Experimental Analysis of the Mechanical Properties and Failure Behavior of Deep Coalbed Methane Reservoir Rocks

Abstract

A comprehensive understanding of the mechanical characteristics of deep coalbed methane reservoir rocks (DCMRR) is crucial for the safe and efficient development of deep coalbed gas resources. In this study, the microstructural and mechanical features of the coal seam roof, floor, and the coal seam itself were analyzed through laboratory experiments. The impact mechanisms of drilling fluid and fracturing fluid hydration on the mechanical properties and failure behavior of coal seam rocks were investigated. The experimental results indicate that the main minerals in coal seams are clay and amorphous substances, with kaolinite being the predominant clay mineral component in coal seam rocks. The rock of the coal seam roof and floor exhibits strong elasticity and high compressive strength, while the rock in the coal seam section shows a lower compressive capacity with pronounced plastic deformation characteristics. The content of kaolinite shows a good correlation with the mechanical properties of DCMRR. As the kaolinite content increases, the strength of DCMRR gradually decreases, and deformability enhances. After immersion in drilling fluid and slickwater, the strength of coal seam rocks significantly decreases, leading to shear fracture zones and localized strong damage features after rock compression failure. The analysis of the mechanical properties of DCMRR suggests that the horizontal well trajectory should be close to the coal seam roof, and strong sealing agents should be added to drilling fluid to reduce the risk of wellbore collapse. Enhancing the hydration of slickwater is beneficial for the formation of a more complex fracture network in deep coalbed methane reservoir.

1. Introduction

According to official Chinese data, the geological reserves of deep coalbed methane with burial depths exceeding 2000 m in China amount to 40.71 trillion cubic meters, significantly surpassing the reserves of coalbed methane within depths of 2000 m [1,2,3]. With the substantial reduction in the development of economically viable shallow coalbed methane blocks, the deep coalbed methane resources with burial depths exceeding 2000 m are expected to play a crucial role in the future large-scale development of China’s coalbed methane industry. In recent years, CNPC has successfully developed several horizontal wells in the deep coalbed methane reservoir, such as the No. 8 coalbed in the Ordos Basin, with daily natural gas production exceeding 100,000 cubic meters. However, due to the considerable burial depth of deep coalbed methane reservoirs, strong rock heterogeneity, and complex mechanical properties, drilling and hydraulic fracturing operations face significant challenges [4,5,6]. Addressing these challenges is essential for the efficient and safe development of unconventional deep coalbed methane resources in the future.

Deep coalbed methane reservoirs are characterized by thick coal seams, high gas content, low matrix permeability, and widespread coal cleat [7,8]. In deep coalbed methane reservoirs, free gas coexists with adsorbed gas, and a substantial amount of free gas exists in a high-pressure compressed state within fractures and micropores [9,10,11]. Therefore, the economic development of deep coal and gas reservoirs requires the use of horizontal wells and large-scale hydraulic fracturing techniques to create a complex network of fractures, providing favorable pathways for the flow of free gas [12]. The experimental analysis of the mechanical behavior of coal seam rocks plays a pivotal role in optimizing drilling engineering and hydraulic fracturing design. Understanding the stress–strain responses, fracture propagation characteristics, and failure mechanisms of coal rocks under various loading conditions is essential for enhancing drilling efficiency and ensuring wellbore stability. Such analyses provide critical insights into the anisotropic and heterogeneous nature of coal seams, which significantly influence the selection of drilling parameters and the design of hydraulic fracturing operations [13,14,15,16]. By accurately characterizing the mechanical properties of coal, including compressive strength, tensile strength, and Young’s modulus, engineers can develop more effective drilling strategies that minimize the risk of wellbore collapse and improve the overall success rate of hydrocarbon extraction. Moreover, the detailed study of fracture mechanics within coal seams informs the optimization of fracturing fluid viscosity, injection rates, and pressure regimes, thereby maximizing fracture network connectivity and enhancing resource recovery. In summary, the rigorous experimental investigation of coal seam rock mechanics is indispensable for advancing both drilling and hydraulic fracturing technologies, ultimately leading to more efficient and safer extraction processes [17,18,19,20].

Numerous experimental studies have investigated the mechanical properties of coal seam rocks. Mishra et al. [21] employed two feedback control modes, namely axial strain control and transverse strain control, to examine the failure behavior of coal samples. Wang et al. [22] conducted a comprehensive comparison of the mechanical behavior and seepage characteristics of coal and coal–rock combinations under triaxial conditions. Building upon a rock mechanics testing system, Xie et al. [23] observed that deep coal rock exhibits greater axial strain and volumetric strain compared to shallow coal rock, indicating significant plasticity. Investigating moisture content changes and crack expansion characteristics, Yao et al. [24] examined coal samples before and after water immersion. Ma et al. [25] explored the influence mechanism of different confining pressure and inclined coal seam thickness on the mechanical properties and failure characteristics of rock–coal–rock composite materials. Wang et al. [26] conducted experimental studies on the physical properties and rock mechanical behavior of complex-structure coalbed methane reservoirs, analyzing the impact of coal deformation on coal–rock combinations. While these studies have significantly advanced our understanding of shallow coal seams at a burial depth of approximately 1000 m, providing a crucial theoretical foundation for optimizing coal mining practices and preventing gas-related hazards, research on rock mechanical behavior in deep coal seams exceeding 2000 m remains relatively limited. Investigating the mechanical properties and failure behavior of deep coal seam gas reservoirs is of paramount importance for the efficient extraction of these resources. Understanding the complex stress conditions and fracture mechanics at depths exceeding 2000 m can significantly enhance drilling and hydraulic fracturing strategies, leading to improved wellbore stability and optimized fracture networks. This research holds immense potential value by enabling the development of tailored extraction techniques that maximize gas recovery while minimizing operational risks and environmental impact. Ultimately, such insights are critical for tapping into the vast reserves of deep coal seam gas, thereby contributing to a more sustainable and economically viable energy supply. Therefore, this study aims to comprehensively explore the mechanical properties and failure behavior of deep coal seams with a burial depth exceeding 2000 m, aiming to fill existing gaps in the current research landscape.

This paper takes deep coalbed methane reservoir rocks as the research object. Initially, the microstructure and principal mineral components of the rocks of the coal seam roof, coal seam and coal seam floor were analyzed. Subsequently, the study focused on investigating the rock mechanical properties and water-absorbing swelling behavior of distinct components, including the coal seam roof, coal seam, and coal seam floor. Finally, the research delved into the changes in mechanical properties of coal seam rocks following immersion in drilling fluid and fracturing fluid, coupled with an analysis of the failure behavior after triaxial compression. The outcomes of this study offer valuable theoretical guidance for the safe and efficient development of deep coalbed methane resources.

2. Mineral Content and Microstructure Analysis of DCMRR

DCMRR exhibit strong heterogeneity, and there are significant differences in the physical and mechanical properties of coal seams, roofs and floors. Analyzing the mineral content and microstructural features of DCMRR is beneficial for understanding the reasons behind the differences in rock properties.

The drilling cores from the No. 8 coal seam in the Yichuan area of the Ordos Basin, China, were used for experimental analysis in this study. The No. 8 coal seam has a burial depth exceeding 2000 m, and the strong heterogeneity of reservoir rocks poses significant challenges for drilling and hydraulic fracturing. As shown in Figure 1, 13 rock samples were obtained from the Y-10-23-56 wellbore in the No. 8 coal seam, where the roof of the coal seam consists of dark gray mudstone and the floor comprises gray-black carbonaceous mudstone. XRD (X-ray diffraction) technology was employed to analyze the mineral content of the 13 rock samples, as illustrated in Figure 2. From Figure 2, it can be observed that the rock minerals in the coal seam roof are predominantly calcite, with a content generally exceeding 90%, while clay mineral content is very low. The rocks in the coal seam section are mainly composed of clay minerals and amorphous substances, with clay mineral content generally exceeding 30%, and significant variations in the content of each mineral. The rocks in the coal seam floor are primarily composed of clay minerals and quartz, with clay mineral content exceeding 60%. As clay minerals are crucial factors affecting rock mechanical properties and rock hydration damage during drilling and fracturing processes, further analysis of the composition of clay minerals was conducted, as shown in Figure 3. From Figure 3, it is evident that kaolinite is the main component of clay minerals in the coal seam, coal seam roof, and coal seam floor. The relative content of kaolinite in the coal seam and coal seam floor is higher than in the coal seam roof. Montmorillonite content in DCMRR clay minerals is extremely low, with small amounts of chlorite and illite/montmorillonite interlayer. Combining Figure 1 and Figure 2, it can be observed that the mineral content variations in the coal seam roof and floor rocks are relatively small, indicating good rock homogeneity. In contrast, the mineral content variations in the rocks of the coal seam section are significant, indicating poor rock homogeneity. Since clay minerals are primarily composed of kaolinite, the hydration dispersion of montmorillonite is not easily triggered. The high kaolinite content in the coal seam and coal seam floor necessitates consideration of the underground accident risk associated with the hydration expansion of kaolinite during the drilling process.

To investigate the microscopic structural characteristics of the coal seam, scanning electron microscopy (SEM) was employed to observe the surface of coal seam rocks. Figure 4 displays several typical microscopic structural features observed in the coal seam rocks. As shown in Figure 4a,b, the coal exhibits a clustered structure with microcracks developing between clusters, and coal powder adheres to the surface of these clusters. From Figure 4c,d, it can be seen that kaolinite minerals in the coal seam present a lamellar structure, filling the pores between clusters. Figure 4e,f suggest that the microscopic structure of deep coal seams may resemble that of biogenic fossils, where pores and microcracks develop, and lamellar kaolinite aggregates may fill these pores and microcracks. The comprehensive analysis from scanning electron microscopy indicates that microcracks and pores develop in the microscopic structure of deep coal seams, facilitating the construction of natural gas permeation pathways. However, it is crucial to avoid the blockage of pores by the hydration expansion of lamellar kaolinite as much as possible.

3. Analysis of the Mechanical Properties of DCMRR

3.1. Experimental Process

The mechanical properties of DCMRR are directly related to the design of drilling and fracturing construction parameters. The frequent occurrence of instability accidents in deep coal rock wellbore walls highlights the importance of understanding the mechanical properties of the coal seam roof, coal seam, and coal seam floor for the rational optimization of on-site construction. A true triaxial compression experiment was conducted to analyze the mechanical properties of nine rock samples from various sections of Well Y-10-23-56 within the No. 8 coalbed methane reservoir. The experimental process was as follows. (1) A wire cutting machine was used to take a cylindrical standard rock sample with a diameter of 2.5 cm and a height of 5 cm. (2) A standard rock sample was installed in a three-axis compressor, and confining pressure (45 MPa) was applied to simulate in situ stress. (3) Axial stress was continuously applied until the rock sample broke. The stress and strain values were recorded during the rock fracture process, and the fracture mode of the sample was observed.

3.2. Experimental Result Analysis

Table 1. Test results of rock mechanical parameters of coal seam roof, coal seam and coal seam floor.

Number	Rock Property	Elasticity Modulus/MPa	Poisson’s Ratio	Compressive Strength/MPa
1	Coal seam roof	23,804.35	0.13	128.78
2	Coal seam roof	22,080.39	0.17	124.67
3	Coal seam roof	24,264.17	0.15	137.60
4	Coal seam	4665.02	0.32	23.04
5	Coal seam	6230.87	0.42	12.02
6	Coal seam	1479.86	0.41	6.07
7	Coal seam floor	17,105.68	0.19	75.87
8	Coal seam floor	21,198.39	0.21	84.39
9	Coal seam floor	19,708.39	0.26	46.16

presents the results of the true triaxial compression tests conducted on the rock samples. From Table 1, it can be observed that the coal seam roof has a higher elastic modulus, smaller Poisson’s ratio, and is relatively hard, with compressive strength far greater than that of the coal seam rock. The coal seam rock has a lower elastic modulus, larger Poisson’s ratio, stronger rock plasticity, and much lower compressive strength. The coal seam floor has a higher elastic modulus, smaller Poisson’s ratio, and greater compressive strength than the coal seam rock but less than the coal seam roof.

Figure 5 shows the typical stress–strain curves obtained from the experiments for the coal seam roof, coal seam, and coal seam floor. From Figure 5, it can be observed that the stress–strain curves of coal seam rock change relatively smoothly without clear fracture points. Compared to the coal seam roof and floor, the stress–strain curve of the coal seam exhibits strong plastic characteristics. The stress–strain curves of the coal seam roof and floor basically overlap in the elastic stage, and the deformability of the coal seam roof and floor is very similar during the elastic deformation stage. Additionally, compared to the coal seam floor, the coal seam roof requires a higher stress to undergo fracture, and its failure characteristics are more in line with elastic behavior, while the failure rate of the coal seam floor is relatively gradual.

Combining the results from Table 1 and Figure 5, it can be concluded that due to the coal seam roof’s highest compressive strength and strong elasticity, during the process of horizontal drilling in a coalbed methane reservoir, it is advisable to stay close to the roof to reduce the risk of wellbore collapse. The coal seam section has very low compressive strength and strong rock plasticity, so precautions must be taken during drilling to prevent wellbore collapse accidents caused by sudden changes in rock type.

The experimental results of true triaxial compression on rock specimens are illustrated in Figure 6, Figure 7 and Figure 8. The left side shows cores obtained through drilling, while the right side displays the fracture morphology of standard rock cores after the triaxial compression experiment. As depicted in Figure 6, under compressive loading, fractures resulting from the failure of rock in the coal seam roof extend longitudinally through the entire core, exhibiting a relatively singular pattern, possibly forming a Y-shaped configuration. Benefiting from the coal cleat and well-developed microcracks in the coal seam, Figure 7 demonstrates that the coal seam rock forms a complex network of fractures after triaxial compression. The distribution of fractures is highly irregular, with longitudinal and transverse fractures intersecting and tending to propagate along the coal cleat planes of the rock. As shown in Figure 8, fractures in the coal seam floor, following compression-induced failure, exhibit a more singular morphology with a shorter extension distance.

The XRD analysis results in the previous section have indicated that kaolinite is the primary component of the clay minerals in both the coal seam and the roof and floor of the coal seam. Consequently, we have analyzed the relationship between the kaolinite content and the mechanical parameters of the DCMRR, as shown in Figure 9, Figure 10 and Figure 11. From Figure 9 and Figure 10, it can be observed that as the kaolinite content increases, the compressive strength and elastic modulus of the DCMRR exhibit a decreasing trend. However, in Figure 11, the increasing kaolinite content leads to a gradual increase in the Poisson ratio of the DCMRR. Power functions can reasonably fit the relationships between kaolinite content and compressive strength, elastic modulus, and Poisson’s ratio of the DCMRR. Overall, from Figure 9, Figure 10 and Figure 11, it can be inferred that the presence of kaolinite in the clay minerals of the DCMRR weakens the overall strength of the rock, and the higher the content, the more pronounced the deformation capacity of the DCMRR.

Due to the sensitivity of kaolinite to water, it is prone to water absorption and physical expansion [27]. Therefore, we conducted distilled water immersion experiments to analyze the volumetric expansion of DCMRR after water absorption. Figure 12 illustrates the volume expansion rates of the coal seam roof and floor, as well as the coal seam core, after immersion in distilled water until stabilization at room temperature. As depicted in Figure 12, there is a notable correlation between the kaolinite content and the water absorption expansion rate of the DCMRR. The higher the kaolinite content, the greater the water absorption expansion rate of the DCMRR. The water absorption expansion rate of the coal seam roof is relatively low, favoring wellbore stability during drilling. However, the water absorption expansion rate of the coal seam floor exceeds 10%, necessitating precautions against the hydrating expansion of kaolinite if the wellbore trajectory intersects with the DCMRR floor during adjustment.

4. Effects of Fracturing Fluid and Drilling Fluid on the Mechanical Properties of Coal Seam Rocks

4.1. Rock Immersion Experimental Procedure

In the field of oil exploration, the working fluids used on site may undergo hydration reactions with reservoir rock minerals, thereby exerting a significant influence on the mechanical properties of the rock [28,29,30]. In this section, we conducted immersion experiments on coal seam rock cores from well Y-10-23-56 using both drilling fluid and fracturing fluid to investigate the mechanisms by which these two different working fluids impact the mechanical properties of coal seam rock. The standard rock cores used for immersion were sourced from the same rock column to ensure that their mechanical properties were closely matched. The fluids used for immersion were directly obtained from the working fluid systems commonly used in drilling and fracturing operations in the Yichuan area of the Ordos Basin, China. Prior to the immersion experiments, both the working fluids and the rocks were heated to reservoir temperature. Subsequently, the working fluids were heated in a water bath, and immersion experiments were conducted at constant temperature for varying durations. The experimental process was as follows. (1) A wire cutting machine was used to cut standard cylindrical rock samples from the same coal seam rock. (2) A standard rock sample was placed in the heating furnace and heated to the formation temperature (80 °C). (3) Drilling fluid and fracturing fluid were placed in a beaker and heated in a water bath to formation temperature. (4) The heated rock was taken out and placed in the fracturing fluid or drilling fluid heated in a water bath for soaking. (5) The rock samples soaked for a certain period of time were taken out, and triaxial compression experiments were conducted.

4.2. Effect of Drilling Fluid on Coal Seam Rocks

Figure 13 and Figure 14 depict the results of triaxial compression experiments on coal seam rock samples immersed for different durations in water-based drilling fluid and polymer drilling fluid, commonly used in drilling operations in the Yichuan area. It is evident from Figure 13 and Figure 14 that, compared to the non-immersed coal seam rock, the fractures in the coal seam rock subjected to compression after immersion in drilling fluid become more intricate. The drilling fluid significantly weakens the strength of the coal seam rock, causing coal cleats that would not have activated under compression loads to open, resulting in more fractures. Under drilling fluid immersion, the distribution of fractures after rock compression exhibits clustering characteristics, potentially forming localized areas of concentrated damage. Immersion in water-based drilling fluid induces the formation of more longitudinal fractures in the coal seam rock under compression, while polymer drilling fluid immersion may lead to the generation of shear fractures. The immersion in different drilling fluid systems leads to distinct clustering features in the morphology of fractures after rock compression.

Figure 15 illustrates the variation curve of compressive strength in coal seam rock over time under immersion in different drilling fluid systems. It is evident from Figure 15 that, with increasing immersion time, the compressive strength of coal seam rock experiences a substantial decrease in the initial four days, followed by a gradual transition into a period of slow decline. In comparison to the water-based drilling fluid system, the polymer drilling fluid system leads to a faster decline in coal seam rock strength, and its impact on coal seam rock strength in the later stages of drilling is more pronounced. During the early stages of horizontal well drilling in the coal seam, drilling fluid immersion significantly influences the strength of the coal seam rock, thereby substantially increasing the risk of collapse in the coal seam wellbore. Therefore, in the drilling process of deep coal seams, drilling fluid systems must incorporate sealing agents to block the coal cleats and microfractures around the wellbore, thereby minimizing the impact of drilling fluid on rock strength as much as possible.

Figure 16 and Figure 17 depict the stress–strain curves obtained from triaxial compression experiments on coal seam rock under immersion in different drilling fluid systems. From Figure 16, it can be observed that the slope of the linear segment of the stress–strain curve under immersion in water-based drilling fluid remains relatively consistent, indicating minimal influence on the mechanical properties of rock during the elastic deformation stage. However, with increasing immersion time, the length of the linear segment decreases, while the length of the curve segment increases, indicating a reduction in the elastic deformation capability and an increase in the plastic deformation capability of coal seam rock under immersion in water-based drilling fluid. In Figure 17, under immersion in polymer drilling fluid, the deformation and failure characteristics of the rock are generally similar, indicating that drilling fluid immersion affects the strength of the rock but has minimal impact on the characteristics of the stress–strain curve. Furthermore, comparing Figure 16 and Figure 17 reveals that, in comparison to polymer drilling fluid, water-based drilling fluid immersion enhances the plastic deformation capability of coal seam rock. The rock undergoes compression at a slower rate, which is advantageous in preventing wellbore collapse accidents. In summary, the results from Figure 15, Figure 16 and Figure 17 suggest that the water-based drilling fluid system currently used in the Yichuan drilling site is more conducive to preventing wellbore collapse accidents.

4.3. Effect of Fracturing Fluid on Coal Seam Rocks

Figure 18 illustrates the results of triaxial compression experiments on coal seam rock after immersion in slickwater for various durations, a fluid commonly used in the Yichuan fracturing site. From Figure 18, it is evident that compared to the unimmersed coal seam rock, the rock immersed in hydraulic fracturing fluid exhibits a more complex pattern of fractures after compression failure. In comparison to the experimental results with drilling fluid, the coal seam rock immersed in fracturing fluid may develop shear bands and shows pronounced localized damage on the surface of the core. Slickwater significantly weakens the strength of coal seam rock, causing the opening of coal cleats and micro-cracks under compression loads, leading to the initiation of more shear fractures around the main fracture.

Figure 19 displays the variation curve of coal seam rock compressive strength over time under slickwater immersion. Figure 20 illustrates the stress–strain curves obtained from triaxial compression experiments on coal seam rock immersed in slickwater. From Figure 19, it can be observed that with increasing immersion time, the compressive strength of coal seam rock significantly decreases in the initial 3 h and then gradually stabilizes. Figure 20 reveals that after 0.5 h of slickwater immersion, the rock strength slightly decreases, but the stress–strain curve remains essentially similar to the unimmersed coal seam rock, with no significant changes in rock deformation and failure characteristics. However, as immersion time increases, the stress–strain curve gradually becomes more gentle, indicating plastic failure characteristics in the rock under compression. Combining Figure 18, Figure 19 and Figure 20, it is evident that the hydraulic effects of slickwater on coal seam rock cannot be ignored during the fracturing construction period. In the early stages of deep coal seam rock fracturing, enhancing the hydraulic effects of slickwater may be advantageous for opening coal cleats and micro-cracks around the main fracture, forming a more complex fracture network in the coal seam rock reservoir.

5. Conclusions

Using core samples obtained from drilling in the deep coalbed methane reservoir of the No. 8 coal seam in the Yichuan area of the Ordos Basin, China, a series of laboratory experiments were conducted in this study to investigate the mechanical properties and failure behavior of the deep coalbed reservoir. According to the results, the major conclusions can be summarized as follows:

(1)

The roof of the coal seam is composed of deep gray argillaceous limestone with a calcite content exceeding 90%, while the floor consists of dark gray carbonaceous mudstone with a clay mineral content exceeding 60%. The rock in the coal seam section is dominated by clay minerals and amorphous substances. The clay minerals in both the roof, floor, and coal seam section primarily consist of kaolinite, and the content of montmorillonite is extremely low, making it less prone to hydration dispersion. Microcracks and pores are developed in the coal seam, facilitating the establishment of natural gas permeation pathways. However, these internal spaces may be filled with sheet-like kaolinite aggregates that are susceptible to hydration expansion.

(2)

The roof and floor of the coal seam are relatively dense and hard, exhibiting strong elasticity and compressive strength, with a more uniform fracture pattern after compression. In contrast, the rocks in the coal seam section have lower compressive strength and show pronounced plastic deformation characteristics. After compression-induced failure, fractures tend to extend along coal cleats, resulting in a distribution of fractures that intersects both longitudinally and transversely.

(3)

The content of kaolinite exhibits a notable correlation with the mechanical properties of DCMRR. As the kaolinite content increases, the compressive strength and elastic modulus of DCMRR decrease, while the Poisson’s ratio increases. The presence of kaolinite weakens the strength of DCMRR, and higher kaolinite content is associated with more pronounced deformation capabilities and greater water absorption expansion in the rock. During the horizontal drilling process, it is advisable to keep the wellbore trajectory as close to the roof of the coal seam as possible to minimize the risk of borehole collapse.

(4)

Under the immersion of drilling fluid and slickwater, the strength of the coal seam rock is significantly reduced. After the rock undergoes compressive failure, shear fracture zones and localized strong damage characteristics may appear. During the drilling process, precautions should be taken to prevent the risk of borehole collapse caused by the immersion of drilling fluid in the rock. Enhancing the hydration effect of slickwater on coal seam rock may contribute to the formation of a complex fracture network in the deep coalbed methane reservoir.