Low Permeability Coal Research Articles

Theoretical study on nonlinear seepage mechanism in fractal dendritic fracture network of low permeability coal with water injection

Coal seam water injection technology is adopted by many mines as an effective means of dust reduction in coal mines. There is a threshold pressure gradient phenomenon in the process of water injection in low permeability coal seam, which makes the flow of pressure water in the fracture structure of coal body present nonlinear seepage characteristics. To reveal the theoretical relationship between the structural parameters of coal and the nonlinear seepage characteristics, firstly, the Bingham fluid constitutive equation is used to describe the non-Newtonian behavior in low-permeability coal. Combined with the fractal tree-like bifurcation fracture network model, a mathematical analytical model of threshold pressure gradient is established. Secondly, the model was verified by high-pressure water invasion and radial seepage experiments, and the sensitivity of the model was analyzed. The results show that the error between the theoretical calculation value and the experimental measurement value is between 8.65 % and 42.4 %, which verifies the validity of the model. The above research results can provide a theoretical basis for improving the water injection effect of low permeability coal seam.

Mechanical Damage to Coal and Increased Coal Permeability Caused by Water-Based Ultrasonic Cavitation

Coalbed methane (CBM), recognized as a sustainable and environmentally friendly energy source, plays a crucial role in mitigating global climate change and advancing low-carbon energy solutions. However, the prevalence of low-permeability coal seams poses a significant challenge to effective CBM extraction. Improving coal permeability has emerged as a viable strategy to address the issue of low-permeability coal. Conventional CBM stimulation methods fall short in overcoming this obstacle. In contrast, the enhanced technique of CBM extraction by water-based ultrasonic cavitation holds great promise due to its use of high energy intensity, safety, and efficiency. Nevertheless, the inadequate theoretical framework for managing this technology impedes its widespread adoption for large-scale applications. This study investigated the impact of water-based ultrasonic cavitation treatment on coal’s properties and permeability through mechanical testing and permeability measurements conducted before and after treatment. This study also explored the process by which this technology, known as WUC-ECBM, improves coal’s mechanical properties and permeability. The findings suggest a potential stimulation technique (WUC-ECBM) for use in CBM extraction, and its physical mechanism.

Effect of gas pressure on the creep and seepage characteristics of low permeability coal

Effect of gas pressure on the creep and seepage characteristics of low permeability coal

Study on the Mechanism of Coal Fracturing by Liquid Nitrogen

As a fracturing medium with excellent properties, liquid nitrogen has multiple fracturing effects on coal, and its potential application in coalbed methane mining has become a hot spot in current research. In this paper, the key scientific problems of fracturing and penetration enhancement by liquid nitrogen in low-permeability coals are discussed in depth, and the pore fracturing process and its fracturing mechanism of low-temperature fracturing coal are analyzed. It is found that the ultra-low-temperature fluid fracturing process has the effect of high-energy fluid fracturing to break the rock, and also has the coupled fracturing effect of low-temperature fracturing and phase-change fracturing to increase the energy-driven replacement. It aims to provide theoretical basis and technical support for the efficient and environmentally friendly exploitation of low-permeability oil and gas fields.

Session 6. Oral Presentation for: Don’t forget your keys when trying to unlock the productivity of low-permeability coals

Session 6. Oral Presentation for: Don’t forget your keys when trying to unlock the productivity of low-permeability coals

Don’t forget your keys when trying to unlock the productivity of low-permeability coals

Low-permeability coal seam gas (CSG) wells have been the subject of laboratory research and modelling studies over the past decade, particularly focusing on the pressure-dependent permeability (PDP) behaviour of coals. These research efforts have progressed diagnostic methods to identify and quantify PDP and provide practical technologies to counter these effects. Firstly, machine learning methods based on drilling and historical well-test data can provide insight into the range of coal permeability during drilling. Next, the process of history-matching the after-closure pressures from a diagnostic fracture injection test (DFIT), using reservoir simulators, can determine best-fit values for fracture compressibility, a key parameter for reservoir models. Finally, these data, along with DFIT reservoir pressure and permeability data, can inform the decision-making process regarding the most applicable completion strategy and aid developmental planning. For areas where vertical or surface-to-inseam (SIS) wells have been unsuccessful, new hydraulic fracturing technologies have been developed to enhance the stimulated reservoir volume (SRV) in coals, using horizontal wells with multi-stage hydraulic fracturing in excess of 20 stages. Recent laboratory and modelling of micro-proppants has extended prior laboratory and modelling studies and provided insight into proppant transport, embedment, and screen-out behaviour. These well stimulation technologies can be co-applied in new or existing CSG fields and are suitable for areas where overlapping tenements limit conventional, steel-based completion strategies. In conclusion, this paper will bring the key findings of these studies together in a cohesive framework and provide the workflows to implement these technologies for better productivity in low-permeability coals.

Influence of Pore Structure Characteristics of Low Permeability Coal on Gas Nonlinear Seepage

Influence of Pore Structure Characteristics of Low Permeability Coal on Gas Nonlinear Seepage

Experimental Study of Non-Darcian Flow Characteristics in Low-Permeability Coal Pillar Dams.

The safe operation of underground reservoirs and environmental protection heavily rely on the water flow through coal pillar dams in coal mines. Meanwhile, research on the flow characteristics in coal pillar dams has been limited due to their low hydraulic conductivity. To address this gap, this study assembled a novel seepage experimental device and conducted a series of carefully designed seepage experiments to examine the characteristics of low-permeability in coal pillar dams. The experiments aim to explore the relationship between water flux and hydraulic gradient, considering varying core lengths and immersion times. Flow parameters were determined by fitting observed flux-gradient curves with predictions from both Darcy and non-Darcian laws. Several significant results were obtained. First, a noticeable non-linear relationship between water flux and hydraulic gradient was observed, particularly evident at low flow velocities. Second, the non-Darcy laws effectively interpreted the experimental data, with threshold pressure gradients ranging 13.60 to 58.64 for different core lengths. Third, the study established that water immersion significantly affects the flow characteristics of coal pillar dams, resulting in an increased hydraulic conductivity and flow velocity. These findings carry significant implications for the design of coal pillar dams within underground coal mine reservoirs, providing insights for constructing more stable structures and ensuring environmental protection.

Modeling of Supercritical CO2 Adsorption for Low-Permeability Coal Seam of Huainan-Huaibei Coalfield, China.

Investigating the coal adsorption behavior on supercritical CO2 (ScCO2) is crucial for long-term CO2 geological storage. In this paper, low-permeability coal samples from the Huainan-Huaibei coalfields in China were selected. The high-pressure isothermal adsorption of CO2 was carried out at 36, 42, and 48 °C. The results of adsorption experiments were analyzed by fitting 9 types of modified adsorption models, including three different adsorption theories. Considering that different adsorption mechanisms may exist for CO2 in coal, 14 mixed adsorption models were established. The accuracy of the coefficient of determination (R2) and root-mean-square error (RMSE) for ScCO2 excess adsorption capacity was analyzed, mainly focusing on the accuracy of the key model parameters such as the adsorption phase density and the theoretical adsorption capacity. These parameters were discussed, combined with the predicted adsorption phase density of CO2 based on the intercept method. The results indicate that among the 9 types of modified adsorption considered, based on the adsorption phase density screening, the deviation of the predicted adsorption capacity from the experimental value was then considered. The Dubinin-Radushkevich (DR) model can effectively fit the adsorption behavior of CO2 at low pressure (<7.5 MPa). The Langmuir (L), Langmuir-Freundlich (LF), Extended-Langmuir (EL), and TOTH models can effectively fit the adsorption behavior of CO2 at high pressure (7.5-20 MPa), while the multimolecular layer models were unsuitable for fitting ScCO2 adsorption. The model fitting results showed that only the monomolecular layer and micropore-filled adsorption models were suitable for fitting the ScCO2 adsorption capacity. The DR-LF model best fits the adsorption data based on its key parameters of adsorption phase density and theoretical adsorption capacity. The established mixed model DR-LF fitting results showed that the CO2 in coal was dominated by microporous filling adsorption. The higher the temperature, the greater the contribution of microporous filling adsorption to the total adsorption. There still exists deviation in the adsorption phase density and theoretical adsorption capacity. The contribution percentage of different adsorption mechanisms of CO2 in coal needs to be further investigated.

Pore structure of low-permeability coal and its deformation characteristics during the adsorption–desorption of CH4/N2

The pore structure of coal plays a key role in controlling the storage and migration of CH4/N2. The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement effect of N2 injection. The deformation characteristic of coal during adsorption–desorption of CH4/N2 is an important factor affecting CH4 pumpability and N2 injectability. The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. Subsequently, a simultaneous test experiment of CH4/N2 adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. Finally, the applicability of low-permeability coal to N2 injection for CH4 displacement technology was investigated. The results show that the micropores and transition pores of coal samples are relatively developed. The pore morphology of coal is dominated by semi-open pores. The pore structure of coal is highly complex and heterogeneous. Transition pores, mesopores and macropores of coal have good connectivity, while micropores have poor connectivity. Under constant triaxial stress, the adsorption capacity of the coal for CH4 is greater than that for N2, and the deformation capacity of the coal for CH4 adsorption is greater than that for N2 adsorption. The axial strain, circumferential strain, and volumetric strain during the entire process of CH4 and N2 adsorption/desorption in the coal can be divided into three stages. Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-difference. A strain model for the adsorption–desorption of CH4/N2 from coal was established by considering the expansion stress of adsorbed gas on the coal matrix, the compression stress of free gas on the coal matrix, and the expansion stress of free gas on micropore fractures. N2 has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas flow, which can significantly improve the effectiveness of low-permeability coal seam gas control and promote the efficient utilization of gas resources.

Study on heat transfer effect of high-temperature nitrogen seepage in low-permeability coal

ABSTRACT High-temperature nitrogen injection into the low permeability coal can be used to improve the efficiency of coalbed methane extraction effectively. In order to study the heat transfer effect of High-temperature nitrogen seepage in low-permeability coal, a coupled hydro-thermo-mechanical model of High-temperature nitrogen injection into coal is established. On this basis, by using COMSOL Multiphysics numerical software to numerically solve the coupled model, the results show that: the higher the High-temperature nitrogen temperature is, the more favorable the heat diffusion of N2 will be. The High-temperature nitrogen pressure and the initial coal permeability have few effects on the heat transfer. The longer time of High-temperature nitrogen injection is, the greater influence range of heat transfer becomes, but the influence gradually decreases. In addition, the larger borehole diameter is, the greater influence is, but the influence is limited. Under the condition of 4MPa, 100°C and 20 days of heat injection, the gas recovery concentration is up to 98%. The research results can guide the field test of High-temperature nitrogen injection into coal seam.

Experimental Study on the Effect of Freeze-Thaw Cycles on the Mechanical and Permeability Characteristics of Coal

The safe and efficient mining of coal seams with low porosity, low permeability, and high heterogeneity under complex geological conditions is a major challenge, with the permeability of coal seams playing a crucial role in coal mine gas extraction. The development of coal seam permeability enhancement technology can help coal mines produce safely and efficiently, while the extracted coal bed methane can be utilized as green energy. To study the effect of freezing and thawing on the evolution of the mechanical and permeability properties of coal, triaxial permeability tests were conducted on low-permeability coal under two different confining pressures. Simultaneously, dry, saturated, and freeze-thaw coal samples were set up for comparison, and the effects of water and freeze-thaw were isolated from each other. The triaxial mechanics and percolation laws of dry, saturated, and freeze-thaw coal rocks were obtained; the results show that saturated coal has the lowest initial permeability, while freeze-thawed coal has the highest initial permeability. Through analyzing the effects produced by water, freezing and thawing on coal specimens, the mechanism of the influence of freeze-thaw on the permeability evolution of coal was revealed. The research results can provide theoretical guidance for the development of gas extraction technology for low-permeability coal seams.

Effect of moisture on microwave ignition of bituminous coal.

The interaction between water and microwave is of vital importance to reveal the microwave ignition mechanism of water-bearing coal. This study used two group of bituminous coal after drying and water saturation treatment, for experimental testing and contrastive analysis. During the experiment, permeability of coal samples was obtained based on nuclear magnetic resonance(NMR) test, then different power of microwaves were applied to coal samples, and the occurrence of hot spots within coal samples was regarded as a sign of microwave ignition. Microwave ignition of water-saturated coal is mainly affected by microwave power and coal permeability. The pore water in low permeability coal is conducive to microwave ignition, while the pore water in high permeability coal will prolong the ignition time. There is a permeability threshold, above which the average ignition time of water-saturated coal samples is longer than that of dry coal samples, but below which the opposite is true. These insights can be used to evaluate the safety of microwave technology when applied to coal engineering.

Mechanism and Practice of Multifracturing Using Dynamic Loads in a Low-Permeability Coal Reservoir.

The geological conditions of coal reservoirs in China are complex, and the reservoir permeability is generally lower. Multifracturing is an effective method of improving reservoir permeability and coalbed methane (CBM) production. In this study, two types of dynamic loads, CO2 blasting and a pulse fracturing gun (PF-GUN), were used to conduct multifracturing engineering tests in nine surface CBM wells in the Lu'an mining area in the central and eastern parts of the Qinshui Basin. The curves of pressure versus time of the two dynamic loads were obtained in the laboratory. The prepeak pressurization time of the PF-GUN was 200 ms, and that of the CO2 blasting was 2.05 ms, which just falls in the optimum pressurization time of multifracturing. The microseismic monitoring results revealed that, in terms of the fracture morphology, both the CO2 blasting and PF-GUN loads produced multiple sets of fractures in the near-well zone. In the six wells used for the CO2 blasting tests, an average of three branch fractures were produced outside of the main fracture, and the average angle between the main fracture and the branch fractures exceeded 60°. In the three wells stimulated by PF-GUN, an average of two branch fractures were produced outside of the main fracture, and the average angle between the main fracture and the branch fractures was 25-35°. The multifracture characteristics of the fractures formed via CO2 blasting were more obvious. However, a coal seam is a multifracture reservoir with a large filtration coefficient; the fracture will not extend after reaching the maximum scale under a certain gas displacement condition. Compared with the traditional hydraulic fracturing technique, the nine wells used in the multifracturing tests exhibited an obvious stimulation effect with an average increase of 51.4% in daily production. The results of this study provide an important technical reference for the efficient development of CBM in low- and ultralow-permeability reservoirs.

Characteristics of Gas Seepage and Pore Structure Response in CO2-ECBM Process of Low Permeability Coal Seam

CO2-ECBM is a method of enhanced coalbed methane extraction followed by cutting greenhouse gas emissions and new energy development. In order to reveal the characteristics of gas flow in porous media and the pore structure response characteristics of coal rocks, the experiments were carried out to simulate the process of CO2 displacement of N2 at a buried depth of 900 m, including monitoring the changes in gas permeability and strain of coal samples along with a comparison of the pore structure of low-temperature liquid nitrogen adsorption on coal samples both before and after displacement were both done. The findings of the experiment are listed below. The N2 permeability of the LiuZhuang sample ranges from 0.0008mD to 0.0014mD, whereas the permeability of QiDong is around 0.0003mD. With an increase in gas injection duration and an expansion of the coal matrix for N2 adsorption, the permeability steadily decreases. The efficient stress compression of the coal pore fracture structure during sample preparation and testing avoids the visible fracture region, which results in poor permeability. The displacement stages of CO2 can be divided into three phases. Free nitrogen flows from the end of the position and the permeability diminishes during the phase of free nitrogen. When CO2 is introduced into the penetration stage, the permeability tends to rise, however when there is no penetration, the permeability test values are frequently low. During the CO2 steady displacement phase, gas permeability gradually declines. Axial and radial strains are progressively raised during the initial stage of the CO2 injection whereas they are gradually reduced during the initial stage of the N2 injection. While CO2 is continuously supplied through the coal body stage, there are modest axial and radial strain changes. The axial and radial stresses are stabilized by the CO2 displacement. The overall pore volume of the coal significantly rises following the displacement. The increase part of the pore volume is primarily focused on the pore of absorption and filling (aperture &lt; 10nm), whereas the decreased part is mainly concentrated in the diffusion pore of the Fick type and the permeability part (aperture &gt; 50nm). The increased in pore volume ratio surface area is centered mostly in the fill pore region (aperture 10 nm) and is four times greater than it was before the displacement. The CO2 injection exerts an expansion impact on the adsorptionfilled and diffusion pores during the CO2-ECBM process, whereas the compression effect on the percolation pores results in a reduction in permeability.

Insight into difference in high-pressure adsorption-desorption of CO2 and CH4 from low permeability coal seam of Huainan-Huaibei coalfield, China

This study aimed to investigate the differential characteristics of high-pressure adsorption-desorption of supercritical CO2 and CH4 at main temperatures of 36 ℃, 42 ℃, and 48 ℃ and the pressure of 0–20 MPa in low permeability coal, a case from the tectonic coal area of Huainan-Huaibei coalfield, eastern China. Combining the variation characteristics of adsorption capacity with pressure, gas free phase density, the prediction of CO2 adsorbed phase density, the correction method of gas adsorption capacity, the sequestration capacity of CO2 in low permeability coal, and the adsorption hysteresis index of adsorbed gas were assessed. The controlling factors of gas adsorption capacity were revealed based on the general discussion of the relation between the gas adsorption capacity and the conditions such as temperature, pressure, coal maceral, coal quality, and pore structure. The results show that the lowest equilibrium pressure associated with the CH4 excess adsorption peak is 9 MPa, which is larger than most of the experimental pressures for the CH4 isothermal adsorption test. CO2 excess adsorption capacity of the different coal samples has characteristic of an obvious peak at the experimental pressure near 6 MPa under different experimental temperatures, all CO2 excess adsorption capacity changes with CO2 free phase density in a two-step pattern, CH4 or CO2 adsorbed phase density can be predicted by the truncation method. Desorption of CH4 or CO2 in coal has an obvious "hysteresis loop", the cumulative adsorption hysteresis index (CAHI) and the incremental adsorption hysteresis index (IAHI) of CH4 based on the calculation of CH4 excess adsorption capacity are controlled by the temperature and pressure, it has the obvious rise during the stage of low pressure (less than 4 MPa), whether IAHI or CAHI of CO2, it shows characteristics of complex changes. Variations in CAHI and IAHI for CH4 or CO2 are governed by the development of pore structure in coal, and the temperature. Coal maceral, coal quality, and coal grade have an effect on gas adsorption in coal by affecting the growth, filling, and deformation of pores in coal; the adsorption capacity of the gas depends both on the surface area of the pores and the pore volume of the pores in the coal. The differential mechanism for the gas adsorption capacity from the low permeability coal requires further understanding.

Changes in Microstructure and Mechanical Properties of Low-Permeability Coal Induced by Pulsating Nitrogen Fatigue Fracturing Tests

Changes in Microstructure and Mechanical Properties of Low-Permeability Coal Induced by Pulsating Nitrogen Fatigue Fracturing Tests

Impact of Stimulated Fractures on Tree-Type Borehole Methane Drainage from Low-Permeability Coal Reservoirs

Tree-type hydraulic fracturing (TTHF) is a promising method applicable to the effective development of methane in low-permeability coal seams. However, a large-scale application of this technique is limited due to the unclear impact of stimulated fractures by TTHF on the effect of post-fracturing methane drainage. To address this issue, a multi-scale methane flow model of coupled thermo-hydro-mechanical (THM) processes in stimulated coal seams by TTHF was developed and verified against laboratory-based measurements. Using this proposed model, a systematic evaluation of the influence extent of hydraulic fractures connecting sub-boreholes in a tree-type borehole on the drainage effect under different fracture apertures, initial permeabilities of the cleat system, and remnant methane pressures was performed. Detailed simulated results showed that the presence of highly permeable fractures induced by TTHF greatly enhanced, as expected, the drainage efficiency of coal seam methane between the ends of adjacent sub-boreholes, and led to a significant increase in the homogeneity coefficient β. Furthermore, increasing the stimulated fracture aperture and initial cleat permeability or reducing the remnant methane pressure also resulted in a larger value of β, but in turn shortened the lead time of the tree-type borehole. The β’s growth rate for different investigated cases compared to identical simulations without stimulated fractures presented an overall trend of increasing at first and then slowly decreasing with sustained drainage time. Meanwhile, large-aperture hydraulic fractures and lower remnant methane pressure are more beneficial to the drainage effect of tree-type boreholes in the initial stages of drainage. These results portrayed herein can be employed to better understand how fractures generated by TTHF play a role in post-fracturing drainage programs and provide theoretical assistance in engineering applications.

Effects of chemical solvents on coal pore structural and fractal characteristics: An experimental investigation

Chemical solvent treatment is one of the most effective means to improve pore structure of low-permeability coal and increase production of coalbed methane. Low-temperature liquid nitrogen adsorption method, X-ray diffraction (XRD) mineral analysis and fractal theory were adopted to study variations in coal pore structural parameters and fractal characteristics affected by hydrochloric acid (HCl), tetrahydrofuran (THF) and carbon disulfide (CS2). Liquid nitrogen adsorption results of coal samples were analyzed using density functional theory (DFT), Brunauer-Emmett-Teller (BET) equation and Barrett-Johner-Halenda (BJH) model. Meanwhile, changes in fractal dimensions of pore surface (D1) and pore structure (D2) were calculated based on Frenkel-Halsey-Hill (FHH) model. Relationship between pore volume, average pore diameter, specific surface area and two fractal dimensions were established. Results show that: (1) Hysteresis lines exist in all isothermal adsorption-desorption processes of coal. Pores of raw coal are mainly semi-open pores with poor connectivity. The types of pores do not change significantly after THF and CS2 treatment, while the number of pores rise. In contrast, affected by HCl treatment, ink-bottle pores and open pores increase and pore fracture network connectivity becomes better. (2) Pore sizes of treated coal mainly distribute in range of 0–10 nm. After HCl treatment, many closed pores are opened, with growth in proportion of micropores. Meanwhile, specific surface area (SSA) of pores increases by 55.21% and 31.36%, respectively. THF extraction results in an increase in pore volume and SSA. After CS2 extraction, dissolution of organic material increases pore size and further improve volume of transition pores and mesopores. (3) HCl treatment causes decrease in D1 value of pores. However, in organic-solvent-treatment cases, solvent residues retain on coal pores, resulting in growth in D1 value and pore surface becoming rougher. Proportion of total pore volume of transition pores and mesopores of acidized coal decreases and D2 value rises. Pore distribution is more uneven. Relevant results could provide theoretical references for revealing mechanism of chemical solvents effects on gas flow in coal.

Study on Crack Penetration Induced by Fatigue Damage of Low Permeability Coal Seam under Cyclic Loading

For low permeability coal seam permeability is weak, low degree of gas migration, prone to gas accidents and other issues. In this paper, a numerical model is established to simulate the process of hydraulic fracturing under monotonic loading and cyclic loading, and a method of increasing permeability of coal seam by cyclic loading hydraulic fracturing technology is proposed. Combined with similar experiments, the influence of cyclic load and cyclic load applied parameters on the fracturing effect of coal and rock mass was analyzed by applying a cyclic load with a pulse pump. The effect of cyclic load pressure technology on coal seam drainage was analyzed by application in 20915 gas control roadways of a coal mine in Guizhou. The results show that after fracturing, the fracture extends along the weak plane of the prefabricated fracture, the pore pressure in the fracture is high pressure, and the pore pressure around the fracture decreases step by step. Due to the compression of the crack, the energy is transferred to the two ends of the crack. The pore pressure has an irregular oval distribution, and there is stress concentration. The pressure value reaches 41.48 MPa. After the cyclic load was applied to the model, the pressure reached the maximum value of 27.64 MPa at 3.37 s. Compared with the monotonic load, the pressure value was reduced by 46.27%. Through pressure and ringing analysis, the fatigue damage of specimens can be realized under cyclic loading. In the experiment, the unconstrained initiation pressure was 2.48 MPa, but after the constraint was applied, the initiation pressure increased to 4.58 MPa, and the pressure increase reached about 55%. After multiple loading and unloading, the peak pressure of the specimen can be reduced and the number of cracks can be increased. In the experiment, the gas extraction rate of ordinary drilling was maintained at about 0.019 m3/min, and the gas extraction rate of ordinary fracturing drilling fluctuated at 0.025 m3/min after 21 days of gas extraction. The pumping capacity of 15 Hz and 20 Hz cyclic loading fracturing boreholes tended to be stable after 15 days, which were about 0.041 m3/min and 0.062 m3/min, respectively. Cyclic loading hydraulic fracturing is better than monotonic loading hydraulic fracturing, and the lower the cyclic loading frequency, the better the fracturing effect.