Mechanism of Gas Control and Fracturing Release in Mid-Shallow High-Rank Coal Reservoirs and Its Engineering Practice

Abstract

To achieve efficient development of medium-depth and shallow high-rank coalbed methane in the Qinshui Basin of Shanxi Province, the authors focused on the microscopic methane release mechanism. Through scanning electron microscopy, nuclear magnetic resonance, and isothermal adsorption experiments, the pore structure, distribution patterns, and influence of hydration effects in this type of coal were revealed. It was clarified that the ineffective utilization of “bound-state” methane within nanopores is the key factor leading to low productivity and efficiency in coalbed methane development. Further, based on molecular simulations, the competitive adsorption characteristics between water and methane molecules were quantified, indicating that about 78% of the methane in the internal pores of 4 nm coal molecular clusters cannot be desorbed through pressure reduction. Meanwhile, the production enhancement mechanism of hydraulic fracturing on coal seam depressurization, permeability enhancement, reduction in low-speed diffusion distance, and enhancement of high-speed linear flow was clarified. Through large-scale pad water injection and stepwise slow production increase, the coal seam can be fully communicated, the reservoir effectively stimulated, and the adsorbed methane sufficiently released. This paper establishes a “channeled” fracturing concept and its supporting technological system for medium-depth and shallow high-rank coal, which has been successfully applied in field operations. The pilot well group achieved stable daily production exceeding 50,000 cubic meters per day, laying a solid foundation for the continuous and stable production increase in medium-depth and shallow high-rank coalbed methane in the Qinshui Basin.

1. Introduction

Coalbed methane (CBM) is particularly important for ensuring China’s energy security, optimizing the energy structure, and accelerating the construction of a modern energy system that is clean, low-carbon, safe, and efficient [1,2]. China’s CBM resources in coal seams with burial depths shallower than 2000 m are 30 × 10¹² m³, ranking third in the world [3]. Commercial development of CBM abroad was achieved in the 1980s through technologies such as vertical well fracturing and drainage, vertical well fracturing in high-rank, low-permeability areas, and multilateral horizontal well technology [4]. China’s CBM development has long faced the “four lows” problem: low single-well production, low reservoir utilization rate, low remaining recoverable reserves, and low profitability. Among these, low average single-well production is the biggest challenge for CBM development in China. China’s CBM industry is currently in a period of matching understanding with technological breakthroughs and is on the verge of a large-scale application boom [3,4]. Two major CBM industrial bases have now been established: the Qinshui Basin and the eastern edge of the Ordos Basin [5].

The Qinshui Basin is located in central–southern Shanxi Province. The high-rank coal in its southern region is characterized by weakly filled multi-scale exogenous fractures and micropores. The development and connectivity of these features directly influence the gas desorption, diffusion, and seepage processes during gas extraction [6]. The high-rank coal in the southern Qinshui Basin exhibits well-developed fine pores with poor connectivity; the connected pores are primarily nanopores [7,8]. The main resistance to coal seam water flow within microfractures and pores stems from capillary forces, while the primary driving force is the pressure difference between gas pressure and fluid pressure in the cleats. Fluid flow within microfractures and pores begins when the resultant force of resistance and driving force become greater than zero [9]. Competitive adsorption between water molecules and methane reduces methane adsorption capacity, exerting a pro-desorption effect. However, due to the hydrophilicity of microfractures and pores, it can also lead to water-blocking phenomena, hindering gas flow. The fluid flow pattern within microfractures is key to the long-term stable production of high-yield wells [10]. Furthermore, coal seams are prone to mechanical damage under stress, generating coal fines. Therefore, controlling coal fines production, preventing potential blockages, and optimizing the drainage and production schedule are necessary [11]. As reservoir pressure decreases, the stress sensitivity of fluid flow channels across various scales gradually becomes a key factor affecting production capacity. Consequently, the efficient development of high-rank coalbed methane resources requires exploring a scientifically sound development approach.

Since 1994, the exploration and development of coalbed methane in the Qinshui Basin have undergone the following four stages: the regional comprehensive exploration stage (1994–2005), the rapid large-scale development stage (2006–2012), the technological exploration stage (2013–2016), and the transformation, upgrading, and efficient development stage (2017–present). In the earlier development phases, major challenges included incomplete development-related theories, low precision in favorable area selection, weak adaptability of reservoir stimulation techniques, and an imperfect drainage system leading to low reservoir control efficiency. To address the issue of low precision in selecting favorable areas for production development [12], a “three-layer four-property” method for optimizing production development zones was proposed. This method is based on three levels—resource volume, gas production potential, and reservoir transformability—and uses gas content, permeability, conduciveness, and recoverability as selection criteria [13]. For evaluating transformability, dominant geological factors affecting transformability were obtained through physical simulation experiments, the inversion of seismic and logging data, and the comparison of microseismic results with reservoir characteristics. A quantitative transformability evaluation model was established to conduct zonal evaluation of reservoir transformability and determine adaptive stimulation technologies and plans for different zones [14]. To tackle the problems of large well spacing in blocks and low reserves mobilized per well, innovative development technologies such as well type and pattern optimization and horizontal well-coupled pressure reduction to revitalize vertical wells were proposed. By optimizing well spacing and employing interleaved hydraulic fracture connections between lateral wells and vertical wells, coupled pressure reduction between horizontal and vertical wells was achieved [15]. In response to the weak adaptability of reservoir stimulation techniques, Huabei Oilfield first proposed the new concept of “dredged-style” development for coalbed methane. Here, “dredging” refers to using hydraulic fracturing to cut through coal reservoirs, shorten the low-speed gas diffusion stage, and maximize reserve control; “guiding” refers to guiding fluid to flow efficiently under high driving pressure, improving reservoir physical properties, expanding the rapid pressure reduction range, and achieving maximum recovery. The core of this concept is to promote the desorption and release of adsorbed methane from coal seams, thereby accelerating methane production, based on fully communicating and modifying the coal reservoir to establish high-permeability conductive channels.

Guided by the concept of dredged-style coalbed methane development, the development outcomes in the southern Qinshui Basin have significantly improved. However, key issues remain, such as the mismatch between stimulation scale and production expectations, unclear mechanisms for the efficient release of methane from high-rank coal, and how to further increase the recovery rate of coalbed methane. To address these issues, this paper, based on research on the pore structure and occurrence characteristics of medium-depth and shallow high-rank coal seams, elucidates the mechanisms for achieving efficient coalbed methane release through different targeted technologies. By integrating theory with engineering and holistically considering the dredged-style fracturing concept, a technical system for the efficient development of medium-depth and shallow high-rank coal in the Shanxi Qinshui Basin is proposed. Field application has yielded favorable development results. Finally, development suggestions are proposed for further optimizing and upgrading the technologies for medium-depth and shallow high-rank coalbed methane development.

2. Pore Structure and Occurrence Characteristics of Mid-Shallow in China High-Rank Coal Seams

2.1. Pore Structure and Distribution Patterns of Coal Rock Before Hydration

Crushed coal samples from Jincheng No. 1 and Changzhi No. 1 were selected, and smooth thin sections were prepared. The surfaces were polished with argon ions and coated with gold. Scanning electron microscopy (SEM) observation experiments were conducted at a resolution of 3000–6000 times, and results within the range of 30–500 μm were selected, as shown in Figure 1. The SEM observations indicate that the coal mass develops continuously distributed pores and fracture structures. A large number of primary minerals can be observed attached to the surfaces and interiors of the fractures, filling them and causing them to lose conductivity (refer to Figure 1a,g). Mineral dissolution and hydrocarbon generation during thermal evolution formed numerous micron-scale vesicular pores [16] (refer to Figure 1b,c,h). Meanwhile, the fracture aperture distribution is wide, with fractures of 10–30 μm visible (refer to Figure 1d,g). Therefore, the mid-shallow high-rank coal in the Shanxi Qinshui Basin is a typical dual-porosity medium. The abundant pores provide favorable sites for coalbed methane occurrence, while locally developed high-permeability channels enable the rapid extraction of coalbed methane. Consequently, fully releasing the adsorbed methane from the micropores and simultaneously connecting the primary cleats and fractures are key to efficiently extracting mid-shallow high-rank coalbed methane.

To further quantitatively characterize the pore structure distribution of mid-shallow high-rank coal in the Qinshui Basin, three standard core plugs with a diameter of 2.5 cm and a length of 5 cm were drilled from each of the Jincheng 1, 2, 3# and Changzhi 1, 2, 3# coal samples. The pore structure characteristics were further characterized using nuclear magnetic resonance (NMR) technology, and the results are shown in Figure 2a,b. T₂ spectra can effectively represent the proportion of pores/fractures in the sample. Among these, the peak width reflects the sorting of a certain type of pore/fracture, the peak height can indicate the corresponding content of pores/fractures, and the continuity of the T₂ spectrum reflects the quality of connectivity between pores and fractures [17,18]. As can be seen from the T₂ spectra of the fluid-saturated coal samples in Figure 3, the tested 1–3# coal samples all exhibit a “three-peak” characteristic. Adsorption pores, seepage pores, and fractures are all developed in both the Jincheng and Changzhi coal samples. Among these, nano-sized adsorption pores account for a high proportion, with their T₂ peak area proportion ranging from about 69.2% to 88.6%. At the same time, the NMR energy amplitude between the peaks on the T₂ curve is 0, which is particularly evident between pores and fractures. This indicates poor connectivity between fractures and pores, that the coal seam interior has a large amount of adsorption space that is not connected to macroscopic fractures, and that the methane within the nanopores is difficult to utilize. This indirectly demonstrates that the adsorption mechanism of methane in coal seams is complex and that considering only Langmuir adsorption cannot fully reveal the extraction mechanism of coalbed methane.

2.2. Pore Structure and Distribution Patterns of Coal Rock After Hydration

Analysis based on pore structure reveals that the natural pore structure characteristics of coal rock are unfavorable for the full desorption and production of methane. Considering the influence of water on coal rock pore structure during the fracturing process, thin sections prepared from No. 1 coal samples were immersed in active water (0.5% KCl solution) for hydration over 48 h. By pre-marking characteristic pore/fracture structures before hydration, in situ observation of the impact of hydration on coal pore structure was achieved via electron microscopy scanning. Results selected from a 10-micrometer range are shown in Figure 3. The findings indicate that hydration causes partial mineral dissolution and expansion, effectively increasing fracture aperture [19] (see Figure 3h), with a fracture width change rate as high as 18.8%. The formation of hydration fractures was also observed, collectively demonstrating that water exerts a certain expansive and ameliorative effect on coal rock fractures.

Meanwhile, core samples from Jincheng No. 4 and No. 5 coal samples were drilled, and their NMR curves were compared before and after saturation with slickwater gel-breaking fluid (hydrated for 48 h), as shown in Figure 4a,b. The results show that the T₂ spectral signal intensity of nanopores increases after hydration, while that of fractures decreases conversely. For the Jincheng No. 4 and No. 5 cores, the T₂ peak area proportion of nanopores increased from 92.38% and 91.51% to 96.62% and 94.92%, respectively, while the T₂ peak area proportion of fractures decreased from 5.97% and 4.41% to 2.11% and 2.99%, respectively. This indicates that hydration can increase the pore size range of nanopores, which is beneficial for methane desorption and release. However, residues from the gel-breaking fluid can also block fractures with a certain aperture, restricting methane migration and production. Therefore, by fully leveraging the hydration advantages of water-based fracturing fluids, the proportion of pad fluid can be increased to improve the nanopores in as much of the coal seam matrix as possible. However, further research and the development of low-damage fracturing fluid systems is also necessary to avoid post-fracturing residue blocking fractures and causing secondary damage to the coal seam.

2.3. Methane/Nitrogen Isothermal Adsorption Patterns

Using coal fines (40–60 mesh) prepared from crushed Changzhi and Jincheng No. 1, 2, and 3 coal samples, methane and nitrogen adsorption experiments were conducted to analyze the adsorption/desorption characteristics and pore distribution features of mid-shallow high-rank coal. The isothermal adsorption test results, as shown in Figure 5, indicate that the Langmuir volume of the Chengzhi coal samples is smaller than that of the Jincheng coal samples, while the Langmuir pressures are nearly identical. Within the pressure range of 0.1–1 MPa, the isothermal adsorption capacity increases significantly. When the pressure is further increased from 1 MPa to 4 MPa, the adsorption capacity gradually reaches a plateau and peaks. The average Langmuir pressure and volume for the Changzhi and Jincheng No. 1–3 coal samples are 1.43 MPa, 1.41 MPa, 21.14 cm³/g, and 28.8 cm³/g, respectively. This indicates that methane in this type of coal seam requires significant pressure reduction to achieve substantial desorption.

Nitrogen adsorption/desorption experiments indicate that the minimum pore diameter for nitrogen adsorption in Changzhi coal samples can reach 0.4 nm, while the pore diameter for desorption is approximately 0.8~1 nm. This type of pore space does not conform to the Langmuir adsorption law for methane adsorption [20]. Furthermore, the adsorption surface areas of Changzhi No. 1 and No. 2 coal samples are significantly smaller than their desorption surface areas, with adsorption/desorption area ratios of 1.96 and 1.83, respectively (see Figure 6). This indirectly demonstrates that over half of the methane adsorbed within the pores of the coal rock remains undesorbed, reflecting the “micropore filling” effect [21,22]. Therefore, further potential exploitation is urgently needed for this type of high-rank coalbed methane, necessitating a clear understanding of the methane adsorption mechanism in nanopores and the development of efficient release technologies.

3. Mechanisms for Efficient Release of CBM in Mid-Shallow High-Rank Coal

3.1. “Hydration-Enhanced Pore Expansion” for Fully Releasing Bound Methane and Maximizing Recovery Potential

To reveal the adsorption/desorption mechanism of methane in nanopores, this section employs molecular simulation to study the desorption characteristics of methane within coal molecular clusters [23]. The Forcite module in MS was used to conduct molecular dynamics simulations with the Compass II force field and a medium solution accuracy. Two symmetrically distributed 4 nm coal molecular unit cells were constructed to represent the pore-fracture structure within the coal matrix. The internal pores of this structure were used to characterize the adsorption space for methane. Molecular dynamics relaxation was performed for 1000 ps under conditions of 1, 4, 7, and 10 MPa, respectively. The results show that the radial distribution function between carbon atoms in methane and carbon atoms in coal stabilizes at a value of 1. As illustrated in Figure 7a,b, this confirms the formation of “filling-type” adsorption within the nanopores of coal molecules. As pressure further decreases, some methane begins to desorb from within the coal molecules and migrate into the fractures. The longitudinal relative concentration curves indicate that the number of methane molecules in the fractures increases as pressure decreases, while the number within the coal molecular unit cells decreases. However, even when pressure is reduced to 1 MPa, the relative concentration remains at a relatively high level. As seen in Figure 7c, a significant amount of methane remains adsorbed within the internal pores of the coal. Statistical results show that approximately 78% of the “bound-state” methane is not desorbed through pressure reduction, forming a stable “micropore filling” within the pores of the coal molecular clusters [21].

When pressure reduction cannot promote the desorption of “bound-state” methane, changing the temperature or introducing other media becomes the only means to alter this situation. However, in the coalbed methane (CBM) fracturing extraction process, large volumes of fracturing fluid enter the coal seam. Some scholars believe that water has a certain inhibitory effect on methane adsorption. To address this, the competitive adsorption mechanism of the methane–water binary system was further analyzed. The isothermal adsorption curves and energy distribution characteristics of methane and water molecules under conditions of 1, 4, 7, and 10 MPa were calculated. The adsorption amounts of methane and water remain comparable under different pressure conditions, with their respective adsorption proportions showing a “seesaw” effect as pressure increases—each accounting for approximately 50% (Figure 8a,b). This demonstrates the existence of competitive adsorption. The energy distribution curves of methane and water (Figure 8c,d) also show that the energy distributions at different adsorption sites are broad, but lower energy corresponds to greater stability. The energy peaks of both molecules are in low-energy states (<2 kcal/mol), with the methane peak being higher than that of water, indicating that methane adsorption is more stable. However, as shown in Figure 8e, during the competitive adsorption process, as pressure increases, the two molecules occupy favorable adsorption sites. As adsorption saturation is reached, competitive adsorption occurs, with some high-energy adsorbed methane being displaced by water molecules. This fully demonstrates that water molecules have a desorption effect on methane and can occupy some adsorption sites, promoting further desorption of methane from nanopores. Therefore, for the fracturing of mid-shallow high-rank coal in the Qinshui Basin, large-scale pad water injection can be adopted to fully leverage the dual mechanical and physicochemical effects of water. While adequately stimulating the coal seam, the fracturing fluid can promote the desorption of “bound-state” methane, enabling the efficient release of both surface-adsorbed and micropore-filled methane in mid-shallow high-rank coal seams.

3.2. “Ultra-Long Pad + Stepwise Slow Ramp-Up Rate” Forms a Complex Fracture Network and Connects Matrix Pores

Two cubic coal samples measuring 10 × 10 × 10 cm³ were prepared using underground mining samples from Lanhua Coal Mine in Qinshui County, Jincheng City, Shanxi Province. A central borehole was drilled and a pipe with a diameter of 3 mm was embedded (borehole diameter: 1 cm, depth: 6 cm), consolidated with AB adhesive. A full three-dimensional true triaxial large-scale physical fracturing simulation experimental apparatus was utilized to study the effect of different ramp-up rate modes on fracture propagation mechanisms in high-rank coal. Triaxial in situ stresses of 4, 6, and 8 MPa were set, and two sets of fracturing experiments were completed using active water fracturing fluid for two ramp-up rate modes. The results are shown in Figure 9 and Figure 10. The results indicate that stepwise slow ramp-up helps replenish energy in the initial stage, wetting the coal rock, and effectively supporting primary natural fractures. Further fluid injection continuously connects cleats/fractures. When the fluid intake limit is exceeded, main fractures form, resulting in a more uniform fracture network. Fast ramp-up injects fluid in a short time, creating a dominant main fracture inside the coal rock. The continuous injection process fails to connect cleats, resulting in poor connectivity and a small fracture stimulation volume. Combined with post-fracturing CT scan results, it can be seen that slow ramp-up effectively communicates the reservoir around the wellbore. After pressure build-up and fracture initiation, a fracture network rapidly forms, exhibiting high overall fracture complexity and good connectivity. Fast ramp-up causes pressure build-up at the wellbore, forming an “X”-shaped damage zone. Under triaxial stress, the coal rock undergoes shear slip, creating an induced fracture network zone. However, connectivity is poor, and effective communication with the matrix is not achieved. In summary, for high-rank coal reservoirs, the “ultra-long pad + stepwise slow ramp-up rate” fracturing process is recommended. This aids in achieving full communication and stimulation of the coal seam, facilitating efficient release and production of coalbed methane.

4. Engineering Practice for Efficient Development of Mid-Shallow High-Rank Coalbed Methane

4.1. Technical System for High-Rank Coalbed Methane Fracturing Stimulation

Guided by the understanding of the gas release mechanism in high-rank coalbed methane, a high-rank coal-specific large-scale pressure-controlled and channeled volumetric stimulation technology has been developed [24,25,26,27]. The specific core technologies include the following: First, high-quality layer energy-focused directional perforation. Primary structured coal with a more developed cleat system and strong brittleness is selected, and perforations of 2 m per stage are made within ±30° horizontally to reduce inefficient energy loss in structural coal. Second, stepwise pressure-controlled channeled fracturing is used. Starting with an initial low injection rate, “energy charging” is performed for under-saturated pore-fracture reservoirs under low-pressure conditions to protect natural cleats and fractures from complete compression and closure under later high pressure. Based on changes in treatment pressure and tubing friction, the injection rate is increased stepwise. The main fracture is controlled to extend distally under initial low net pressure, and the net pressure is gradually increased in later stages to promote fracture extension or redirection toward surrounding cleats and fractures, expanding the cleats from the inside out to form a volumetrically integrated fracture network of primary and secondary fractures. Third, large-volume and large-scale volumetric fracturing are used. The treatment injection rate is increased from the early 6–8 m³/min to 15–20 m³/min, with 2–3 clusters per stage, to enhance fracturing energy and net pressure. The fluid volume per stage is increased from 800–1000 m³ to 2000–3000 m³, and the sand volume per stage is increased from 50–80 m³ to 100–200 m³. Fracture monitoring shows that the stimulated volume can be increased by 1–2 times. Fourth, multi-scale proppant placement with combined particle sizes was used. This involves the use of 70/140 mesh fine sand upfront for filtration control and fracture wall conditioning, 70/140 mesh and 40/70 mesh quartz sand to prop microfractures and branch fractures, and 20/40 mesh sand to prop the main fractures. Resin-coated sand is tailed in to prevent sand production. Depending on the sand-carrying capacity of the fracturing fluid, a reverse-sequence hybrid sand addition mode is used under active water fracturing fluid conditions, while a forward-sequence sand addition mode is adopted for medium-to-high-viscosity fracturing fluids.

4.2. Production Practice of “Channeled” Fracturing for High-Rank Coalbed Methane

Since 2021, the comprehensive application of horizontal well efficient development engineering technology has been promoted. Huabei Oilfield Company has rapidly increased the commercial gas production in the Qinshui Basin, with daily commercial gas production exceeding 7 million cubic meters. In 2024, the annual commercial gas production surpassed 2.6 billion cubic meters, establishing the largest mid-shallow coalbed methane field in China. The indicators for newly built capacity have significantly improved. The average capacity fulfillment rate has increased from less than 50% to over 90%. The average daily production per horizontal well has risen from 6000 m³/day to 8600 m³/day. The proportion of new wells exceeding 10,000 m³/day has increased to 50%. Among these, the 22 horizontal wells put into production in 2023 in the Qinnanxi–Mabidong block achieved an average daily production of 9500 m³/day, demonstrating strong stable production capability. In particular, the three wells in the Maping 77 well group achieved a daily production of 50,000 m³. Significant breakthroughs have been made in the early low-efficiency Zhengzhuangbei and Qinnandong blocks. Through technological upgrades, the horizontal well production in the Zhengzhuangbei block, characterized by dense coal rock and ultra-low permeability, reached 9000–18,000 m³/day. The horizontal wells in the Qinnandong block, with low gas saturation and fragmented coal structure, achieved 5000–15,000 m³/day, which is 8–50 times the production of early vertical wells. Notably, the Changping 12 well in the Qinnandong block achieved a breakthrough of 15,000 m³/day in single-well daily gas production (Figure 11 and Figure 12). This has greatly boosted confidence in the development of coalbed methane in fractured and soft coal seams and has laid a solid foundation for the sustained production increase and stable production of mid-shallow high-rank coalbed methane in the Qinshui Basin.

5. Prospects for Efficient Development Technologies of Mid-Shallow High-Rank Coalbed Methane

5.1. Focus on Developing “High-Rank Coal Geology–Engineering Integrated Sweet Spot Evaluation Technology”

It is necessary to ensure the optimal selection of sweet spot areas in target coal seams to achieve effective fracture creation during fracturing and high post-fracturing production. To realize the potential of mid-shallow high-rank coalbed methane resources, it is necessary to break through traditional single-scale “sweet spot” prediction methods. Geological and engineering factors should be comprehensively considered to establish a geology–engineering integrated “sweet spot” prediction model. This model must fully integrate geological parameters such as total hydrocarbon content, porosity, natural fracture content, and gas-bearing properties, as well as engineering characteristics like Young’s modulus, Poisson’s ratio, dilation angle, etc., to form a “dual-sweet spot” evaluation system. Based on this, a three-dimensional model should be constructed to accurately guide well selection, layer selection, stage selection, and cluster spacing optimization.

5.2. Deepen and Strengthen the “Competitive Adsorption Mechanism for Efficient Micropore Methane Release”

It is necessary to further clarify the competitive adsorption effects of other media on methane to achieve the ultimate release of “bound-state” methane in high-rank coal reservoirs. It is also necessary to deepen the understanding of the competitive adsorption kinetics mechanisms involving carbon dioxide, nitrogen, and desorption-promoting additives with “bound-state” methane, and analyze the formation mechanism of the “micropore filling” effect. Finally, it is necessary to conduct detailed characterization of the molecular structures of different types of high-rank coals, investigate the interactions between specific functional groups and methane, and develop targeted desorption-promoting strategies for different blocks.

5.3. Accelerate the Establishment of a “Low-Temperature, Low-Damage Integrated Variable-Viscosity Fracturing System”

It is necessary to ensure the realization of the “channeled fracturing concept,” and further develop a low-temperature, low-adsorption, low-damage fracturing fluid system. This system should enable efficient sand carrying during the fracturing of mid-shallow low-temperature coalbed methane reservoirs, effectively avoiding rapid fluid loss in the early stages of coal seam fracturing. This will allow fractures to propagate further into the coal seam, achieving a larger controlled volume. Simultaneously, it should effectively increase net pressure, ensure fracture width, utilize main low viscosity to fully communicate the matrix with the cleat/natural fracture system, and employ multi-stage variable viscosity to address different operational complexities. Finally, the system should produce minimal gel-breaking and flowback residues and cause low damage to the coal seam. The main approach to achieving this low-temperature, low-damage fracturing fluid system is as follows: introducing molecular fragment design concepts and employing targeted polymerization methods to design and synthesize medium-to-high-performance, low-molecular weight, non-oil-phase emulsion friction reducers. The system should feature low adsorption and low damage characteristics, helping to achieve the engineering philosophy of “creating multiple effective fractures”.

5.4. Continuously Improve the “Concept of Collaborative Fracturing Stimulation for Horizontal Well Clusters”

It is necessary to fully draw on the shale gas “well factory” development model to form a collaborative fracturing development technology for clustered horizontal well groups. It is also necessary to make full use of stress interference between fracture tips and horizontal well sections to create a complex fracture network with high aperture and large area. It is necessary to increase the integration degree of the fracturing network and the fracturing well group, fully mobilize the reservoir, and achieve a development mode shift from single-well production control to well-group reserves control. Finally, it is necessary to utilize clustered horizontal well groups for “maximum reserves control and effective reserves mobilization”, deepen the integrated design of geology, engineering, and production, construct a large-scale “artificial gas reservoir,” achieve high production with fewer wells, and improve economic efficiency.

6. Conclusions

(1)

The methane occurrence characteristics of the pore structure in high-rank coal from the Qinshui Basin, Shanxi were analyzed through scanning electron microscopy and nuclear magnetic resonance experiments. The improvement effect of hydration on the nanopores of the coal seam matrix was revealed; simultaneously, based on nitrogen adsorption experiments, it was demonstrated that the micropore filling effect inhibits the efficient release of methane.

(2)

Utilizing molecular simulation, numerical simulation, and large-scale physical modeling of fracturing experiments, the efficient release mechanism of hydraulic fracturing on mid-shallow high-rank coalbed methane was revealed, showing that it shortens the low-speed diffusion stage and enhances high-speed linear seepage. It was clarified that adopting the construction technology of large-scale pad water injection and stepwise slow production increase can fully leverage the dual mechanical and physicochemical effects of water. This established a stimulation approach that fully modifies the coal seam, achieving efficient release of both “bound-state” and surface-adsorbed methane in the coal seam.

(3)

Based on the efficient release mechanism of mid-shallow high-rank coal, a technical system centered around “channeled” fracturing stimulation has been developed. This system has achieved favorable field application results in the Qinshui Basin, Shanxi, with the MP well group maintaining a stable daily production exceeding 50,000 m³/d. This strongly supports confidence in the efficient development of mid-shallow high-rank coal seams and solidifies the foundation for the continuous production increase and stable production of mid-shallow high-rank coalbed methane in the Qinshui Basin.

(4)

Technical directions for the comprehensive upgrade of the “channeled” fracturing stimulation system were proposed, including: developing the “high-rank coal geology–engineering integrated sweet spot evaluation technology”, solidifying the “competitive adsorption mechanism for efficient micropore methane release”, establishing the “low-temperature, low-damage integrated variable-viscosity fracturing system”, and refining the “concept of collaborative fracturing stimulation for horizontal well clusters”.