Quantitative Mechanisms of Long-Term Drilling-Fluid–Coal Interaction and Strength Deterioration in Deep CBM Formations

Abstract

During deep coalbed methane (CBM) drilling, wellbore stability is significantly influenced by the interaction between drilling fluid and coal rock. However, quantitative data on mechanical degradation under long-term high-temperature and high-pressure conditions are lacking. This study subjected coal cores to immersion in field-formula drilling fluid at 60 °C and 10.5 MPa for 0–30 days, followed by uniaxial and triaxial compression tests under confining pressures of 0/5/10/20 MPa. The fracture evolution was tracked using micro-indentation (µ-indentation), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), establishing a relationship between water absorption and strength. The results indicate a sharp decline in mechanical parameters within the first 5 days, after which they stabilized. Uniaxial compressive strength decreased from 36.85 MPa to 22.0 MPa (−40%), elastic modulus from 1.93 GPa to 1.07 GPa (−44%), cohesion from 14.5 MPa to 5.9 MPa (−59%), and internal friction angle from 24.9° to 19.8° (−20%). Even under 20 MPa confining pressure after 30 days, the strength loss reached 43%. Water absorption increased from 6.1% to 7.9%, showing a linear negative correlation with strength, with the slope increasing from −171 MPa/% (no confining pressure) to −808 MPa/% (20 MPa confining pressure). The matrix elastic modulus remained stable at 3.5–3.9 GPa, and mineral composition remained unchanged, confirming that the degradation was due to hydraulic wedging and lubrication of fractures rather than matrix damage. These quantitative thresholds provide direct evidence for predicting wellbore stability in deep CBM drilling.

1. Introduction

Amid the global energy transition and the pursuit of “dual-carbon” targets, coal-bed methane (CBM) has become a critical low-carbon energy resource. Efficient extraction of deep CBM reservoirs is thus receiving growing research attention. For deep CBM wells, wellbore stability is a fundamental requirement for safe and continuous drilling operations. Among the controlling factors, the mechanical behavior of coal plays a decisive role, with its importance increasing significantly at greater depths [1,2,3]. However, as exploration extends to deeper formations, deep CBM development faces several technical challenges. Notably, the evolution of coal’s mechanical properties under high-temperature and high-pressure (HTHP) conditions, and its implications for production efficiency, remains poorly understood [4,5,6]. Therefore, a comprehensive study of drilling-fluid–coal interactions and their effects on coal mechanics is not only essential for optimizing drilling processes, reducing costs, and improving efficiency but also a scientific prerequisite for the safe and sustainable exploitation of deep CBM resources [7].

Experimental studies have consistently shown that drilling-fluid filtrate can severely damage coal seams. Filtrate invasion significantly reduces coal permeability, weakens mechanical integrity, and enhances stress sensitivity [8,9,10]. Further research has identified water-sensitive minerals, such as illite and kaolinite, as key contributors to the deterioration of coal mechanical properties upon fluid exposure. Prolonged interaction with drilling fluids leads to progressive declines in uniaxial compressive strength and elastic modulus, with degradation rates increasing with immersion time [11]. The extent of mechanical weakening also depends strongly on fluid composition; notably, when mud salinity falls below formation-water salinity, the damaging effect intensifies [12]. However, most existing studies have examined only short-term fluid exposure under ambient conditions, failing to account for the combined thermal and mechanical stresses encountered in deep CBM reservoirs.

Advances in numerical modeling have enhanced our understanding of drilling-fluid–coal interactions. Within the continuum-mechanics framework, researchers have developed constitutive models that account for coal heterogeneity, anisotropy, and damage evolution to characterize strength degradation during fluid exposure [13,14,15,16]. Hou et al. [17,18,19] introduced a fully coupled thermo-hydro-mechanical (THM) model capable of predicting how different drilling fluid formulations affect coal strength evolution, providing a valuable tool for deep CBM development. Gentzis et al. [20] further advanced the field by proposing a dual-porosity, dual-permeability model to analyze fluid invasion effects on coal seepage behavior. Despite these advances, several challenges remain. Discrepancies between laboratory and in situ conditions, uncertainties in model parameters, and a lack of long-term exposure data under realistic wellbore conditions continue to constrain model accuracy [21,22,23,24].

Despite significant progress, critical gaps persist in understanding the long-term effects of drilling fluids on coal under realistic reservoir temperature-pressure conditions. Current studies primarily focus on short-term experiments and simulations, leaving a notable lack of systematic research on prolonged high-temperature, high-pressure (HTHP) exposure effects, particularly regarding the evolution of coal mechanical properties, microstructural alterations, and fracture development. This knowledge gap complicates accurate long-term mechanical parameter prediction and wellbore-stability assessment, posing substantial challenges for deep CBM development [25,26,27]. To bridge this gap, this study conducts experiments on the interaction between drilling fluid and coal under long-term HTHP (60 °C, 10.5 MPa) conditions. By systematically tracking mechanical-property degradation over time and employing microscale characterization techniques (e.g., micro-indentation, nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM)), we aim to unravel the underlying mechanisms of coal weakening. The results and insights can provide a theoretical basis for assessing wellbore stability during deep CBM drilling, thereby optimizing drilling fluid formulations and drilling processes to improve the efficiency and safety of deep coalbed methane development.

2. Coal Specimen Preparation and Drilling-Fluid Formulation

2.1. Coal Specimen Preparation

Representative coal blocks were extracted from an active CBM drilling site, ensuring direct geological correspondence with the target formation. To minimize variability, samples were sourced exclusively from a single, lithologically homogeneous seam. As shown in Figure 1, cylindrical cores (25 mm diameter × 50 mm height) were drilled perpendicular to the bedding plane using a computer-controlled coring lathe. A low-speed wire-cut electro-discharge technique was employed to prevent thermally induced microcracking during coring. Subsequent machining included precision cutting, initial end-face grinding, and final polishing with a diamond wheel grinder. Dimensional compliance was verified against Chinese National Standard GB/T 40961-2021 [28], with strict tolerances.

Following preparation, the cylindrical coal specimens were dried in a forced-convection oven at 60 ± 2 °C for 48 h to eliminate free moisture, establishing a consistent dry baseline for immersion testing. Oven temperature and specimen mass were monitored at 2-h intervals to ensure thermal stability and confirm mass stabilization (defined as Δm < 0.1 mg over any 2-h window). After cooling in a desiccator, each specimen was weighed on an analytical balance (resolution: 0.1 mg) to record its initial dry mass, the reference value for all subsequent water-uptake calculations. For traceability, unique identifiers were assigned, and key data (geological origin, geometric dimensions, and baseline mass) were logged in a relational database, ensuring efficient management throughout the experimental campaign.

2.2. Drilling Fluid Preparation

The drilling fluid was prepared according to the actual ratio in the field, which is shown in

Table 1. Drilling fluid composition (wt%).

Anti-Collapse Polymeric Plugging Agent	Low-Viscosity Anionic Carboxymethyl Cellulose	Anti-Collapse Fluid-Loss Reducer	Encapsulating Polymer	Anti-Collapse Gilsonite
0.1%	1.0%	1.0%	1.0%	3.0%

. As shown in Figure 1, The rheological properties of the freshly mixed fluid were characterized using a six-speed rotational viscometer at ambient temperature. Dial readings at 3, 6, 100, 200, 300, and 600 r/min yielded corresponding apparent viscosities of 2, 3, 19, 31, 41, and 72 mPa·s, respectively. Based on these measurements, the apparent viscosity and plastic viscosity were calculated separately, and were 36 mPa·s and 31 mPa·s, respectively.

3. Experimental Scheme

Specimens were mounted on a corrosion-resistant sample rack housed within a high-temperature and high-pressure reaction autoclave. Freshly prepared drilling fluid was introduced into the vessel under laminar flow conditions at a controlled rate until complete submersion of the specimens. The autoclave was sealed and conditioned to 60 °C and 10 MPa, simulating typical deep coalbed methane (CBM) drilling conditions. Specimens were exposed for 1, 3, 5, 10, 20, and 30 days to assess the progressive impact of drilling fluid on mechanical behavior and pore structure evolution. After each exposure interval, the heating and pressurization systems were deactivated, and specimens were extracted only after temperature and pressure stabilized at ambient conditions. Residual drilling fluid was removed by three sequential rinses with deionized water, followed by gentle blotting with lint-free filter paper. After cleaning, the coal samples were precisely weighed again to record their post-immersion mass. The water absorption rate of the coal samples was then calculated, and its variation over immersion time was analyzed to evaluate the softening effect of drilling fluid on the coal. It should be noted that in this experimental study, constrained by the functional limitations of the existing experimental equipment, the “soaking first, then loading” sequence was adopted as a compromise solution to simulate downhole conditions as effectively as possible under current constraints. The research emphasizes elucidating the relationship between the microstructural evolution and macro-mechanical responses of drilling fluids and coal rock fracture systems under long-term contact conditions.

Subsequently, uniaxial and triaxial compression tests were performed using an MTS 815.04 servo-hydraulic system at confining stresses of 0, 5, 10, and 20 MPa. Each experimental condition was tested in duplicate to account for natural specimen heterogeneity (see

Table 2. Mechanical Testing Scheme.

Confining Stress/MPa	Immersion Time/day
0	0, 1, 3, 5, 10, 20, 30
5
10
20

(Note: Mechanical tests were conducted under 4 different confining stresses for different immersion days, with 2 samples tested for each condition, totaling 56 samples).

). To investigate the underlying degradation mechanisms, post-mechanical characterization included: Micro-indentation to assess matrix elastic modulus, NMR for quantitative fracture analysis, and SEM for qualitative evaluation of fracture propagation. This multimodal dataset provides a mechanistic understanding of how fluid–rock interactions influence the macroscopic mechanical behavior of coal.

4. Test Equipment and Test Method

4.1. High-Temperature and High-Pressure Reactor

The HTHP reactor (Self-Developed in Wuhan, China) is an apparatus designed to replicate the thermomechanical conditions prevalent in deep CBM. Equipped with precision control systems, it maintains temperatures ranging from ambient to 200 °C (±1 °C) and pressures of 0–100 MPa (±0.1 MPa), enabling accurate simulation of in situ stress states. The fluid circulation system maintains a uniform flow of drilling fluid, ensuring complete and consistent immersion of coal specimens.

4.2. Uniaxial/Triaxial Compression Testing Equipment

Uniaxial and triaxial compression tests were conducted using an MTS 815.04 servo-hydraulic rock-testing system (Made in Minnesota, USA). The apparatus achieves a maximum axial compressive load of 4600 kN and a maximum tensile load of 2300 kN, with confining stresses adjustable up to 140 MPa. The system accommodates both stress-controlled and strain-controlled loading regimes, with a feedback resolution exceeding 0.1% of full scale. Axial load, displacement, and confining stress are continuously recorded using high-precision transducers at sampling rates reaching 1 kHz, enabling accurate derivation of load–displacement relationships for subsequent constitutive modeling under ambient or elevated thermomechanical conditions.

4.3. Millimeter Indentation Test Equipment

Micro-indentation tests were conducted using the millimeter-scale indentation system. The setup consists of a modified WANCE 503A (Made in Wuhan, China)electromechanical universal testing frame, equipped with a precision actuator that drives a diamond indenter into and out of polished sample surfaces. During testing, a miniature load cell and linear variable differential transformer (LVDT) continuously record applied load and penetration depth, respectively. The acquired load–displacement data are processed using the Oliver–Pharr method [29] to determine key mechanical properties of the coal matrix, including elastic modulus and hardness.

5. Test Results

5.1. Compressive Strength of Coal Under Different Immersion Times

Figure 2 displays representative stress–strain curves for coal specimens subjected to varying immersion durations, while Figure 3 summarizes the evolution of mechanical parameters under different confining stresses.

Coal rock contains inherent fractures, joints, and variations in microhardness, leading to strength discrepancies between parallel samples under identical conditions. This study tested the strength of parallel samples under confining pressures of 5–20 MPa, using the average value as the final result. All trend descriptions in Figure 3 are based on these averages, with error bars added to illustrate data variability. The observed variation aligns with the range recommended by the International Society for Rock Mechanics for sedimentary rock parallel samples, reflecting the inherent heterogeneity of coal rock. Therefore, this discreteness does not undermine the statistical reliability of the degradation trend.

As depicted in Figure 3a, the uniaxial compressive strength (UCS) exhibits a monotonic decline with increasing exposure time. Under unconfined conditions, UCS decreases sharply from 36.85 MPa (intact) to 9.33 MPa after 30 days, representing a 74.68% reduction. Under confining stress conditions, while the compressive strength increase significantly, the relative strength reductions remain considerable. Specifically, compressive strength decreases by 53.37% (from 66.51 MPa to 31.01 MPa) at 5 MPa confining stress, by 49.34% (from 74.32 MPa to 37.65 MPa) at 10 MPa, and by 43.25% (from 90.37 MPa to 51.28 MPa) at 20 MPa confining stress. Notably, most strength degradation occurs within the first five days of exposure, with compressive strength reductions ranging from 40.27% to 69.28% across all confining stress levels. Beyond this period, further strength loss is minimal (≤4.04%), indicating that drilling-fluid–coal interactions predominantly weaken the coal within the initial five-day interval, after which mechanical properties stabilize.

From the perspective of confining stress effects, the strength of coal under the same immersion duration increases with higher confining stress. For instance, after 30 days of immersion, the compressive strengths under confining stresses of 5 MPa, 10 MPa, and 20 MPa were approximately 31.01 MPa, 37.67 MPa, and 51.28 MPa, respectively, all significantly higher than the uniaxial strength of 9.33 MPa. Moreover, as the confining stress increased, the strength reduction rate after 30 days of immersion decreased from 74.68% (uniaxial) to 43.26% (20 MPa). These results demonstrate that confining stress enhances the mechanical properties of coal, effectively mitigating the weakening effect induced by drilling fluid exposure and stabilizing its long-term mechanical performance.

5.2. Cohesion and Internal Friction Angle of Coal Under Different Immersion Times

Figure 3c presents the time-dependent variations in Mohr–Coulomb strength parameters (cohesion c and friction angle φ) under different confining stresses. Both parameters exhibit a progressive reduction with prolonged exposure to drilling fluid, showing consistent behavior with the uniaxial compressive strength trend. Specifically, cohesion experiences a significant decrease from 14.51 MPa to 5.43 MPa after 30 days of immersion, representing a 62.58% reduction in shear strength contribution. The friction angle shows a more gradual decline from 24.95° to 18.65° over the same period, corresponding to a 25.25% reduction in frictional resistance.

The strength degradation predominantly occurs during the initial five-day exposure period, with cohesion decreasing by 59.13% and the friction angle reducing by 20.60%. Subsequent changes from day 5 to day 30 are negligible, with additional reductions of ≤4.64% for both parameters. This clearly indicates that the drilling-fluid–coal interaction predominantly weakens the mechanical properties during the first five days of exposure before reaching a stabilized state where only minimal further deterioration occurs, with cohesion showing significantly greater susceptibility to fluid-induced weakening compared to the more stable frictional component.

5.3. Elastic Modulus and Poisson’s Ratio of Coal Under Different Immersion Times

Figure 3c demonstrates the time-dependent evolution of elastic modulus under different confining stresses, revealing a consistent degradation pattern where elastic modulus decreases monotonically with prolonged drilling fluid exposure while showing significant stress-dependence, as evidenced by the 49.74% reduction under uniaxial conditions (1.93 GPa to 0.97 GPa over 30 days) compared to substantially lower reductions of 12.99% (2.54→2.21 GPa), 10.38% (2.60→2.33 GPa) and 9.59% (2.71→2.45 GPa) at 5, 10 and 20 MPa confinements, respectively, clearly illustrating the protective effect of confining stress against modulus degradation, with the most severe weakening occurring during the initial five-day period (44.56% reduction for uniaxial and 9.05–12.33% for confined cases) followed by stabilization where subsequent reductions (5–30 days) remain below 1.57%, confirming that both the magnitude and temporal characteristics of elastic modulus degradation are strongly influenced by confinement conditions while maintaining the observed pattern of initial rapid weakening and subsequent stabilization.

Figure 3d presents the evolution of Poisson’s ratio in coal under varying confining stresses during fluid immersion. The results reveal two key characteristics: (1) Poisson’s ratio remains remarkably stable throughout the 30-day immersion period at each confinement level, with values ranging from 0.20–0.22 (uniaxial), 0.24–0.30 (5 MPa), 0.27–0.31 (10 MPa) to 0.30–0.33 (20 MPa); (2) a pronounced positive correlation exists between Poisson’s ratio and confining stress, as evidenced by the systematic increase in mean values from 0.21 (uniaxial) to 0.31 (20 MPa). This behavior reflects the enhanced lateral deformation capacity induced by confining constraints, while demonstrating exceptional temporal stability of this mechanical parameter during prolonged fluid exposure.

5.4. Water Absorption Rate of Coal Under Different Immersion Times

Figure 4 illustrates the evolution of water absorption rate in coal under varying confining stresses at different immersion times. Experimental results demonstrate that while the water absorption rates at individual time points show variations (ranging from 5.13% to 7.08% after 1 day of immersion and 7.30% to 8.815% after 30 days), the overall trend reveals a clear pattern. The average water absorption rate exhibits rapid growth from 6.11% to 7.53% during the initial 5-day immersion period, followed by gradual stabilization within the range of 7.78% to 7.90%. This observation suggests an approximately inverse correlation between coal strength and water absorption rate—higher absorption rates correspond to lower mechanical strength.

6. Discussion and Analysis

To elucidate the intrinsic mechanisms underlying the weakening effect of drilling fluid immersion on coal’s macroscopic mechanical properties, a comprehensive experimental investigation was conducted, including (1) millimeter-scale indentation tests to characterize matrix mechanical evolution, (2) XRD analysis for mineralogical composition changes, (3) NMR pore structure analysis, and (4) SEM imaging of fracture development. This multi-scale approach systematically examines the microstructural alterations in coal subjected to drilling fluid under high-temperature and high-pressure conditions. Furthermore, a quantitative correlation analysis was systematically performed to establish the functional relationship between compressive strength and water absorption characteristics in coal, providing a basis for predictive strength assessment.

6.1. Elastic Modulus and Mineral Composition Changes in Coal Matrix

Micro-indentation tests were systematically performed to assess the time-dependent evolution of coal matrix mechanical properties following different immersion periods, with particular attention given to selecting intact regions away from fractures or structural discontinuities. As illustrated in Figure 5, the elastic modulus of the coal matrix demonstrates remarkable stability throughout the immersion period, maintaining consistent values between 3.5 and 3.9 GPa regardless of exposure duration. These results conclusively show that the deterioration of bulk mechanical properties in coal subjected to drilling fluid exposure does not originate from matrix-scale degradation.

To elucidate the mechanisms underlying coal’s mechanical degradation during drilling fluid exposure, X-ray diffraction (XRD) was performed to characterize temporal changes in the coal matrix’s mineralogical composition (Figure 6,

Table 3. Main mineral components of coals with different immersion days.

Sample Number	Mineral Mass Fraction (%)
	Amorphous	Quartz	Kaolinite	Montmorillonite	Plagioclase	Siderite	Dolomitic
Not soaked-1	88.5	3.8	7.8	/	/	/	/
Not soaked-2	91.5	2.9	5.3	/	/	/	0.3
Soak for 1 day-1	86.2	6.7	7.1	/	/	/	/
Soak for 1 day-2	88.5	4.4	6.4	/	0.7	/	/
Soak for 3 days-1	88.5	2.3	8.5	/	/	0.7	/
Soak for 3 days-2	87.6	5.0	6.8	0.7	/	/	/
Soak for 5 days-1	93.2	1.9	5.0	/	/	/	/
Soak for 5 days-2	93.1	1.9	4.9	/	/	/	/
Soak for 10 days-1	87.2	4.7	8.2	/	/	/	/
Soak for 10 days-2	91.2	1.4	7.4	/	/	/	/
Soak for 20 days-1	89.7	2.7	7.6	/	/	/	/
Soak for 20 days-2	90.0	4.3	5.8	/	/	/	/
Soak for 30 days-1	88.7	3.4	6.9	/	/	1.0	/
Soak for 30 days-2	90.4	2.7	6.3	/	/	0.6	/

). Quantitative phase analysis revealed a composition dominated by amorphous organic matter (86.2–93.2 wt%), with subordinate crystalline phases including quartz (1.4–6.7 wt%) and kaolinite (4.9–8.5 wt%), along with trace amounts of montmorillonite, albite, and siderite (<1 wt% combined). Crucially, comparative XRD patterns (Figure 7) demonstrated no statistically significant variation in mineral proportions following extended fluid exposure, confirming the exceptional stability of the coal matrix’s fundamental composition. The invariant mineral composition directly explains the preserved matrix stiffness observed in micro-indentation tests (Figure 5). Consequently, the macroscopic mechanical degradation documented in Section 5 arises principally from drilling fluid interactions with pre-existing fracture networks, rather than through any matrix-scale alteration processes. This fluid–fracture interaction mechanism dominates the observed bulk mechanical response while leaving the fundamental matrix properties essentially unaffected.

6.2. Development of Coal Fractures

To examine fracture development in coal during drilling fluid interaction, scanning electron microscopy (SEM) was performed. Scanning electron microscopy (SEM) analysis (Figure 8) revealed that unsoaked coal samples contained limited surface fractures, while fracture density showed progressive increase with prolonged immersion time. Quantitative nuclear magnetic resonance (NMR) measurements (Figure 9) provided volumetric fracture analysis, demonstrating an initial rapid increase in fracture volume fraction from 0.39% to 1.08% during the first five days of immersion, followed by stabilization between 0.94% and 1.13% with continued exposure. Fracture propagation serves as the dominant mechanism governing the mechanical degradation of coal, which consequently exhibits a characteristic two-stage evolution pattern: an initial rapid reduction phase followed by gradual stabilization of mechanical properties.

Although XRD analysis did not detect any crystalline mineral alterations, fracture walls may still undergo nanoscale oxidation or hydroxylation due to prolonged contact with oxygen- and salt-containing drilling fluids. Such ultra-thin surface layers (thickness <1 µm) fall below the detection limit of XRD but can persistently reduce the frictional resistance of fracture surfaces by lowering surface energy and interfacial bonding strength. This mechanism aligns with the observed slight decrease of 0.5–0.7° in the internal friction angle after 5 days and coincides with the cessation of fracture volume growth, indicating that chemical alteration primarily affects the fracture walls rather than the bulk matrix.

The elastic modulus of the coal matrix (3.5–3.9 GPa) remained nearly unchanged over 30 days, indicating no hydration-induced softening of the skeleton. In contrast, the macroscopic elastic modulus decreased by 45%, suggesting that the weakening originated from the loss of stiffness in the fracture system. NMR measurements revealed an increase in the fracture volume fraction from 0.39% to 1.08%, with newly formed fractures concentrated in the 0.1–1 µm width range, corresponding to the micro-fracture scale. SEM observations showed that these micro-fractures propagated along grain boundaries and coalesced into localized macroscopic fractures measuring 10–50 µm, leading to the overall reduction in specimen stiffness. Thus, the dominant weakening mechanism is the propagation of millimeter-scale fracture networks induced by “micro-fracture initiation and coalescence,” rather than degradation of the matrix itself.

6.3. Development Mechanism of Coal’s Fractures

The experimental results demonstrate that the weakening of coal mechanical properties is primarily attributed to fracture propagation, which is mainly driven by the “hydraulic wedging” effect of high-pressure drilling fluid. First, during the interaction between drilling fluid and coal, the high-pressure fluid infiltrates existing fractures, generating a pressure field that exerts tensile forces on fracture walls, thereby promoting crack tip propagation. Second, during triaxial compression testing, the confined drilling fluid within saturated coal specimens becomes pressurized under axial loading, further enhancing crack extension through hydraulic wedging.

Additionally, the lubrication effect of drilling fluid contributes to mechanical degradation. The viscous drilling fluid penetrating fractures reduces interfacial friction by lowering the coefficient of friction between fracture surfaces, consequently diminishing the overall load-bearing capacity of the coal matrix. This is corroborated by Figure 3b, which shows a reduction in the internal friction angle from 24.95° to 18.65° with prolonged immersion.

In summary, the deterioration of coal mechanical properties under drilling fluid exposure stems from two synergistic mechanisms: (1) fracture propagation induced by hydraulic wedging, and (2) reduced fracture friction due to fluid lubrication.

6.4. Correlation Between Water Absorption Rate and Strength Parameters of Coal

As analyzed above, water absorption by coal reduces fracture friction properties while increasing water pressure within internal pores and fractures. Under stress loading, the pressurized fluid promotes further fracture propagation, leading to significant changes in the mechanical properties of the coal. Therefore, a thorough analysis of the relationship between water absorption rate and coal strength weakening helps predict the mechanical behavior of coal under different water absorption conditions.

Figure 10 illustrates the relationship between water absorption rate and strength under varying confining stresses. The result shows a negative correlation between water absorption rate and coal strength. Given the low confining pressure conditions in this study, a linear fitting model is recommended for conservative estimation. If higher accuracy is required, the initial fracture volume may be incorporated, or a piecewise linear model can be adopted. Subsequent nonlinear three-parameter correction will be performed using porosity data acquired while drilling. The linear regression formula is provided in

Table 4. Linear fitting relationship between coal rock strength and water absorption under different confining pressures.

Confining Pressure	Linear Fitting Relationship (x = Water Absorption, y = Strength)	R2
0	y = −170.84x + 25.19	0.3783
5	y = −324.74x + 57.8	0.4315
10	y = −433.66x + 73.42	0.3596
20	y = −808.25x + 117.52	0.836

. The analysis reveals that the strength of coal gradually decreases with increasing water absorption. Notably, under high confining stresses, the strength reduction becomes more pronounced with rising water absorption, as evidenced by the fitted curve slope decreasing from −170.84 MPa/% to −808.25 MPa/%. This indicates a significant intensification of water-induced strength degradation at elevated confining stresses. The primary mechanism involves the enhanced “water-wedging” effect under high confining stress: the increased water pressure within fractures promotes further crack propagation, thereby deteriorating the mechanical properties of coal. Overall, higher water absorption corresponds to lower coal strength, while increased confining stress increases the weakening effect of water absorption. The fitted lines clearly quantify the relationship between water absorption rate and coal strength under different confining stresses, providing a basis for predicting the mechanical behavior of coal in wellbores during deep coalbed methane development.

7. Conclusions

This study systematically investigates the evolution of mechanical properties of coal under long-term exposure to high-temperature and high-pressure drilling fluid through immersion experiments. The main findings are as follows:

• With increasing immersion time, the peak strength, elastic modulus, cohesion, and internal friction angle of coal rock all show a declining trend: Within the first 5 days, the uniaxial compressive strength decreased from 36.85 MPa to 22.0 MPa (−40%), the elastic modulus from 1.93 GPa to 1.07 GPa (−45%), cohesion from 14.5 MPa to 5.9 MPa (−59%), and internal friction angle from 24.9° to 19.8° (−21%). After this period, the parameters stabilized. Confining pressure has a protective effect on the mechanical properties of coal rock. Even after 30 days under 20 MPa confining pressure, the strength loss reached 43%, but the absolute value remained at 51.3 MPa. The relative decline in strength decreased with increasing confining pressure.
• The overall degradation of coal rock mechanical properties caused by high-temperature and high-pressure drilling fluid immersion is primarily due to two fracture-fluid interaction mechanisms: Hydraulic wedging-induced fracture propagation: High-pressure drilling fluid creates a pressure field within fractures, promoting their propagation. Under axial pressure, the fluid acts as a “hydraulic wedge,” further accelerating fracture expansion. NMR measurements showed an increase in fracture volume fraction from 0.39% to 1.08%. Fracture surface lubrication-induced reduction in internal friction angle: High-pressure drilling fluid infiltrates fractures, forming a “liquid film” that reduces the friction coefficient of fracture surfaces. This is reflected in the gradual decrease in internal friction angle with prolonged immersion time.
• Water absorption exhibits a negative linear correlation with strength (R² ≥ 0.84), and confining pressure significantly amplifies the water-induced weakening effect: For every 1% increase in water absorption, strength decreases by 170.84 MPa under no confining pressure and by 808.25 MPa under 20 MPa confining pressure, representing a 4.7-fold increase in the slope. This quantitative relationship provides direct evidence for predicting wellbore stability in deep coalbed methane drilling.
• It should be noted that the current study employed static immersion to reveal the “limiting” weakening trend of long-term drilling-fluid–coal interaction. However, under actual field conditions, coal formations are subjected to dynamic loads such as drilling vibrations, cyclic tripping operations, and bottomhole pressure fluctuations. Future research should focus on cyclic loading–unloading experiments or simulated rotary drilling tests to compare the degradation rates under dynamic versus static conditions. This will help validate the applicability thresholds (e.g., the 5-day criterion) and quantitative relationships established in this study under realistic drilling disturbances, thereby providing a more comprehensive time-load coupled basis for predicting wellbore stability in deep coalbed methane (CBM) reservoirs.