure modes of the rock exhibit varying degrees of complex multi-fracture failure.

(2) With increases in CO₂ contact time, temperature, and pressure, the triaxial compressive strength of shale exhibits a gradual decreasing trend, the elastic modulus shows a gradual increasing trend, and the compressive failure of shale cores tends to be complex multi-fracture failure.

(3) Under the experimental conditions of CO₂ contact, the triaxial compressive strength of shale decreased by an average of 3.77% (maximum decrease: 6.58%), while the elastic modulus increased by an average of 8.54% (maximum increase: 11.95%). Additionally, nonlinear fitting was performed to quantitatively describe the variation laws of shale mechanical parameters with time, temperature, and pressure under CO₂ contact.

(4) The influence of gaseous CO₂ on the mechanical parameters and compression failure of shale cores is significantly weaker than that of supercritical CO₂.

The indoor experimental conditions in this study are close to the actual field operating conditions, and the research results can be directly applied to the optimization design of CO₂ fracturing and geological storage in the Chang 7 shale oil reservoir to serve engineering practice. In the construction design, the prediction of fracturing pressure and the design of maximum injection pressure must fully consider the influence of CO₂ in reducing the reservoir fracture pressure and generating complex fracture networks. Compared with conventional hydraulic fracturing, CO₂ fracturing can adopt lower construction displacement and pressure while achieving ideal reservoir stimulation effects, which helps reduce the cost and construction difficulty of CO₂ fracturing operations. Compared with waterflooding, the maximum injection pressure for CO₂ displacement should be lower to avoid fracturing-induced CO₂ breakthrough (uncontrolled CO₂ flow through fractures). The modification of shale reservoirs into a complex fracture network may exert a negative impact on the stability of CO₂ geological storage within the reservoir while also facilitating CO₂ adsorption and storage. After all, the evaluation of CO₂ storage capacity and safety primarily depends on the stability and sealing capacity of the caprock rather than the reservoir itself.

In this study, only a preliminary exploration has been conducted under experimental conditions of short real-time contact time and low contact temperature and pressure with the CO₂ phase limited to gaseous and supercritical states. Further investigations are required to explore the influence of longer real-time contact time, higher real-time contact temperature and pressure, as well as the effect of liquid CO₂ on the mechanical properties of shale. In the experiments, although the inherent variability of the cores themself and the variability of the applied load during compression may exert a certain impact on the compression failure of the cores, this effect is considered relatively minor. Due to the limited number of experimental cores, it is challenging to systematically summarize the complex failure laws of shale under CO₂ contact; thus, additional relevant experimental studies are still needed.