Coal structures significantly influence the efficiency of hydraulic fracturing, which is vital for enhancing coalbed methane production. Experiments, models, and microseismic monitoring show how coal structures affect fracture propagation: (1) Different structural combinations affect the initiation pressure and time, with variations in the way cracks expand depending on the structure. (2) An increase in the thickness of clastic coal leads to fracture extension towards it, while the native coal structure facilitates fracture expansion. Cracks propagate along the largest horizontal principal stress direction due to the increased bonding and flexibility of clump coal when exposed to fracturing fluid. (3) In the "two hards sandwiching one soft" structural combination, where hard coal is thick and soft coal is thin, cracks propagate through the soft coal. The hard coal provides effective crack generation, yet the presence of soft coal notably increases fracturing fluid loss.
Research Article
Research on fracture propagation process of coal seam hydraulic fracturing based on simulation analysis and microseismic monitoring
https://doi.org/10.21203/rs.3.rs-4976157/v1
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Fracture propagation
coal structure
hydraulic fracturing
numerical simulation
Microseismic detection
The propagation law of hydraulic pressure in coalbed methane wells, as well as the path of gas migration and production, are directly impacted by the characteristics of coal seam hydraulic fracturing (Ren et al. 2024;Zhaoet al. 2022). At the moment, field testing, numerical simulation, physical simulation tests, and theoretical analysis are the primary research methodologies used to study hydraulic fracturing in coal rocks. Hydraulic fracturing technology studies are based on theoretical analysis, numerical and physical simulation experiments are necessary to confirm the accuracy of theories and can offer technical guidance and support for field testing. In order to examine the impacts of variables including fracture length, fracture aspect ratio, number of fractures, fracture spacing, production pressure, and intrinsic permeability anisotropy on production efficiency, Wang et al (2023). developed a variety of three-dimensional models. Five common coal models were developed by Wang et al (2017). in order to investigate how spontaneous fractures affect the spread of hydraulic fracturing. The study simulated three natural fracture modes that influence the spread of hydraulic fractures and discovered the presence of joints causes fractures to expand. A finite element-discrete element combination technique was used by Grasselli et al (2015). to present preliminary findings from their investigation of the interplay between fluid-driven fractures and naturally occurring rock mass discontinuities. Fracture propagation tests on artificial coal samples were carried out using large-scale genuine triaxial experimental setups, which were designed to mimic field circumstances (Guo et al. 2015;Hou et al. 2016;Zhai et al. 2022;Yu et al. 2024;Zhang et al. 2024;Wu et al. 2024). The findings demonstrated that one important factor influencing the direction of hydraulic fractures is the approach angle between the hydraulic fractures and the natural fractures; the greater the angle, the greater the likelihood that the hydraulic fractures will penetrate the natural fractures. Based on experimental analysis, Liu (2020) and Chen et al (2017). examined the fluid flow patterns under various circumstances, including varying fracturing media, fracture surface roughness, contact area, and aperture changes. The geometric forms of hydraulic fracture extension in reservoirs with and without natural cracks were examined by P.L.P. Wasantha et al (2017). The findings demonstrated that in rocks lacking natural fractures, the symmetry of fracture extension is greatly impacted by natural fractures. It was discovered that the geometry of hydraulic fractures is influenced by the stiffness and approach angle of natural fractures as well as the distance between the wellbore and the natural fractures. The findings demonstrate that, in rocks devoid of natural fractures, natural fractures have a major impact on the symmetry of crack propagation. The geometric shape of hydraulic fractures was shown to be influenced by the stiffness and approach angle of natural fractures, as well as the distance between the fractures and the wellbore. A linked numerical analytical approach to seepage-stress was developed by Song et al (2024). to simulate complex hydraulic fracturing in porous media. They developed a phase-field model framework to explain how fluid flow is affected by the crack opening system. The route taken by hydraulic fracturing cracks. An associated phase-field model with hydraulic fracturing describes the crack propagation path. They used an interleaved iterative technique to address the three-field coupling problem, which was based on the Newton-Raphson iteration algorithm. In order to track and analyze the spatial morphology of fractures caused by hydraulic fracturing in a coal seam borehole of an underground coal mine, Li et al (2021). used microseismic (MS) monitoring equipment. Additionally, after the hydraulic fracturing technique, they investigated and examined the changes in the coal body's stress and water content at various distances from the well. The findings demonstrated a strong relationship between hydraulic fracture operations and microseismic activity. Microseismic events are produced by hydraulic fracture propagation, and the energy of these events increases with the size of the fracture. Zhao et al (2023). monitored the spatiotemporal characteristics of fracture propagation during the hydraulic fracturing process of coalbed methane wells in the Zhengzhuang area, southern Qinshui Basin, using microseismic technology. The quantification of the three-dimensional morphology of fractures in vertical wells following fracture can be achieved by imaging processing and microseismic fracture scanning data.
The process of hydraulic fracturing crack propagation in coal rock masses has been the subject of much domestic and international research. However, the proliferation of hydraulic fracturing cracks in complex coal body structure combinations is presently the subject of less research, and the laws governing the propagation of cracks in various coal body structures are not systematically understood. This work uses indoor physical fracturing tests, numerical modeling, and microseismic detection technology to inquire about the regularity of crack extension morphology in various coal body structure combinations, based on the research of the aforementioned researchers.
China's southern Qinshui Basin is a key region for the production and exploration of coalbed methane. Situated on the southern Qinshui Basin's northwest dipping slope lies the Shizhuang block (Guo et al 2018;Han et al 2021) (Fig. 1). Primary structural coal and fractured coal make up the majority of the coal body structure. Furthermore, granular coal and mylonitized coal are generated in the interlayers within the coal seam and in the vicinity of the bottom. With an average of more than 0.09×10⁻¹⁵ m2, the coal seam's permeability mostly varies from 0.01×10⁻¹⁵ to 0.3×10⁻¹⁵ m2 (Ni et al 2024). With an average of 5.5%, the porosity distribution spans from 4.08–6.83%. The average reservoir pressure is 3.5 MPa, with a variation between 2.1 and 6.9 MPa (Han et al 2021). Vertical Principal Stress: Averages 20.04 MPa and spans 12.35 to 31.07 MPa. Maximum Horizontal Principal Stress: Averages 21.09 MPa and spans from 9.55 to 36.66 MPa. The minimum horizontal principal stress has an average value of 17.03 MPa and a range of 8.41 to 29.17 MPa (Liu et al 2020;Niu et al 2024).
3.1 Hydraulic fracturing of coal seam based on physical fracturing experiment
3.1.1 Experimental Equipment.
Hydraulic fracturing simulation equipment is utilized in fracturing experiments. With real-time monitoring of both fracture pressure and fracture propagation pressure, this device has a triaxial confining chamber with pressure application capabilities. Temperature ranges of 0 to 150°C, overburden pressures of 0 to 70 MPa, injection pressures of 0 to 45 MPa, and injection rates of 0 to 45 ml/min are among the experimental parameters that can be obtained from it. Figure 2 displays the fracture device's schematic diagram. Three pumps—designated A, B, and C—are the essential components of the experimental setup. They are employed to apply axial pressure, confining pressure, and fluid injection pressure, respectively, and each pump records data on its own. Axial pressure is provided by Pump B, and fluid injection pressure is provided by Pump C, and Pump A provides confining pressure using clean water. Pump C is used to inject water prior to the test in order to remove air from the confining chamber and pipes, preventing interference and having no effect on the test outcomes.
3.1.2 Test sample.
The mechanical parameters of various coal structures were examined in accordance with standard GB/T 23561.8–2024 in order to serve as a foundation for the mechanical parameter settings in the numerical simulation of coal structure models (2022). The results show that the Young's modulus of primary structure coal is 3.7 GPa and its tensile strength is 0.58 MPa, suggesting strong deformation resistance. The Young's modulus of crushed coal is 0.21 GPa, which is much lower than that of primary structural coal, indicating low deformation resistance. Fragmented coal has a Young's modulus of 2.4 GPa and a tensile strength of 0.35 MPa, putting it in between primary structure coal and crushed coal in terms of deformability.
In the experiment, cylindrical coal sample specimens from primary-structure coal and fragmented coal were prepared utilizing a wire cutter CNC machine. To minimize mechanical damage, the specimens' diameters—roughly 50 mm by 25 mm, 50 mm by 50 mm, and 50 mm by 75 mm—were prepared perpendicular to the bedding plane. In order to prevent fracture damage, the specimens' two ends were kept as flat as possible—less than 0.03 mm. By crushing raw coal samples collected from the field and sifting out coal powder with a mesh size of 60–80, fragmented coal samples were created. The powder was pressed under a forming pressure of 200 KN into broken specimens measuring approximately 50 mm by 25 mm, 50 mm by 50 mm, and 50 mm by 75 mm, using water as an adhesive. Zhang Rong's (2019) study indicates that this sample preparation technique is workable and may, to a limited extent, guarantee the mechanical and physical characteristics of crushed coal.
| serial number | Type of coal body assemblage | Pressure on the axis (MPa) | Limiting force (MPa) | Rate of injection (ml/min) |
|---|---|---|---|---|
| 1 | NC(75mm)+CC༈25mm༉ | 12 | 10 | 30 |
| 2 | NC(50mm)+CC༈50m༉ | 12 | 10 | 30 |
| 3 | NC(25mm)+CC༈75mm༉ | 12 | 10 | 30 |
| 4 | FC(75mm)+CC༈25mm༉ | 12 | 10 | 30 |
| 5 | FC(50mm)+CC༈50mm༉ | 12 | 10 | 20、30 |
| 6 | FC(25mm)+CC༈75mm༉ | 12 | 10 | 20、30 |
| Notes: 1)Native structure coal(NC༉、Fragmented coal༈FC༉、Crushed coal༈CC༉;༈75mm༉Different coal body structure thickness. | ||||
3.2 Extended finite element method for hydraulic fracturing in coal seams
3.2.1 Principle of numerical simulation experiment.
Small model scales and differences between the experimental and geological contexts are among the physical experiment's problems. The finite element method in the ABAQUS software can be used to model hydraulic fracturing in intricate coal seam formation (Guo et al 2016;Dehghan et al 2017;Li et al 2020). The cohesive element model is used to simulate hydraulic crack extension in order to examine the fracture morphology throughout the fracturing process of various coal seam structural combinations and disclose its influence on the length, width, and height of the fractures. The first damage criteria is applied using the secondary stress criterion. According to Eq. (1), damage to the cohesive sections starts when the three directions' sum of the squares of the stress ratios becomes close to 1.
\(\:{\left\{\left.\frac{⟨{\epsilon\:}_{n}⟩}{{\epsilon\:}_{n}^{0}}\right\}\right.}^{2}\) +\(\:{\left\{\left.\frac{⟨{\epsilon\:}_{s}⟩}{{\epsilon\:}_{s}^{0}}\right\}\right.}^{2}\)+\(\:{\left\{\left.\frac{⟨{\epsilon\:}_{t}⟩}{{\epsilon\:}_{t}^{0}}\right\}\right.}^{2}=1\) (1)
In the formula, \(\:{\epsilon\:}_{n}\) represents the applied stress in the normal direction of the cohesive unit, measured in MPa; \(\:{\epsilon\:}_{s}\) and \(\:{\epsilon\:}_{t}\) are the applied stresses in the two tangential directions of the unit, also measured in MPa; \(\:{\epsilon\:}_{n}^{0}\), \(\:{\epsilon\:}_{s}^{0}\), and \(\:{\epsilon\:}_{t}^{0}\) are the critical stresses for failure in the normal direction and the two tangential directions of the unit, respectively.
Crack propagation criteria use the B-K energy criterion, i.e.,
where Gn, Gs, and Gt are the normal, first shear, and second shear energy release rates of the cohesive elements, respectively, in Pa·m; \(\:{\text{G}}_{\text{s}}^{\text{c}}\) and \(\:{\text{G}}_{\text{n}}^{\text{c}}\) are the critical energy release rates of the cohesive elements in the normal and shear directions, respectively, in Pa·m; η is a constant related to the material properties; and \(\:{\text{G}}^{\text{c}}\) is the critical fracture energy release rate for mixed-mode cracks, in Pa·m.
As illustrated in Fig. 3, the cohesive components at the crack tip node will debond and the crack will progressively spread forward when the energy release rate at the crack tip node surpasses the B-K critical energy release rate.
3.2.2 Numerical simulation experiment model and parameters.
Two-layer and three-layer coal seam structure combination fracturing models were developed based on the nature of coal seam structures in the Shizhuang Block of the Qinshui Basin. The models have dimensions of 50 meters by 100 meters. Regarding the combos: 6 meters for the original coal, 2 meters for the crushed coal; 4 meters for the fractured coal, 4 meters for the crushed coal; 4 meters for the original coal, 2 meters for the crushed coal, and 2 meters for the fractured coal. Due to the well's rather thin coal seams, the overall model thickness was simplified by ten times in order to better see the variations in vertical fracture propagation inside the coal seam. Three-dimensional, eight-node solid elements were used to mimic various coal seam formations in the finite element model. These hexahedral elements have the ability to analyze and numerically simulate the fracture fluid flow and fracture propagation inside the cohesive elements in three dimensions.
The YOZ plane, that runs parallel to the orientation of the lowest horizontal main stress (X direction), is the plane where fractures propagate. A sophisticated algorithm is used in the mesh division approach to enhance calculation accuracy to some degree. Figure 4 displays the finite element models for each of the three coal seam structural combinations (Table 2).
As hydraulic fracturing occurs, the fracturing fluid spreads through hydraulic fractures to the surrounding areas under the pressure of the fracturing fluid injection, entering microcracks and generating a fracture network structure, as depicted in Fig. 5.
| parameters | Primary structural coal | fractured coal | pulverized coal |
|---|---|---|---|
| modulus of elasticity/GPa | 3.7 | 2.46 | 0.21 |
| Poisson's ratio | 0.24 | 0.29 | 0.35 |
| tensile strength/MPa | 0.58 | 0.35 | 0.15 |
| Initial penetration/10-15m2 | 0.79 | 1.09 | 0.002 |
| overriding pressure/MPa | 12 | 12 | 12 |
| Minimum horizontal principal stress/MPa | 8 | 8 | 8 |
| Maximum horizontal principal stress/MPa | 11 | 11 | 11 |
| coefficient of leaching/mm/min1/2 | 0.0035 | 0.005 | 0.00001 |
| Initial water saturation/% | 100 | 100 | 100 |
3.3 Identification of fracturing process based on microseismic monitoring technology
Microseismic monitoring technology is a geophysical technique that evaluates the effects of production activities on foundations, reservoirs, and the transformation of subterranean oil reservoirs by analyzing micro-seismic events, such as vibrations and coal seam rupture (Feng et al 2023;Zhang et al 2023). To assess the fracturing effect in real time and comprehend the artificial fracturing scenario throughout the enhanced oil recovery process, this technology analyzes and computes the geometric properties of fracture networks, including direction, length, height, and other data.
(1) Fundamental idea
Hydraulic fracturing requires the placement of extremely sensitive seismic sensors in strategic locations around coalbed methane wells. The breaking liquid is pushed into the coal reservoir, causing tensile and shear fractures. Seismic waves are released from the coal reservoir during the fracturing process. The micro-seismic events brought on by the variations in internal tension inside the rock are then detected and recorded by the seismic sensors (Zhao et al 2016;Li et al 2019).
(2) Microseismic event location principle
Based on polarization information, the fundamentals of the grid search method for microseismic event location are as follows: 1) The direction of the microseismic event is determined by gathering and analyzing the P-wave data; 2) The distance between the microseismic event and the geophone is further determined by combining the P-wave and S-wave propagation speeds and the first arrival time difference between P- and S-waves; 3) After integrating all the data gathered by the geophones, precise calculations are made to determine the exact location of the microseismic event point (Fig. 6). By making good use of polarization information, this technique raises the precision and dependability of location determination (Chen et al 2020).
4.1 Analysis of the formation process of cracks in different coal body structure combinations
4.1.1 The combination of native structure coal and fragmented coal.
Primary structure coal and crushed coal are the two distinct coal structures on which hydraulic fracturing tests were performed. Three distinct combinations—75 mm of primary structure coal and 25 mm of crushed coal, 50 mm of primary structure coal and 50 mm of crushed coal, and 25 mm of primary structure coal and 75 mm of crushed coal—were utilized in order to prevent inaccuracies from a single set of trials. Based on Fig. 7, with an axial pressure of 12 MPa and a limiting force of 10 MPa as stress conditions, the pressure of starting and beginning time for different compositions of coal are as follows: for 75mm primary structure coal and 25mm fragmented coal, the beginning pressure is 15.76 MPa and the beginning time is 64 seconds; for 50mm primary structure coal and 50mm fragmented coal, the beginning pressure is 12.12 MPa and the beginning time is 54 seconds. And for 25mm primary structure coal and 75mm fragmented coal, the beginning pressure is 10.34 MPa and the beginning time is 34 seconds. This suggests that the higher the primary structure coal thickness and the lower the crushed coal thickness, the higher the mechanical strength of the coal sample combination and the higher the beginning pressure during hydraulic fracturing tests, leading to an extended beginning time, as long as the coal seam's overall thickness stays unchanged. There are also disparities in the fracture test findings because different coal formations differ significantly in terms of pore structure, compressive strength, and tensile strength. The difference in the relationship between the rise in injection pressure and injection time for the coal sample combinations is not significant, though, prior to the fracture of the coal sample specimens.
4.1.2 Combination of fragmented coal and fragmented coal.
The same three combinations are employed for carrying out hydraulic fracturing studies on fractured coal and fragmented coal body structure combinations. A curve representing the relationship between injection pressure and time is displayed, as illustrated in Fig. 8, to help further study the distinctive patterns of injection pressure changes over time during hydraulic fracturing.
As shown in Fig. 8(a ~ d), Similar to the primary structure coal and fragmented combination specimen, the hydraulic fracturing experiment for the combination of 75mm fractured coal and 25mm fragmented coal can also be broken down into three stages, each with a different fracture initiation pressure. Nonetheless, there are two stages that may be distinguished between the two combination specimens of 25mm fractured coal and 75mm fragmented coal and 50mm fractured coal and 50mm fragmented coal. The first stage exhibits a linear increase in injection pressure with time prior to 50 seconds, just like the primary structure coal and fragmented coal combination specimen. There is no evident fracture initiation pressure during the second stage, which incorporates the second and third stages. The fracturing fluid pours out along the cracks in the coal sample specimen, causing the injection pressure to briefly increase before it starts to decrease. The coal sample specimen's injection pressure quickly drops when the fracturing flow of fluid is removed.
According to Fig. 8(d), under the same axial and limiting force conditions, the initial cracking pressures of the three composite specimens are 15.25 MPa, 13.58 MPa, and 12.98 MPa, with initial cracking times of 70s, 81s, and 106s, respectively. Based on the experimental results, The greater the thickness of the fractured coal, the smaller the granular coal thickness when the coal seam thickness remains constant. Resulting in an increase in initial cracking pressure but a decrease in initial cracking time. This is contrary to the experimental results of the combination of original structural coal and granular coal specimens. Compared to original structural coal, fractured coal has relatively developed porosity, making it easier for clear water fracturing fluid to flow. The cracking of cracked coal is more likely when water pressure is applied. In the meantime, the coal body's ability to deform and the flow of fracturing fluid are hampered by a specific thickness of broken coal. For some combinations of coal structures, a higher thickness of broken coal will lead to a longer cracking time.
4.2 The rules of fracture initiation under the influence of coal-rock structure combinations. Different coal body types and thicknesses can have varying effects on the propagation of cracks, as demonstrated by physical fracturing simulation, which also analyzes the relationship between water injection pressure and fracture formation. However, because the small physical size of the cylindrical or cubic samples used in general physical fracturing experiments means that the fracturing fluid discharge is less than that of the construction site, it is challenging to analyze data like the length, width, and height of fractures during the fracturing process of various coal body structure combinations. Large-scale coal body structure models' fracture findings can be more faithfully reflected by numerical simulation, which offers adjustable parameter settings.
4.2.1 Combination of native structure coal and fragmented coal.
The model is composed of two layers, one measuring 6 meters thick for native coal and the other 2 meters thick for crushed coal. The midway point of the vertical direction of the model is where the fracturing fluid injection point is located. Fracture initiation and perforation occur perpendicular to the orientation of the main pressure. The injection rate of the fracturing fluid is 0.5 m3/s, and its viscosity is 1 Pa·s. The model takes 300 seconds to calculate. The fracture opening patterns at various points throughout the fracturing simulation were generated, as illustrated in Fig. 9, taking into account that the extension and expansion of fractures during hydraulic fracturing are dynamically changing.
Fracture development and propagation can be categorized into three stages, as seen in Fig. 10: fracture crossing layers, rapid propagation, and continuous propagation. Stage of Rapid Propagation: When fracturing fluid is injected, cracks in the main structural coal first begin to occur quickly. A fracture with dimensions of 7.3 mm in breadth, 16 m in height, and 8 m in length occurs at t = 6 seconds. The fracture propagation range progressively widens and extends in the course of the maximal principal stress as the injection duration increases. Stage of Fracture Crossing Layers: The fracture crosses from the main structural coal interface to the granular coal interface at t = 41.18 seconds. The fracture propagates in the vertical main stress orientation instead of the greatest principal stress direction. The fracture hole is bigger at the interface where the granular coal and primary structural coal meet. There is a 68.3% rise in fracture width, reaching 12.31 mm. There is a 243.5% rise in fracture height, reaching 54.96 meters. The length of the fracture has increased by 237.5% to 27 m. According to analysis, the coal next to the granular coal section is softer and more likely to fracture when injection pressure is applied continuously, which leads to the spread of fractures. Furthermore, the fracture extends along the interfaces of the various coal structures at the intersection of the major structural coal and granular coal interfaces. Stage of Continuous Propagation: At t = 101 seconds, the crack mostly spreads in the primary structural coal as well as along the path of the highest possible main stress inside the granular coal. Along the path of the greatest principle stress, the fracture in both the primary structural coal and the granular coal keeps getting bigger. Granular coal and main structural coal are both somewhat prone to fracture development.
4.2.2 Fractured and crushed coal combinations.
From top to bottom, the model is made up of 4 m of fractured coal and 4 m of granular coal. The middle point of the vertical direction of the model is where the fracturing fluid injection point is located. Three stages of the overall fracture's creation and propagation can be distinguished, as seen in Fig. 11: fast propagation, continuous propagation, and fracture deflection. Stage of rapid propagation: The crack starts to form, with a bigger gap close to the wall of the borehole. The fracture widens and propagates at the point where the cracked coal and granular coal come into contact at t = 4 s, creating a fracture that is 6.98 mm wide, 8 m high, and 4 m long. Stage of continuous propagation: At t = 31 seconds, the crack mostly spreads throughout the granular coal as long as the fracturing fluid is introduced. According to analysis, the granular coal is less resistant to deformation than the upper fractured coal, which has a larger elastic modulus. As of right now, the fracture is 11.75 m long, 26.01 m high, and its breadth is 10.75 mm. It primarily extends in the direction of the largest major stress. Stage of fracture deflection: The fracture propagation direction deflects to the vertical principal stress direction between t = 121 and t = 300 seconds.
4.2.3 Primary structural, crushed coal and fractured coal assemblages.
The model consists of three layers, from top to bottom: primary structural coal (4 m), granular coal (2 m), and fractured coal (2 m), respectively. Similarly, the middle point is designated as the injection site for the fracturing fluid in the model's vertical direction. Three stages of fracture development and expansion can be distinguished, as seen in Fig. 12: fast expansion, fracture deflection, and fracture through the layer. Quick Growth Phase: With a fracture width of 6.54 mm, a height of 10.01 m, and a length of 4 m, cracks in the native structure of coal and granular coal expand at t = 6s. Stage of Fracture Deflection: As more fracturing water is persistently injected, the range of fracture expansion expands. The crack widens to 9.5 mm, reaches a height of 30 m, and lengthens to 15.01 m at t = 62 s as it continues in the direction of the vertical principal stress. It's more than tripled both in terms of height and length. According to the study, the fracture deflected because of the granular coal layer in the middle. Fracture Through Layer Stage: The fracture keeps growing in the direction of the maximal primary stress from t = 145s to 300s. In the orientation of the vertical major stress, the crack in the fractured coal seam marginally extends at 300 s. According to the investigation, the coal body sticks together once the fracturing fluid penetrates the fractured coal, enhancing its flexibility and preventing the fracture from spreading inside of it.
4.2.4 Characteristics of Pore Pressure and Fracture Morphology Changes in Different Fracturing Stages.
(1) Different Fracturing Stage Divisions for Various Coal Seam Structures. As illustrated in Fig. 13, the hydraulic fracturing process is divided into four stages based on the pattern of variation in pore pressure with fracturing time: continuous increase, wellbore initiation, fracture propagation, and fracture penetration (Zhao et al 2023). This division is based on the analysis of the outcomes from hydraulic fracturing simulations of three distinct coal structure combinations. Using primary structure coal and fragmented coal together as an example: Stage of Continuous Increase (0 ~ 1.675s): The water pressure keeps rising during this phase. As the fracturing fluid diffuses along the high-pressure water pipe and injection pipe, the water pressure first rises gradually. The rate of pressure increase quickens when more fracturing fluid is pushed in. The sample has not yet broken at this time. At 1.675 seconds, the sample cracks, and the highest fracturing pressure is 16.14 MPa. Stage of Wellbore Initiation (1.675 ~ 2.35s): Hydraulic fractures begin when the water pressure reaches the fracture pressure and then suddenly drops from 16.14 MPa to 7.88 MPa. The sluggish development of fractures is indicated by the modest rebound and variations that follow. Fracture Propagation Stage (2.35 ~ 181s): During this phase, the fractures are filled with fracturing fluid by constantly pumping it in. New fractures are created once the initiation pressure is reached; these fractures show up as erratic rises in water pressure. Stage of Fracture Penetration (181–300s): At this point, the water pressure fluctuates around 10 MPa as it approaches a dynamic equilibrium under the predetermined ground stress and flow conditions. The fracturing process is complete since the pressure is gently fluctuating.
(2) Pore Pressure and Fracture Morphology Changes. Pore pressure rises gradually during the fracturing process when the cracking fluid enters the coal seam, first rising, then dropping, and eventually stabilizing. A minor rising trend may be seen in the curves for combinations of original structure coal, granulated coal, and fragmented coal. The maximum pore pressure values for the three different types of coal structure combinations are 16.14 MPa, 14.33 MPa, and 10.23 MPa, respectively, as shown in Fig. 13. According to the research, this pressure is a significant amount higher than the confining pressure, which prevents fractures from developing vertically. Fractures generally expand along the path of the largest horizontal primary stress at the interface of soft coal because the direction of fracture propagation is diverted there. The injection pressure can rebound and rise when the bedding interface stops the hydraulic fracture's vertical propagation (Guo et al 2024). The impact range of fracture propagation becomes important as the thickness of fragmented coal grows, given a particular coal thickness. The impact range of fracture propagation is small when the granulated coal is thin.
Figure 14 shows that the amalgamation of fragmented coal and granulated coal has the largest slope of the curve before 15.87 seconds, followed by the combination of original structure coal and granulated coal, and the combination of original structure coal, granulated coal, and fragmented coal has the lowest slope. According to the investigation, this is because original structure coal and fragmented coal have dropped in thickness while granulated coal has increased in thickness. While the combination of primary structure coal, granulated coal, and fragmented coal continues to see an increase in fracture width as the fractures propagate, the combinations of primary structure coal, granulated coal and fragmented coal quickly increase to their maximum and stabilize after 15.87 seconds. When the duration of fracturing exceeds 300 seconds, the injection point fracture widths are 12.93 mm, 12.13 mm, and 11.74 mm, in that order.
When comparing three different combinations of coal body morphologies, Fig. 15(a) shows that the crack height develops gradually while the crack width increases rapidly. The largest crack widths measured are 33.14 mm, 31.26 mm, and 12.93 mm, in that order. The crack width steadily shrinks as the crack height increases after reaching its maximum value. Another peak in crack width is shown in the curves representing the initial structure of coal and granular coal at a height of 40 meters. According to the research, coal's original structure is where the first maximum crack width originates. A trans-layer fracture phenomenon results from the crack penetrating the granular coal interface as more fracturing fluid is introduced. The original structure's fracture width steadily shrinks when the crack moves to the granular coal reservoir, forming the second maximum crack width in the granular coal. Nonetheless, compared to the original structural coal, the maximum crack width that forms in the granular coal is smaller. The principal cracks that resulted from the fracturing have heights of 79.84 meters, 55.9 meters, and 39.1 meters, respectively. The combination of granular coal and original structure coal during fracturing generates the biggest fracture width and length, whereas the combination of granular coal, fragmented coal, and original structure coal forms the shortest crack width, as shown in Fig. 15(b).
4.3 Microseismic technology to monitor hydraulic fracturing process
4.3.1 Analysis of on-site fracturing of coalbed methane wells.
The No. 3 coal seam in the southern Shizhuang block has been chosen, and the typical well TS32-04D2 will be the focus of the analysis. This well is situated on a slope belt that slopes northwest through the southern portion of the Qinshui Basin. The slope belt has a straightforward structure, minimal faults, and a general 5°dip angle of the strata with a southwest trend. The coal seam has low permeability and porosity, yet because of bedding, cleavage, and natural fractures, the reservoir pressure gradient is smaller than that of the normal pressure system, which causes a rapid filtering loss (Ni et al 2020). The TS32-04D2 well's vertical structure is arranged in the following sequence from top to bottom: primary structure coal, clastic coal, and fractured coal. This identification of the coal body structure is based on the logging curve (Fig. 16). According to Liu et al (Liu et al 2020). clastic coal is classified as "soft coal" in the coal body structure classification system, while Primary structure coal and fractured coal are referred to as "hard coal." As such, the TS32-04D2 well's vertical construction is of the "two hards sandwiching one soft" variety.
The TS32-04D2 well has a depth ranging from 707.00 to 714.60 meters. To reduce coal seam contamination, a fracturing method using clean fracturing fluid with sand carried by high-volume pumping is adopted. Additionally, to reduce the frictional resistance of construction pump pressure, the method of pumping in with a smooth casing is used, which consists of test fracturing and main fracturing. The test fracturing has a pumping rate of 4.0 cubic meters per minute, with a fracturing fluid volume of 35 cubic meters and an instantaneous pump-off pressure of 13.88 MPa. During the sand fracturing, the sand fracturing pumping rate is 8.0 cubic meters per minute, with a net fluid injection volume of 630 cubic meters and 50 cubic meters of quartz sand, and the highest sand ratio is 20%. The pressure variation pattern in the fracturing construction curve, as shown in Fig. 17, is similar to the trend of pore pressure change over time obtained by numerical simulation, which first increases, then decreases, and finally tends to stabilize. It can also be divided into four stages: continuous increase, wellbore fracturing, fracture propagation, and fracture penetration. The fracturing pressure during the pre-injection stage is not obvious and relatively low, with a fracturing pressure of 16.09 MPa. After the instantaneous stop of the pump, the pressure drop during the pressure measurement process is from 6.00 to 3.54 MPa. Based on the change of construction pressure, the fracture curve belongs to the descending type. In theory, the descending type has better fracture effects. The fracture morphology is single, and it can develop a long-type advantageous hydraulic fracture (Han et al 2021).
4.3.2 Microseismic fracture monitoring.
A crack monitoring system is used to perform thorough monitoring of the produced cracks, based on the idea of crack monitoring. While the TS32-04D2 well is a fracturing well, the TS32-07 well is a monitoring well. The orientation, length, and height of the created cracks are provided by the crack monitoring results. The TS32-04D2 well is a vertical hydraulic fracturing well. Microseismic monitoring of the fracture is done, and the depth of the perforation section serves as both the monitoring depth and the monitoring center. The majority of the microseismic events during the middle stage of the main fracturing are distributed on the southwest side of the perforated section, as depicted in Fig. 18(a), a top view, suggesting that the fracturing fractures extend to this region. As of right now, there is no clear crack orientation, and the distribution of microseismic events exhibits the features of a complex fracture network without the morphological traits of a primary break. The side view diagram (Fig. 18b) micro-seismic events shows that the fracture cracks had penetrated deeper into the coal seam, both downhill and upward. The coal structure in this part of the fracture is a "hard coal-soft coal-hard coal" combination, indicating that while the hard coal is thick, the soft coal is thin. This implies that fissures in the hard coal have the ability to penetrate the soft coal. On the other hand, they are difficult to penetrate (Liu et al 2021;Wang et al 2017).
A significant number of microseismic events can be observed abruptly extending to the northeast at the late stage of the main fracturing, as seen in the top view of Fig. 19(a), suggesting that the fracture is extending in that direction. The cracks going northeast are mostly broken through and extend above the coal seam, as can be observed from Fig. 19(b)'s side view. The cracks create high-conductivity fractures in the hard coal because the upper portion of the coal seam is primarily composed of hard coal with a distinct strip structure. The upper portion has more developed cracks, and as additional fracturing fluid is injected, the pressure inside the coal seam rises above the pressure that the roof can resist deformation, thereby breaking through the coal seam and extending to the position above the coal seam. Following the conclusion of the primary fracturing, the fractures on the northeastern side persist above the coal seam and are primarily distinguished by an increase in the affected range of the fracture network breadth without a corresponding rise in length. On the southwest side, the fracture morphology is unaltered. This is because hard coal has a better fracturing effect than soft coal, which causes a significant loss of fracturing fluid. As a result, the fracture mostly expands in the breadth and height directions rather than the length direction.
Numerous microseismic events were identified, resulting in a diverse array of network cracks. The fracture network has a total length of 290 meters with an overall angle of NE51°. The eastern wing of the fracture is 190 meters long, and the western wing is 100 meters long. The total height of the fracture is 56 meters.
(1) The results of hydraulic fracturing tests for various combinations of coal body structures show that the structure combination affects the coal reservoir's pressure and duration of initiation. The onset and spreading of fractures vary in coal reservoirs with the same coal body form but differing stacking techniques. As the injection flow rate increases, the strain rate increases, resulting in a higher strength of the combined coal sample and an increase in initiation pressure. In engineering practice, when soft coal has a comparatively thick layer, it is best to stay away from the soft coal as it significantly affects reservoir modification. When the soft coal is thin and the hard coal is thick, the impact on reservoir modification is minimal. When there is little to no variation in thickness between soft and hard coal, it is preferable to modify the hard coal, as soft coal still has a certain degree of influence.
(2) For three different coal body structure combinations, under the same displacement conditions, a decrease in the thickness of primary structure coal results in a significant reduction in the fracture propagation range; a greater thickness of primary structure coal leads to a larger fracture propagation range, mainly in the course of the vertical principal stress. An increase in the thickness of fragmented coal causes the fractures to propagate from fragmented coal to granular coal in the course of the vertical principal stress. Fragmented coal bonds when it encounters fracturing fluid, enhancing plasticity, and the fractures mainly propagate in the course of the major horizontal stress's maximum.
(3) The geological properties of coal seams possess a notable impact on the start and spread of hydraulic fracturing cracks. In this portion of coal fracture, the coal body's structural configuration is "hard coal - soft coal - hard coal," indicating that two hard coals are encased in a soft coal. The hard coal's cracks tend to pierce through the soft coal when the two types of coal are thin and thick, respectively. On the other hand, they are difficult to penetrate. Hard coal has a greater effect on forming cracks, while the presence of soft coal causes a significant amount of fracturing fluid filtrate loss. As a result, cracks mostly widen and height directions rather than lengthen.
Author contributions T J contributed to the research idea, methodology and model design as well as the writing of the paper. C T has contributed to the research ideas, methods and paper revision. D L and Y L contributed to the model design and methodology. Data collection and analysis were performed by W H、M H and H F.
Funding This research was supported by the National Great Science and Technology Specific Project (2016ZX05066001-002, 2017ZX05064-005), National Science Foundation for Young Scientists of China (Grant No. 41702171), Program for Excellent Talents in Beijing (2017000020124G107), Project of Education Department of Liaoning Province (LJKMZ20220744), Doctoral research Start-up Project of Liaoning Petrochemical University (2021XJJL-025). We thank China United Coalbed Methane Co., Ltd. for providing the data and help for this paper.
Data availability No datasets were generated or analysed during the current study.
Ethical approval The authors undertake that this article has not been published in any other journal and that no plagiarism has occurred.
Consent to participate Informed consent was obtained from all individual participants included in the study.
Consent for publication All authors whose names appear on the submission give their consent to publish this paper in case it is accepte.
Competing interests The authors declare no competing interests.
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No competing interests reported.