Physical Simulation Experiment for Visualizing Pulverized Coal Transport in Propped Fractures

Abstract

The issue of pulverized coal in coalbed methane wells during the discharge and mining process spans all stages, and it is a key factor constraining the continuous and stable discharge and production capacity of coalbed methane. Among these stages, the single-phase water flow stage features a high incidence of pulverized coal. Consequently, this paper presents a physical simulation experiment within the propped fractures during the single-phase water flow stage. The results of this study reveal the following: (1) Within the propped fracture channel, when pulverized coal is deposited along the flow line without causing blockage, the front end of the deposition exhibits a strip-like dispersion, evolving into “block deposition”, “flame-like accumulation”, “linear accumulation”, and “dispersed point-like accumulation”. (2) Agglomerated fracturing fluid can effectively mitigate the permeability damage caused by pulverized coal to the propped fractures. Both the driving speed and particle size of pulverized coal significantly influence pulverized coal transportation. The injury rate of propped fracture conductivity increases with increasing driving speeds, while the output of pulverized coal first increases and then decreases with increasing driving speed. Moreover, larger pulverized coal particle sizes result in notably greater damage to propped fracture conductivity than smaller particle sizes. Correspondingly, larger particles exhibit significantly lower output of pulverized coal compared to smaller particles, and transportation and output time are prolonged for larger particles. These findings underscore the importance of particle size and driving speed in the transportation dynamics of pulverized coal. The research results provide a theoretical basis for developing strategies for the prevention and control of pulverized coal during the single-phase water flow stage, thereby offering substantial scientific and practical value for the economic and efficient development of coalbed methane.

1. Introduction

Pulverized coal output is more common during coalbed methane (CBM) production, leading to the fragility of propped fractures, stuck pumps, and buried pumps, which ultimately reduce the production capacity of CBM wells [1,2,3,4]. This problem considerably limits the productivity of coal seam gas wells [5]. Consequently, understanding the transportation dynamics of pulverized coal and implementing effective prevention and control measures are pivotal for the economic and efficient development of CBM. Variations in pulverized coal production characteristics occur across different geological and engineering conditions [6,7]. Notably, significant fluctuations in pulverized coal output occur during various stages of discharge mining [8]. Among these stages, the single-phase water flow phase is particularly prone to pulverized coal occurrences, wherein the coal particles primarily originate from the erosion of cracked walls by proppants, resulting in small particle sizes [9]. Therefore, studies on pulverized coal transportation dynamics as well as prevention and control countermeasures should pay special attention to the single-phase water flow stage.

The transportation and deposition patterns of coal dust within propped fractures during the single-phase water flow stage have been extensively investigated. The transportation of pulverized coal requires a specific critical start-up flow rate, which varies depending on the characteristics of the coal particle size [10,11]. Some researchers have considered the adhesive properties of coal particles and utilized hydrodynamic theory to establish a mechanical model for the initiation of pulverized coal deposition within coal seam fractures during the single-phase water flow stage. Their findings indicate that the initiation velocity exhibits an initial decline followed by an increase with increasing particle sizes. Moreover, through an analysis of the forces acting on individual static pulverized coal particles within a cutting system, three primary modes of pulverized coal transport under the influence of single-phase water flow have been identified as rolling, lifting, and sliding [12,13,14]. Numerous factors influence the transportation and settling of pulverized coal, including the internal structure of the porous medium, transport pathways, ambient pressure, permeability, seepage velocity, flow field, particle size, particle size distribution, and fluid viscosity [15,16,17]. To investigate the laws governing the transport and settling of pulverized coal, certain scholars have utilized flow conductivity testers and self-designed experimental apparatus (including a visualization parallel plate and high-transparency tempered glass) to conduct physical simulation experiments on coal transport within cracks [18,19,20,21,22]. These experiments explain why suspended pulverized coal tends to deposit more readily at the edges of porous media. The key findings are as follows: as the particle size of pulverized coal increases, its trajectory tends to lower, increasing the likelihood of deposition; higher fluid flow rates and viscosity enhance the suspension capacity of pulverized coal, making it less prone to deposition; pulverized coal can travel greater distances within cracks with smaller curvature and better connectivity, resulting in higher coal output; however, pulverized coal can also obstruct transportation channels. However, these studies mainly focus on the fluid flushing of pulverized coal or pulverized coal mixed solutions driving within cracks, neglecting visualized physical simulation experiments of pulverized coal mixed solutions entering proppant-filled zones. Furthermore, there is a lack of experimental studies on the impacts of sedimentation, aggregation, and dispersed transport of pulverized coal on the permeability of coal reservoirs and the inflow capacity of proppant fractures.

Based on the above research background, the present study conducted a visualized physical simulation experiment to elucidate the transport dynamics of pulverized coal within propped fractures during the single-phase water flow stage, employing a self-constructed driving device. The transport and deposition processes of pulverized coal under varying carrier fluids, driving velocities, and pulverized coal particle sizes were comprehensively investigated. Our objective is to reveal the mechanisms governing pulverized coal transport during the single-phase water flow stage of CBM wells. This research provides theoretical foundations for the formulation of strategies aimed at preventing and controlling pulverized coal during this critical stage of CBM well operations. Such insights hold significant scientific value and practical implications for the economically efficient development of CBM.

2. Materials and Methods

2.1. Experimental System

A physical simulation experimental device for studying pulverized coal transport (Figure 1) was used to conduct a visualized experiment elucidating the transport behavior of pulverized coal within propped fractures during the single-phase water flow stage. The experimental setup primarily consisted of a high-pressure nitrogen bottle, mass flow meter, pulverized coal solution tank, magnetic stirrer, pressure sensor, Plexiglass sheet, propped fracture, transparent tempered glass plate, cold light source, high-speed video camera, computer, and conical flask. The propped fracture comprised a transparent tempered glass flat plate measuring 150 cm in length and 20 cm in width, featuring a quarter slot inside with dimensions of 20 cm in length and 2.63 cm in width, filled with proppant to simulate the propped fracture. A cold light source positioned behind the plate, combined with a hair glass, enhanced camera visibility for clearer imaging.

2.2. Experimental Material

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Pulverized coal and proppant

Pulverized coal particles ranging from 80 to 100 mesh (0.15 to 0.18 mm) and 100 to 150 mesh (0.106 to 0.15 mm) were employed to analyze the impact of different pulverized coal particle sizes on coal transport. Natural quartz sand particles with a size of 16 to 20 mesh (0.85 to 1.18 mm) were chosen as the proppant. The mass fraction of SiO₂ in the quartz sand exceeded 99.97%, with a hardness of 7, exhibiting a brittle property without joints, and a density of 2.65 g/cm³. Prior to each test, the quartz sand was thoroughly rinsed in high-purity distilled water until it was free of suspended impurities, and then the sand was spread over an area measuring 20 cm × 2.63 cm.

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Carrier fluid

Four types of transport fluids (

Table 1. Types and properties of carrier fluids.

Type of Carrying Fluid	Surface Tension (mN∙m−1)	Contact Angle (°)	Viscosity (mPa∙s)
Distilled water	61.75	56.13	0.5
Active water fracturing fluid	66.76	67.50	0.2
Agglomerated fracturing fluid	25.08	8.00	0.1
Dispersed fracturing fluid	27.06	18.25	0.4

) were prepared to analyze the effects of fracturing fluids with different properties on pulverized coal transport. Among them, the active water fracturing fluid had a KCl concentration of 1.5%. The agglomerated fracturing fluid (1.5% KCl + 0.05% KO) caused the pulverized coal to settle and accumulate at the solution’s bottom, while the dispersed fracturing fluid (1.5% KCl + 0.06% SA) allowed the pulverized coal to disperse and remain suspended in the solution. The KO in the agglomerated fracturing fluid was a compound surfactant consisting of anionic surfactant fast T (sodium dioctyl sulfosuccinate) and nonionic surfactant OP-10 (octylphenol polyoxyethylene ether-10) in a ratio of 9:1. The SA in the dispersed fracturing fluid was a compound surfactant composed of the anionic surfactant sodium dodecyl sulfate and the nonionic surfactant dodecyl alcohol ethoxylated ether in a ratio of 1:2.

2.3. Experimental Procedures

The experimental procedure was as follows:

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The proppant was evenly spread across the observation area, the experimental line was connected, and air tightness was verified.

(2)

The equipment was powered on, and 1 L of carrier fluid (with a pulverized coal mass fraction of 1%) was added to the pulverized coal solution tank, which was then placed on a constant-temperature magnetic stirrer set at 800 r/min.

(3)

The mass flow meter was adjusted to the desired gas flow rate.

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The valve of the nitrogen cylinder was opened, and the pressure-reducing valve was adjusted. Once the mass flow meter stabilized, the water inlet of the observation area was activated. This prompted the nitrogen gas to drive the carrier fluid into the propped fracture. The carrier fluid without pulverized coal was driven in to determine the permeability of the propped fracture under these conditions. Subsequently, the carrier fluid containing 1% pulverized coal was introduced, the transportation of pulverized coal within the propped fracture was observed, and the flow rate of the output fluid was recorded. Timing started when the outlet flow rate stabilized, with data recorded every 250 mL.

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The drained output fluid was centrifuged, filtered, dried, and weighed to determine the amount of pulverized coal output.

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The valve of the nitrogen bottle was closed, the Plexiglas plate and the pulverized coal solution tank were cleaned, and the damage rate of the propped fracture conductivity was calculated.

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The replacement rate (100 mL/min, 200 mL/min, 300 mL/min), carrier fluid (distilled water, active water fracturing fluid, agglomerated fracturing fluid, and dispersed fracturing fluid), and particle size of pulverized coal (80–100 mesh, 100–150 mesh) were replaced, and the above steps were repeated.

3. Results and Discussion

3.1. Characterization of Pulverized Coal Deposition in Propped Fractures

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Pulverized coal accumulation method

The pores formed between the stacked proppants serve as pathways for both pulverized coal and the transporting fluid. As the mixed solution of pulverized coal enters the propped fractures, the fluid tends to traverse cracks with lower resistance, creating a flow line. First, when pulverized coal is deposited along the flow line but has not yet caused a blockage, it forms a strip at the deposition front, dispersing in the flow direction. With the progression of displacement experiments, increasing amounts of pulverized coal deposit along the flow line, eventually resulting in blockage formation. This evolution progresses from “block deposition”, to “flame accumulation”, “linear accumulation”, “dispersed accumulation”, and other accumulation patterns (Figure 2). However, under the same conditions, significant variations exist in the accumulated amount of pulverized coal within the observation unit, showing the trend of dispersed fracturing fluid > distilled water > active water fracturing fluid > agglomerated fracturing fluid.

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Bonding between pulverized coal and proppant

As the mixed solution of pulverized coal infiltrates the propped fracture, some pulverized coal is either deposited or adheres to the proppant due to reduced flow rates or entrapment by the proppant. The choice of carrier fluid influences the degree of pulverized coal adhesion to the proppant, consequently affecting crack permeability. During the experiment, the bonding between fine pulverized coal particles and proppant was more pronounced in distilled water and active water fracturing fluid, whereas it was relatively minimal in dispersed fracturing fluid and agglomerated fracturing fluid (Figure 3).

3.2. Analysis of the Migration Patterns of Pulverized Coal in Various Carrier Fluids

Under the experimental conditions of 16–20 mesh proppant, 100–150 mesh pulverized coal particle size, and a repulsion rate of 300 mL/min, significant variations were observed in the injury rate of propped fracture conductivity, the amount of pulverized coal output, and the repulsion time of equal-volume solution when different transport fluids (distilled water, active water fracturing fluid, agglomerated fracturing fluid, and dispersed fracturing fluid) were employed for pulverized coal transport experiments (Figure 4).

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Injury rate of propped fracture conductivity

The fracture conductivity injury rate is higher when the carrier fluid is distilled water than when the other three carrier fluids are employed. Notably, the injury rate of fracture conductivity in the case of dispersed fracturing fluid starts to increase significantly when the third cup of fluid is produced by driving, and the injury time is earlier than that of active water fracturing fluid and agglomerated fracturing fluid. Furthermore, the damage rate of fracture conductivity for agglomerated fracturing fluid and active water fracturing fluid shows a significant increase upon production of the fourth cup of liquid during displacement. The trend in the increase in the damage rate of the fracture conductivity follows this sequence: agglomerated fracturing fluid > active water fracturing fluid > distilled water > dispersed fracturing fluid. However, the cumulative injury rate of fracture inflow capacity for agglomerated fracturing fluid remains lower than that of the other three transport fluids. This indicates that agglomerated fracturing fluid can effectively mitigate the permeability injury caused by pulverized coal to the propped fractures.

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Pulverized coal output

The three stages of pulverized coal deposition—pulverized coal deposition on the surface of proppant particles (pulverized coal deposition), the narrowing of channels between proppant particles (mild clogging), and the clogging of proppant fractures by pulverized coal (severe clogging)—are illustrated in Figure 5. The deposition and plugging of pulverized coal occur more rapidly with increasing drive flow rates. The powder content in all four transport fluids reaches its peak value in the second cup of the drive output, followed by a sharp decline in the fourth cup. This decline is attributed to the higher flow rate, which leads to more severe blockage in the propped fractures and the deposition of pulverized coal, forming a barrier. Under the same replacement flow rate conditions, the output of pulverized coal exhibits this sequence: dispersed fracturing fluid > active water fracturing fluid > distilled water > agglomerated fracturing fluid.

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Equal-volume solution displacement time

The time required for discharging the equal-volume solution serves as an indicator of the smooth transportation of the carrier fluid within the proppant fracture from the sideline. The expulsion time for the four types of carrier fluids demonstrates an increasing trend, with agglomerated fracturing fluid requiring the shortest time, followed by active water fracturing fluid, and finally distilled water and dispersed fracturing fluid. Distilled water and dispersed fracturing fluid exhibit a significant increase in expulsion time upon production of the third cup of fluid during displacement, whereas active water fracturing fluid and agglomerated fracturing fluid show a significant increase in expulsion time only upon production of the fourth cup of fluid during displacement. This suggests that distilled water and dispersed fracturing fluid experience clogging at an earlier stage, impeding the flow of the carrier fluids.

The analysis above indicates that agglomerated fracturing fluid can alter the wettability of pulverized coal and enhance its cohesion. This leads to the accumulation and settling of pulverized coal particles during the migration process, reducing the amount of suspended pulverized coal in the carrier fluid. Consequently, the likelihood of blocking the propped fractures is reduced, allowing agglomerated fracturing fluid to exert a more effective prevention and control effect on pulverized coal.

3.3. Analysis of the Migration Patterns of Pulverized Coal at Various Displacement Speeds

Under the experimental conditions, where the proppant size ranges from 16 to 20 mesh, the pulverized coal particle size is between 100 and 150 mesh, and the transport fluid is agglomerated fracturing fluid, notable differences are observed in the injury rate of proppant fracture inflow capacity, the amount of pulverized coal output, and the time taken for equal-volume solution repulsion during experiments conducted at different repulsion velocities (100 mL/min, 200 mL/min, and 300 mL/min) (Figure 6).

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Injury rate of proppant fracture inflow capacity

At a replacement speed of 100 mL/min, the injury rate of fracture inflow capacity ranges from 0.58% to 5.08%, with variation rates of 3.9, 2.0, and 1.1 times, demonstrating a gradually slowing increasing trend. With an expulsion rate of 200 mL/min, the variation range of fracture inflow capacity injury rate is 2.35–7.78%, with variation rates of 1, 1.49, and 2.23 times, indicating a steady upward trend. When the expulsion rate is 300 mL/min, the variation range of fracture inflow capacity injury rate is 5–44.12%, with variation rates of 1, 2.19, and 4.03 times, showing a significant increasing trend. The cumulative values of fracture inflow capacity injury rate for the three driving speeds are 12.55%, 15.97%, and 65.06%, suggesting that the injury rate of fracture inflow capacity increases with the rise in driving speed.

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Pulverized coal output

At a driving speed of 100 mL/min, the variation range of pulverized coal output is 59.9–80.9 mg, with variation rates of 1.1, 0.7, and 0.97 times, indicating an initial increase followed by a gradual decrease. With a driving speed of 200 mL/min, the change range of pulverized coal output increases to 107.4–155.9 mg compared with that at 100 mL/min, with variation rates of 1.1, 1.2, and 0.7 times, showing an increasing trend followed by a decrease. At a driving speed of 300 mL/min, the variation range of pulverized coal output significantly increases to 54.9–161.7 mg compared with that at 200 mL/min, exhibiting a sharp increase followed by a rapid decrease, with variation rates of 1.3, 0.9, and 0.4 times. The cumulative values of pulverized coal output for the three driving speeds are 276.6 mg, 517.2 mg, and 485.2 mg, indicating that pulverized coal output tends to increase and then decrease with the increase in driving speed.

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Equal-volume solution displacement time.

At a displacement rate of 100 mL/min, the displacement time of the equal-volume solution ranges from 169 s to 177 s, demonstrating a slowly increasing trend. This suggests a gradual increase in the migration of the carrier fluid within the propped fractures. When the displacement speed is 200 mL/min, the displacement time of the equal-volume solution varies from 85 s to 90 s, also showing a slowly increasing trend, although more gently than at 100 mL/min. This indicates that with the increase in displacement speed, the migration of the carrier fluid within the propped fractures slows down, suggesting that the displacement speed can counteract the influence of some adverse factors on fracture conductivity. At a displacement speed of 300 mL/min, the displacement time of the equal-volume solution ranges from 60 s to 102 s. The displacement time increases noticeably from the beginning of the second cup and rises sharply from the third cup to the fourth cup, indicating significant blockage occurring in the propped fractures.

The analysis above indicates that excessively fast displacement speeds can result in premature blockage formation, leading to damage to fracture conductivity. Conversely, excessively slow displacement speeds can prolong the migration and output time of the carrier fluid, which is unfavorable for drainage operations. Both fast and slow displacement speeds can impact the output of pulverized coal to some extent. Therefore, determining the critical displacement speed for the prevention and control of pulverized coal is paramount.

3.4. Analysis of the Migration Pattern of Pulverized Coal with Various Particle Sizes

Under the experimental conditions of 16–20 mesh proppant, a replacement rate of 200 mL/min, and agglomerated fracturing fluid as the transport fluid, notable differences in the injury rate of proppant fracture conductivity, the amount of pulverized coal output, and the displacement time of the equal-volume solution occur between the coal transport cases with different pulverized coal particle sizes (80–100 mesh and 100–150 mesh, Figure 7).

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Injury rate of propped fracture conductivity

When the particle size of pulverized coal is 80–100 mesh, the variation range of crack inflow capacity injury rate is 3.4–56.8%. There is a sharp increase in the injury rate of propped fracture conductivity in the fourth cup, indicating severe blockage in the propped crack, and a sharp decrease in the crack inflow capacity. When the particle size of pulverized coal is 100–150 mesh, the variation range of the crack inflow capacity injury rate is 2.4–7.8%, showing a slowly increasing trend. However, the variation is more gradual than that under 80–100 mesh, suggesting that the injury caused by 100–150 mesh pulverized coal to the propped fractures’ inflow capacity is much smaller than that of 80~100 mesh pulverized coal.

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Pulverized coal output

The variation range of the pulverized coal output at a pulverized coal particle size of 80–100 mesh is 52.4–69.7 mg, showing a trend of gradual increase, followed by a decrease. This indicates that the propped fractures experience pulverized coal deposition and slight clogging as the replacement experiment progresses, with the pulverized coal clogging the propped fractures to form serious blockage when replacing the fourth cup of liquid, leading to a sudden decrease in the output of pulverized coal. When the particle size of pulverized coal is 100–150 mesh, the variation in the pulverized coal output ranges from 107.4 to 155.9 mg, also showing a slow increase followed by a sudden decrease, but the amount of pulverized coal output is significantly higher than that under pulverized coal of 80–100 mesh.

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Equal-volume solution displacement time

When the particle size of pulverized coal is 80–100 mesh, the variation range of the equal-volume solution displacement time is 88–197 s, with a sharp increase observed in the fourth cup, indicating severe blockage in the propped fractures. When the particle size of pulverized coal is 100–150 mesh, the change range of the displacement time of the equal-volume solution is 58–90 s, showing a slowly increasing trend. There is no sharp increase in the fourth cup, indicating the absence of serious blockage in the propped fractures. The displacement time is notably reduced compared with that under 80–100 mesh.

The analysis above indicates that when the particle size of pulverized coal is larger, the damage to the propped fracture inflow capacity is significantly greater than that under smaller particle sizes. Additionally, the output of pulverized coal is lower, and the transportation output time is longer.

4. Conclusions

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Within the propped fracture channel, when pulverized coal is deposited along the flow line without causing blockage, the front end of the deposition exhibits strip-like dispersion, which then evolves into “block deposition”, “flame accumulation”, “line accumulation”, and “dispersed point accumulation”. However, under the same conditions, significant differences exist in the amount of accumulated pulverized coal in the observation unit, with dispersed fracturing fluid showing the highest accumulation, followed by distilled water, active water fracturing fluid, and agglomerated fracturing fluid.

(2)

Compared with distilled water, active water fracturing fluid, and dispersed fracturing fluid, agglomerated fracturing fluid causes the least damage to fracture conductivity, yields the least amount of pulverized coal output, and exhibits the shortest migration time, making it suitable for preventing and controlling pulverized coal. Agglomerated fracturing fluid can alter the wettability of pulverized coal and increase its cohesion, allowing for accumulation and settling during migration. Consequently, suspended pulverized coal in the carrier fluid is reduced, thereby decreasing the likelihood of blocking propped fractures.

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Excessively high replacement rates can lead to premature blockage formation, damaging the inflow capacity of propped fractures. Conversely, a very low replacement rate can prolong the output time of transport fluid, which is unfavorable for discharge mining operations. Therefore, determining the critical replacement rate is crucial for preventing and controlling pulverized coal.

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Larger particle sizes of pulverized coal result in significantly greater damage to the flow-conducting ability of propped fractures compared to that produced by smaller particle sizes. Additionally, under larger particle sizes, the output quantity of pulverized coal is notably lower, and the transportation and output time of larger particles are significantly longer than those of smaller particles.