Research on Evolutionary Laws of Mechanical Properties and Pore Structure during CO₂ Pre-Injection Fracturing in Shale Reservoirs

Abstract

CO₂ pre-injection fracturing is a promising technology for shale reservoirs development, with multiple advantages for improving the complexity of fractures, the production of crude oil, and the sequestration of CO₂. Previous research mostly focused on the CO₂ effect on macroscopic mechanical properties of shale. However, there are many phenomena closely related to shale micro mechanical behavior. Therefore, this study presents a systematic investigation into the effects of CO₂ on both macro and micro mechanical properties, as well as pore-fracture structures during CO₂ pre-injection fracturing in shale reservoirs. The results show that CO₂ can significantly decrease the tensile strength, uniaxial compressive strength, and elastic modulus of shale. With the increasing CO₂ treatment time, the macro mechanical properties of shale decrease gradually. The microscopic experiments show that this significant decrease may be due to two mechanisms. The first is the significant decrease in the micro-mechanical properties of shale. The results of indentation analysis show that the microscopic elastic modulus and hardness of shale decrease by 51.3% and 63.3% after CO₂ treatment. The second is the changes of the original shale framework. Pore-fractures structure analysis showed that after CO₂ treatment, a large number of dissolution pores are generated in the shale matrix. Meanwhile, there are propagation of original fractures and opening of structural weak planes, which lead to the form of new fractures. Under the action of these two mechanisms, the macro mechanical strength of shale is reduced significantly. Therefore, in the field application, proper soaking following CO₂ injection could lead to a significant overall reduction in mechanical strength, potentially lowering formation breakdown pressure, easing the requirements for treatment equipment, and enhancing fracturing effects.

1. Introduction

The focus of hydrocarbon exploration and development has begun to shift to unconventional oil and gas resources over recent years, as the development of conventional oil and gas fields gradually enter the mid-late stages [1,2,3]. At present, shale reservoir is considered the most practical unconventional resource. It has tremendous development potential and is a strategic alternative that can effectively increase the reserve and production of hydrocarbons [4,5,6,7]. The United States has delivered the large-scale cost-effective recovery of shale reservoir by horizontal wells and volume fracturing technologies, which triggered the shale revolution, can help to achieve energy independence, and profoundly affects the world energy landscape [8,9,10].

After the generation and expulsion of hydrocarbons, a significant quantity of hydrocarbons are still trapped in organic-matter-rich shale beds and adjacent interbeds, and the resultant potential hydrocarbon reserves are greatly considerable [11,12,13,14,15]. The exploration summary of China National Petroleum Corporation states that the geological resources of shale reservoir in China are abundant and have great development potential [16,17,18,19]. The large-scale cost-effective development of shale reservoir is a decisive contributor to the sustainable development of the petroleum industry. The shale revolution in the US has demonstrated the great potential of shale resources. The question of how to realize the effective utilization of shale resources, specifically to gain required well productivity and improve economic benefits, is a great challenge for the large-scale cost-effective development of shale reservoir in China [20,21,22].

Compared with shale reservoirs in the United States, shale reservoirs in China have different reservoir characteristics, with relatively narrow distribution areas, high vertical and plane heterogeneity, high clay mineral content, well-developed nano-scale pores, and extremely low porosity and permeability [23,24]. In these areas, the hydraulic fracturing easily leads to formation damage, which reduces the fracturing stimulation performance. CO₂ fracturing can avoid water sensitivity and water block effects, as it replaces water-based fracturing fluids with CO₂ [25,26,27]. Therefore, it has been widely applied in shale reservoir development in recent years [28,29].

Laboratory experimental analysis and field practice show that CO₂ changes into the supercritical state at the formation temperature and pressure and presents the characteristics of a density close to that of the liquid state and a diffusion capacity close to that of the gaseous state. Under such circumstances, CO₂ can enter nano-scale pore space that cannot be accessed by conventional water-based fluids, open natural weak planes, such as bedding and natural fractures, and increase the complexity of fractures [30,31,32,33]. However, during the treatment process, CO₂ suffers from high friction, low proppant-carrying performance and fast leak off, which results in a small fracture width, insufficient fracture-propping and limited stimulated reservoir volume. To solve such problems, the CO₂ pre-injection fracturing technology is proposed, in which water-based fracturing fluids, such as slickwater or gel, are first injected into the formation to create the main fractures deeply penetrating into the formation and effectively supported by proppants. Then, CO₂ slugs are pumped to open natural weak planes such as bedding and natural fractures in the formation and build complex fracture networks. Finally, water-based fracturing fluids are injected again to further extend the fractures and carried CO₂ to the formation more deeply. This fracturing technology has the advantages of both hydraulic fracturing and CO₂ fracturing.

Previous research on CO₂ pre-injection fracturing technology mostly focuses on the enhancement of oil recovery, such as miscibility, extraction, and formation energy complement [26,34]. The influences of the chemical dissolution of CO₂ dissolved in formation fluids on the mechanical properties and pore-fracture structure of shale are still unclear [34,35]. Furthermore, previous research mostly focuses on the macroscopic mechanical scale of shale. However, during CO₂ fracturing operation, many phenomena are closely related to shale mechanical behavior at the microscopic scale, such as micro-fractures opening, proppant embedding, and proppant crushing [36,37,38]. Given this, targeted experiments are performed in this research using samples from the Qingshankou Formation. The interactions among CO₂, water and shale are clarified, the resultant effects on shale mechanical properties (both macroscopic and microscopic scale) are analyzed. The evolution pattern of shale pore structures under CO₂ is investigated, so as to provide a theoretical foundation for the application of CO₂ pre-injection fracturing.

2. Materials and Methods

2.1. Sample Preparation

The shale samples used for this research are collected from the Qingshankou Formation. The Qingshankou Formation is one of the most important hydrocarbon source rock formations in the Songliao Basin, with a thickness of approximately 260–500 m. The type of the Qingshankou Formation is mainly black shale, with a few sandstone and limestone interbeds. The average proportion of clay minerals of the collected samples was 47.15% and the average proportion of quartz, feldspar, and carbonate minerals is 26.56%, 12.39%, and 5.38%, respectively. The geological resources of member one and two of this formation are approximately 54.5 × 10⁸ t, showing huge exploration and development potential [16,19,20].

2.2. Experimental Apparatus and Methods

For this paper, target experiments were carried out to analyze the CO₂ effects on both macro and micro mechanical properties of shale. The evolution pattern of shale pore-fracture structures was also investigated. The experimental apparatus and methods are shown as follows.

The evolution of macro-mechanical properties of shale during CO₂ treatment was experimentally characterized using the Rapid Triaxial Rock Test System (GCTS Testing Systems, Tempe, AZ, USA). The shale sample was machined into core cylinders of two different sizes (25 mm in diameter and 50 mm in length, or 50 mm in diameter and 30 mm in length) using a wire cutting instrument to ensure that the core end faces were perpendicular to the axis of the sample and flattened smoothly. This process also ensured that there was no irregular bulge around the core cylinder. The shale samples were soaked in CO₂ at 90 °C, 30 MPa, and then their uniaxial compressive and tensile strength were measured.

The evolution of micro-mechanical characteristics of shale during CO₂ treatment was experimentally characterized using the nano-micro indentation test device (Bruker, Billerica, MA, USA). For this experiment, the indentation grids were set to be 5 × 5, and the indentation spacing was set to 50 μm. A micro-mechanical analysis of shale was carried out in a quasi-static mode using Berkovich indenter, and the indentation position and corresponding micro-morphology were investigated via scanning electron microscopy (SEM). The schematic diagram of this analysis is shown as Figure 1.

The micro-fracture morphology of shale during CO₂ treatment was imaged using the Zeiss Xradia 510 Versa high-resolution three-dimensional X-ray microscope (Zeiss, Oberkochen, Germany). The shale samples were machined into core cylinders with a diameter of 25 mm and length of 25 mm by the wire cutting instrument, and the upper and lower end faces were flattened. Then the core samples were soaked in CO₂ at 90 °C, 30 MPa, then the micro-fracture propagation before and after soaking was characterized in the case of the voltage of 140 kV and the power of 10 W.

The micro-pore structure of shale during CO₂ treatment was imaged using the Thermo Scientific Apreo high-performance field emission scanning electron microscope (Thermo Fisher Scientific, Waltham, MA, USA). The resolution of the instrument can reach 1 nm under an acceleration voltage of 1 kV. The shale sample was machined into a specimen of approximately 1 cm × 1 cm × 1 cm, which is then fixed on the sample holder and put through mechanical polishing and argon ion polishing successively to ensure a clean, smooth, pollution-free specimen surface. After that, the prepared shale specimens were soaked in CO₂ at 90 °C, 30 MPa, and then observed under the conditions of an acceleration voltage of 10 kV and beam current of 1.6 nA to capture the changes in the microscopic pore structure of the shale surface during CO₂ treatment.

3. Results

3.1. Evolution of Macro-Mechanical Parameters of Shale before and after CO₂ Treatment

To probe into the influences of CO₂ on the macro-mechanical properties of the Qingshankou Formation shale, the response patterns of mechanical parameters, such as tensile strength, uniaxial compressive strength, elastic modulus and Poisson’s ratio, of the Qingshankou Formation shale during CO₂ treatment were analyzed through uniaxial compression tests and Brazilian splitting tensile strength tests. The calculation equations of each mechanical parameter are detailed in Equations (1)–(4) [39,40].

$${\sigma }_{\mathrm{c}}=\frac{P_{\max }}{A}$$

(1)

where σ_c is uniaxial compressive strength, MPa; P_max is the maximum load of rock specimens, N; A is the loaded area of the specimens, mm².

$${\sigma }_{\mathrm{t}}=\frac{2P}{\mathsf{\pi }DL}$$

(2)

where σ_t is tensile strength, MPa; P is the load at which rock failure occurs, N; D is the diameter of specimens, mm; L is the thickness of specimens, mm.

$$E=\frac{({\sigma }_1-{\sigma }_3{)}_{\left(50\right)}}{{\epsilon }_{\mathrm{h}\left(50\right)}}$$

(3)

$$\mu =\frac{{\epsilon }_{\mathrm{d}\left(50\right)}}{{\epsilon }_{\mathrm{h}\left(50\right)}}$$

(4)

where E is elastic modulus, GPa; μ is Poisson’s ratio; (σ₁ − σ₃)₍₅₀₎ is 50% of the maximum principal stress difference of the specimen, MPa; ε_h(50) is the axial compressive strain corresponding to (σ₁ − σ₃)₍₅₀₎; ε_d(50) is the lateral compressive strain corresponding to (σ₁ − σ₃)₍₅₀₎.

The tensile strength and uniaxial compressive strength of the Qingshankou shale before and after CO₂ treatment are shown in Figure 2. It is evident that CO₂ treatment reduces the shale strength. The post-CO₂ treatment strengths (both the tensile and uniaxial compressive strengths) are considerably lower than those of the untreated shale. As the CO₂ treatment time increases, the mechanical properties of shale gradually reach a stable state.

Under natural conditions, the uniaxial compressive strength of shale is approximately 73.53 MPa. After 3 days, 7 days, and 14 days of CO₂ treatment, the uniaxial compressive strengths are 60.21 MPa, 53.21 MPa, and 48.32 MPa, respectively, with the corresponding reductions of approximately 18.12%, 27.64%, and 34.29%, respectively. The variation of uniaxial compressive strength of shale with CO₂ treatment time is fitted using Equation (5). CO₂ treatment significantly affects the uniaxial compressive strength of the Qingshankou Formation shale in the extended treatment duration, with a substantial decrease observed during the initial three days of treatment. As the treatment time exceeds three days, the uniaxial compressive strength of shale continues to decline, yet with a decreasing magnitude, and the mechanical properties tend to be stable. Driven by pressure, CO₂ intrudes into the core, then corrodes some minerals, changes the original pore structure, induces further fracture expansion and creates new fractures at weak planes of the core, which thus significantly reduces the mechanical strength of shale.

$${\sigma }_{\mathrm{c}}=73.75461\times {\left(\mathrm{t}+1\right)}^{-0.15461}$$

(5)

where t is the duration of CO₂ treatment, d.

Under natural conditions, the tensile strength of shale is approximately 6.53 MPa. After CO₂ treatment for 3, 7, and 14 days, the tensile strength drops to 6.01 MPa, 5.59 MPa, and 5.28 MPa, respectively, with corresponding reductions by approximately 7.96%, 14.40%, and 19.14%, respectively. The variation pattern of shale tensile strength with CO₂ treatment time is fitted as Equation (6). Clearly, CO₂ treatment has notable influences on the tensile strength of the Qingshankou Formation shale—with the extension of treatment time, the tensile strength of shale decreases significantly during the first three days. As the treatment time exceeds three days, the tensile strength of shale continues to decline, yet with a decreasing magnitude, leading to a stabilization of the mechanical properties. Since the tensile strength of shale is mainly dependent on the cementation strength of its mineral particles, and CO₂ dissolves a large number of cement minerals in shale (such as carbonate minerals and feldspar) to decrease the cementation strength, the tensile strength of shale significantly drops after CO₂ treatment.

$${\sigma }_{\mathrm{t}}=6.574\times {\left(\mathrm{t}+1\right)}^{-0.07729}$$

(6)

The variation pattern of elastic modulus of the Qingshankou Formation shale before and after CO₂ treatment is shown in Figure 3a. The natural elastic modulus of shale is approximately 8.56 GPa. After CO₂ treatment for 3, 7, and 14 days, the elastic modulus falls to 6.78 GPa, 5.56 GPa, and 4.87 GPa, respectively, with reduction rates of 20.79%, 35.05%, and 43.11%, respectively. The variation pattern of elastic modulus of shale with CO₂ treatment time is expressed in Equation (7). In terms of the Poisson’s ratio of the Qingshankou Formation shale before and after CO₂ treatment, the variation pattern is illustrated in Figure 3b. Under the natural state, the Poisson’s ratio of shale is approximately 0.113. After CO₂ treatment for 3, 7, and 14 days, the Poisson’s ratio of shale increases to 0.141, 0.145, and 0.149, respectively, with the corresponding growth rates of 24.78%, 28.32%, and 31.86%, respectively. By mathematical fitting, the variation pattern of Poisson’s ratio of shale with CO₂ treatment time is characterized as Equation (8).

$$E=8.64476\times {(\mathrm{t}+1)}^{-0.2044}$$

(7)

$$\mu =-0.03461\times {\mathrm{e}}^{\frac{-\mathrm{t}}{1.90356}}+0.14767$$

(8)

The above analyses show that as CO₂ treatment time increases, the macro-mechanical strength of shale decreases greatly. The overall duration of fracturing operations is relatively short, limiting the reaction time and range of CO₂ within the formations. Consequently, the decline in the mechanical strength of shale is minor, which has relatively fewer influences on fracturing performance compared to longer treatment durations. However, if soaking is performed properly after CO₂ injection, the CO₂ reaction time is prolonged, and the CO₂ affected range is expanded, a sufficient overall reduction of the mechanical strength is expected to significantly reduce the breakdown pressure of the formations, lower the requirements for treatment equipment and improve the performance of the fracturing process.

3.2. Evolution of Micro-Mechanical Parameters of Shale before and after CO₂ Treatment

The displacement-load curve of the indenter during loading-unloading of the indentation test is plotted (Figure 4). The hardness and elastic modulus of the sample at the indentation position can be calculated from the displacement-load curve, and the calculation formulas are detailed in Equations (9)–(11) [41,42].

$$E_{\mathrm{r}}=\frac{\sqrt{\mathsf{\pi }}}{2\beta }\frac{S}{\sqrt{A_{\mathrm{c}}}}$$

(9)

$$\frac{1}{E_{\mathrm{r}}}=\frac{1-{{\mu }_{\mathrm{i}}}^2}{E_{\mathrm{i}}}+\frac{1-{\mu }^2}{E}$$

(10)

$$H=\frac{P_{\max }}{A_{\mathrm{c}}}$$

(11)

where E_r is the converted modulus of elasticity, Pa; β is a constant related to the indenter’s geometry; S is the contact stiffness, N·m⁻¹; A_c is the contact area between the indenter and the sample, m²; μ_i is the Poisson’s ratio of the Berkovich indenter; E_i is the elastic modulus of the Berkovich indenter; μ is the Poisson’s ratio of the sample; E is the elastic modulus of the sample, Pa; H is the hardness of the sample, Pa; P_max is the maximum indentation load, N.

The SEM analysis results of the indentation grid and a single indentation of shale specimens (approximately 1 cm × 1 cm × 1 cm) are shown in Figure 5, respectively. It can be seen that the indentation depths and shapes at different positions on the shale surface are considerably varied. The Qingshankou Formation shale has a high mineralogical heterogeneity, so clay accumulations may occur during indentation where there are quartz and carbonate mineral particles with higher hardness at the indentation position, overlying clay minerals with lower hardness. Furthermore, where there are highly brittle quartz and feldspar mineral particles, such minerals reach the yield strengths and crack to form micro-fractures on the sample surface with the increasing load.

The indentation displacement-load curve for a single indentation grid point of the Qingshankou Formation shale is shown in Figure 6. There is a marked differentiation in displacement under the same load at different indentation positions. Due to the high microscopic heterogeneity of shale, when the indenter is against different mineral particles, the measured displacement-load curves are significantly different, due to the significant differences in the hardness of mineral particles. The tests reveal the high heterogeneity of the micro-mechanical properties of the samples, with indentation depths into the shale surface varying notably at different grid points under the same load.

In addition, part of the displacement-load curves of the Qingshankou Formation shale is found with the pop-in phenomenon. SEM analysis suggests that the main reason for pop-in events may be that the indentation point is located on brittle minerals such as quartz and feldspar. As the load increases and the minerals reach their yield strength, the mineral particles are broken and micro-fractures appear on their surfaces, which leads to pop-in. Furthermore, the presence of micro pores and fractures in shale, and their potential propagation, may also contribute to pop-in.

The SEM analysis results of the indentation of the Qingshankou Formation shale after CO₂ treatment are shown in Figure 7. In comparison to the original shale sample, the CO₂-treated sample presents a considerably different indentation morphology. The reaction of CO₂ with water forms carbonic acid, which dissolves feldspar, carbonates, and other minerals in shale, resulting in the formation of a large number of dissolution pores and fractures. A large number of indentation positions are located on such dissolution pores or fractures, and thus, the indentation morphology is significantly affected.

The displacement-load curve of indentation of the Qingshankou Formation shale after CO₂ treatment is shown in Figure 8. The presence of a multitude of newly-formed dissolution pores and fractures on the shale surface leads to a substantial increase in the incidence of pop-in during loading and pop-out during unloading. The indentation depth increases greatly after CO₂ treatment compared to the original samples, indicating that CO₂ treatment changes the shale’s micro-pore structure and thus, effectively reduces the shale mechanical strength.

The elastic modulus and hardness of the Qingshankou Formation shale samples before and after CO₂ treatment are calculated using the Oliver–Pharr equation [41,42], and are compared in Figure 9 and Figure 10. CO₂ treatment has significant impacts on the micro-mechanical properties of shale. In the natural state, the average elastic modulus of shale is 22.8 GPa, while it decreases to 11.1 GPa after CO₂ treatment, with a reduction rate of approximately 51.3%. Similarly, the average hardness, which is 0.834 GPa in the natural state, drops to 0.306 GPa after CO₂ treatment, with a reduction rate of approximately 63.3%. The micro-mechanical properties of shale are substantially diminished following CO₂ treatment, correspondingly leading to a significant degradation in the shale’s macro-mechanical properties.

3.3. Influences of CO₂ on the Development of Shale Micro-Fractures

The micro computed tomography (CT) scanning was performed to capture the morphological characteristics of micro-fractures in shale cores before and after CO₂ treatment (Figure 11). With the extension of CO₂ treatment time, fractures in shale gradually extend, and vertical and transverse fractures tend to intersect, which significantly increases the complexity of fractures. In their natural state, the original fractures in the Qingshankou Formation shale are narrow, short, and isolated, with no interconnections. After CO₂ treatment, the fracture morphology in shale undergoes substantial changes, and the fracture network complexity of shale climbs up. The newly-formed fractures can be broadly categorized into two types: extended fractures and entirely new fractures.

In the process of CO₂ treatment, the interaction between CO₂ and the shale facilitates the gradual propagation of original fractures. This results in a gradual increase in fracture width and depth, which in turn significantly enhance fracture flow capacity and lead to a gradual decrease in the shale mechanical strength. During CO₂ treatment, cores break along weak planes, such as bedding and cementation planes, generating new fractures. Meanwhile, the original fracture further propagates and branches to considerably improve the fracture complexity and connectivity and degrade the shale mechanical strength. These observations demonstrates that CO₂ can destruct the rock structure, increase the number and complexity of natural fractures, effectively reduce rock strength, and improve the fluid flow capacity of reservoirs. This ultimately enhance the fracture network complexity during re-fracturing, stimulating a greater volume of reservoir rocks.

Furthermore, CO₂ transitions to supercritical state in formations, and supercritical CO₂ has a high diffusion capacity to rapidly diffuse and flow in shale micro-nano pore networks of the shale. Therefore, before the shale is fractured, a large number of micro-pores and micro-fractures are produced in the shale, due to the influences of dissolution and seepage. With the extension of CO₂ treatment time, fractures are connected with each other to form a relatively macro-scale fracture network, which greatly destructs the original shale framework and significantly reduces the mechanical strength of shale. Consequently, more complex fracture networks can be formed in the subsequent fracturing process.

3.4. Effects of CO₂ on the Shale Micro-Pore Structure

As the highest resolution of micron CT is limited to 0.7 μm, it is impossible to characterize and analyze the changes in nano-scale pore structures of shale. Therefore, in order to study the influences of CO₂ on the micro-morphology of the Qingshankou Formation shale, the micro-morphology of shale samples (approximately 1 cm × 1 cm × 1 cm) before and after treatment was imaged and analyzed under the same field of view.

The CO₂ effects on the micro-morphology of the shale matrix are shown in Figure 12a through Figure 12d. After mechanical and argon ion polishing, some relatively small original pores with diameters of approximately 1–5 μm are observed on the shale surface. After CO₂ treatment, the pore size of the shale increases significantly, with pore diameter expanding from 1–5 μm to 5–25 μm. In the areas with concentrated albite (NaAlSi₃O₈), K-feldspar (KAlSi₃O₈), and calcite (CaCO₃), the minerals that can react with CO₂, the large-area dissolution occurs and results in large dissolved pores on the shale surface. The possible reactions are detailed in Equation (12) through Equation (15). The interaction between CO₂ and matrix minerals of shale greatly changes the surface morphology. Concurrently, CO₂ dissolves the cement, which causes some mineral particles to fall off and migrate. These migrating minerals accumulate on the shale surface and plug some pores. The emergence of new dissolution pores and the migration of minerals greatly change the original pore structure of the shale and thus, significantly reduce its macro-mechanical strength.

2NaAlSi₃O₈ + 2CO₂ + 3H₂O → 2Na⁺ + 2HCO₃⁻ + 4SiO₂ + Al₂Si₂O₅(OH)₄

(12)

2KAlSi₃O₈ + 2H⁺ + 9H₂O → 2K⁺ + 4H₄SiO₄ + Al₂Si₂O₅(OH)₄

(13)

CaCO₃ + H⁺ → Ca²⁺ + HCO₃⁻

(14)

CaMg(CO₃)₂ + 2H⁺ → Ca²⁺ + Mg²⁺ + 2HCO₃⁻

(15)

4. Discussion

At present, the overall duration of fracturing operations in wells is relatively short, and the reaction time and range of CO₂ injected into the formation are limited. Therefore, CO₂ fracturing cannot give full play to the CO₂-water-shale reaction to sufficiently change the original pore structure of shale, reduce the mechanical strength of shale or improve the fracturing performance. However, to curtail drilling and fracturing expenses, the development of shale oil is increasingly adopting a factory-like approach, known as well-factory mode, which involves batch drilling and completing multiple wells from a single well pad. In this context, CO₂ pre-injection fracturing can be sequentially employed to perform reservoir transformation and extend CO₂ treatment duration.

By initially injecting high-viscosity fracturing fluids to create the main fracture that penetrates deeply into the formation, followed by the sequential injection of CO₂ slugs into each well, a complex fracture network is created in the vicinity of the main fracture near the wellbore. Subsequently, the wells are subjected to a period of shut in, allowing the CO₂-water-shale reaction to weaken the mechanical strength of the shale. After CO₂ fracturing, water-based fracturing fluids are injected into each well successively to further extend the fractures formed by CO₂ fracturing, open strength-weakened natural fractures and place proppants to effectively support fractures. The sequential fracturing of multiple wells on the same pad allows for an extended CO₂ soaking time, fully exploit the reaction of CO₂-water-shale and reduce the shale mechanical strength. This approach not only diminishes the difficulty of subsequent fracturing operations but also improves the fracturing performance. Therefore, CO₂ pre-injection fracturing is considered an effective reservoir stimulation method for the efficient recovery of shale oil, although its field applications are still faced with many challenges.

5. Conclusions

This research provides an in-depth analysis of the impact of CO₂ on the mechanical properties of shale by characterizing macro/micro mechanical properties. The variation regularity of shale pore-fracture structures during CO₂ treatment was investigated via the CT micro-fracture analysis and SEM micro-pore structure analysis. The conclusions are as follows:

(1)

CO₂ treatment leads to a significant reduction in the tensile strength, uniaxial compressive strength, and elastic modulus of shale, with the degradation of these mechanical parameters progressively increasing with treatment time. However, as reactive minerals become depleted, the mechanical parameters of shale tend toward stabilization. The Poisson’s ratio increases with extended CO₂ treatment duration, ultimately reaching a stable value. After 14 days of CO₂ treatment, the uniaxial compressive and tensile strengths of shale drop by 34.29% and 19.14%, respectively. CO₂ treatment can effectively reduce the mechanical strength of the Qingshankou Formation shale and improve the fracturing performance.

(2)

CO₂ treatment has significant effects on the micro-mechanical properties of shale. The phenomena of pop-in during loading and pop-out during unloading increase greatly in the displacement-load curves, and a notable compaction stage occurs in the curve, which shows that the original shale framework changes after CO₂ treatment, and the volume of micro-pores and fractures in shale increases. After CO₂ treatment, the elastic modulus and hardness of shale decrease by 51.3% and 63.3%, respectively. The micro-mechanical properties of shale decrease greatly, which leads to the degradation of shale macro-mechanical strength.

(3)

After CO₂ treatment, the propagation of original fractures is accompanied by the opening of structural weak planes, resulting in the formation of additional new fractures. Under the effects of CO₂ treatment, micro-fractures in shale continue to extend. The fracture width increases significantly, and the fractures connect with each other to form complex fracture networks. Moreover, CO₂ treatment promotes the development of the pore structure in shale. After CO₂ treatment, a large number of dissolution pores occur in shale. These new micro-pores and micro-fractures greatly change the original framework of shale, considerably reduce its mechanical strength, lower the difficulty of subsequent fracturing operations and improve the fracturing performance.

(4)

With the increasing CO₂ treatment time, the macro-mechanical strength of shale decreases greatly and a large number of dissolution pores occur in shale. However, in field, the overall duration of fracturing operations is relatively short, so the reaction time and range of CO₂ injected into formations are limited, and the decline of the mechanical strength of shale is small, which has relatively fewer influences on fracturing performance compared with lengthy treatment times. If proper soaking is performed following CO₂ injection, the CO₂ reaction time can be prolonged, and the CO₂ affected range can be expanded. Then a sufficient overall reduction of the mechanical strength can be expected to significantly reduce the breakdown pressure of formations, lower the requirements for treatment equipment and improve the fracturing performance.