Fracturing In Coal Seams Research Articles

Coupling Model of Stress-Damage-Seepage and Its Application to Static Blasting Technology in Coal Mine.

Most coal mine field application processes are carried out using empirical formulas because of the insufficient understanding of the fracture development law of the static blasting technology. This lack of understanding results in poor coal seam gas extraction. In this study, a stress–damage coupling model was established to investigate the construction parameters of the static blasting technology using COMSOL simulation software. Then, a stress–damage–seepage coupling model was designed to study the evolution of the fracture field (seepage field) during the static blasting process using realistic failure process analysis simulation software. Finally, the influencing factors and fracturing effects were analyzed comprehensively. The research results show the following. (1) Comparing the simulation results with previous field tests reveals that the seepage law of the numerical simulation of the static blasting technology is consistent with the field test results, verifying the rationality of the stress–damage–seepage coupling model. (2) The development of coal seam fractures is affected by the expansion pressure, elastic modulus, and guide hole arrangement; the guide hole arrangement can play a role in guiding the development direction of fractures and enhancing the effect of fracturing. (3) The coal body mainly experienced the following five stages of fracturing: coal body compaction, microdamage formation, microfracture formation, large fracture formation, and fracture propagation. In addition, because of the rapid release of soundless cracking agents during the large fracture formation stage, the gas flow decreased in a short time. (4) The static blasting technology causes the coal seam permeability coefficient to increase. Compared with conventional extraction, the effective influence radius in the horizontal direction increases by 5.1 times, and the effective influence radius in the vertical direction increases by approximately 3 times. The static blasting technology can increase the number of coal seam fractures and significantly reduce the coal seam gas pressure, thereby enhancing coal seam permeability and realizing safe coal mining.

Experimental study of gas-water two-phase flow patterns in fracture: Implication for enhancing coalbed methane production

It has been long considered that the gas and water in coal seam fractures are produced in the form of stratified flow; however, recent researches and production practices have proved that two-phase flow in fractures shows various flow patterns. In this paper, visualized experiments were conducted to investigate the diversity of two-phase flow pattern in fracture and their controlling factors. The results show that the flow pattern can be bubbly, stratified, slug, wavy, annular-mist and mist flow under different combinations of gas and liquid flow rates. Among them, the slug flow is characterized by strong disturbance, which may easily cause serious reservoir damage during the drainage process of coalbed methane (CBM) wells. It has been observed that the variations of flow rate, fracture surface roughness, aperture, tortuosity as well as liquid viscosity and surface tension not only determine the proportions of each flow pattern but affect the intensity of slug flow greatly. Based on the observations, it is found that slug flow could be mitigated during the processes of reservoir stimulation and drainage. During the reservoir stimulation process, low-damage fracturing fluid, multi-graded proppant and surrounding rock fracture network stimulation are conducive to the prevention of slug flow. Meanwhile, during the drainage process, a two-phase flow pattern identification template can be used to predict flow pattern distribution, and a continuous but slow drainage strategy could be adopted to prevent slug flow and promote CBM production. This study can guide the establishment of CBM development strategy. • Multiple two-phase flow patterns have been observed in fractures through visualized experiments. • Flow pattern distribution and slug flow strength are impacted by flow rate, fracture and fracturing fluid properties. • Optimizations of stimulation technology and drainage schedule are crucial for inhibiting slug flow during CBM development.

A numerical method of hydraulic fracturing in coal seam for enhancing gas drainage

Hydraulic fracturing technology, as one of the measures to enhance the coal seam permeability artificially and increase the coalbed-methane gas extraction rate, has its remarkable advantages. Although the interaction between fluid and coal has been studied comprehensively in the process of fracturing and gas drainage, fewer researchers focus on the combination of solid-gas-water three-phase flow, which brings great deviations to the design of hydraulic fracturing enhanced underground gas drainage. In this paper, a fracture-pore equivalent model is proposed, which establishes the equivalent relationship between the fracture aperture under hydraulic fracturing and the permeability coefficient of coal in the process of gas extraction, thus realizing the weak coupling between gas-solid-water. The hydraulic fracturing takes into account the deformation and fracture of solids driven by fluids. And the gas extraction considering simultaneously adsorption and desorption effect and the permeability coefficient caused by hydraulic fracturing is realized by the coupling of solid stress field and pore seepage field. The accuracy of this method is verified by comparing with the theoretical solution of the KGD model. Taking the fracturing boreholes in a mine’s field test as the background, the change of the cracks distribution with the injection pressure is analysed quantitatively. And the relationship between gas extraction rate and time is verified by the field monitoring data. The results show that the model has positive significance for the optimal design of gas extraction scheme under hydraulic fracturing.

Synergistic polymer fracturing fluid for coal seam fracturing

Coal seam fracturing can effectively reduce gas leakage and coal powder damage while ensuring a healthy, stable and sustainable development of the coal industry. In this study, a fracturing fluid thickener-quaternary polymer (ADP) was synthesized by the synergistic effect and characterized by 1H NMR and Fourier transform infrared spectroscopy. The viscosity and microstructure of the polymer at different concentrations were observed. A rheological measurement suggests that ADP has good viscoelasticity and shear resistance. Additionally, the filtration performance and gel breaking analysis showed that ADP meets the coal industry standards. Moreover it was found that ADP broken gel fluid impacted the surface of coal samples in Yinjiagou Coal Mine, Yangcheng City, Shanxi Province. For example, it reduced the methane affinity of coal (12.69%) and increased the hydrophilicity. It is significantly rare in the literature where polymer broken gel fluid bring beneficial effects on coal. The damage of gel breaking fluid to coal rock permeability in this area is only 14.9%. Thus the polymer fracturing fluid is expected to become a suitable fracturing fluid for coal seam fracturing.

Investigation on the CBM Extraction Techniques in the Broken-Soft and Low-Permeability Coal Seams in Zhaozhuang Coalmine

To enhance the coalbed methane (CBM) extraction in broken-soft coal seams, a method of drilling a horizontal well along the roof to hydraulically fracture the coal seam is studied (i.e., HWR-HFC method). We first tested the physical and mechanical properties of the broken-soft and low-permeability (BSLP) coal resourced from Zhaozhuang coalmine. Afterward, the in situ hydraulic fracturing test was conducted in the No. 3 coal seam of Zhaozhuang coalmine. The results show that (1) the top part of the coal seam is fractured coal, and the bottom is fragmented-mylonitic coal with a firmness coefficient value of less than 1.0. (2) In the hydraulic fracturing test of the layered rock-coal specimens in laboratory, the through-type vertical fractures are usually formed if the applied vertical stress is the maximum principal stress and is greater than 4 MPa compared with the maximum horizontal stress. However, horizontal fractures always developed when horizontal stress is the maximum or it is less than 4 MPa compared with vertical stress. (3) The in situ HWR-HFC hydraulic fracturing tests show that the detected maximum daily gas production is 11,000 m3, and the average gas production is about 7000 m3 per day. This implies that the CBM extraction using this method is increased by 50%~100% compared with traditional hydraulic fracturing in BSLP coal seams. The research result could give an indication of CBM developing in the broken-soft and low-permeability coal seams.

Hydrogen sulfide occurrence states in China's coal seams

The occurrence states of hydrogen sulfide in coal seams are crucial in preventing and controlling hydrogen sulfide emission in coal mines and the safe development of coal bed methane. In this study, the research status of the occurrence states of free-state, adsorbed-state, and water-soluble hydrogen sulfide in coal seams was systematically analyzed. H2S anomaly areas in China's coal seams are mainly located in the Carboniferous-Permian and Jurassic series of northern, eastern, central, and northwest regions of China. Bacterial sulfate reduction accounts for most of the hydrogen sulfide anomalies of low-rank coal, while thermochemical decomposition thermal desorption spectroscopy and thermochemical sulfate reduction may also result in hydrogen sulfide anomaly in medium- and high-rank coal. In contrast, magmatism-induced hydrogen sulfide anomalies are rarely found. Absorbed-state hydrogen sulfide anomalies are prevailing, while water-soluble and free-state hydrogen sulfide anomalies are relatively scarce. Coal seam's porosity mainly controls the hydrogen sulfide adsorption, pressure, coalification degree, pore volume, and specific area, while water-soluble hydrogen sulfide is influenced by pressure, sulfate-reducing bacteria, burn, porosity, fractures, water temperature, and hydrodynamic conditions. The fractures in coal seams, their burial depth, coal quality, coal rank, roof, and floor lithology are the main factors controlling the free-state hydrogen sulfide preservation. The absorbed-state hydrogen sulfide in coal seams is mainly mitigated by varying the ventilation mode, increasing the ventilation capacity, spraying alkali fog into the air, and injecting alkali liquid into coal seams for governance.

Longwall Top-Coal Caving Mechanism and Cavability Optimization with Hydraulic Fracturing in Thick Coal Seam: A Case Study

Longwall top-coal caving mechanisms and cavability optimization with hydraulic fracturing are analysed in this study. Based on the geological and geotechnical conditions of the Dongzhouyao coal mine, it is revealed that top-coal failure mechanisms are dominated by both compressive and tensile stresses. Ahead of the face line, shear failure initiates at the lower level of the top-coal and propagates to the upper level. Compressive stress-induced damage leads to obvious deterioration in tensile strength, causing the onset of tensile failure in the top-coal behind the face line. Accumulated plastic strain (APS) is selected as a top-coal cavability indicator. The cavability degrades gradually at the higher elevation of the top-coal while it is greatly strengthened as the top-coal approaches closer to the face line. In a thick coal seam without hydraulic fractures, the maximum APS occurs at the middle section of the face length in the Longwall top-coal caving (LTCC) panel. After hydraulic fracturing, top-coal cavability is significantly enhanced. But the spatial distribution of the APS transitions from uniform to non-uniform type due to the existence of hydraulic fractures, causing great variety in the cavability along the panel width. With increasing fracture intensity and fracture size, the failure zone expands significantly ahead of the longwall face, which means the cavability becomes increasingly favourable.

A MULTI-FIELD COUPLED SEEPAGE MODEL FOR COAL SEAM WITH FRACTURES OF POWER LAW LENGTH DISTRIBUTIONS

In the process of gas mining, the fracture distribution with power law length and the pore structure with adsorption effect have an important influence on the coal seam permeability. In recent years, the research on the internal structure of coal seam and the fluid flow mechanism has attracted a large number of researchers. In this paper, by considering the coal matrix deformation caused by adsorption, a pore-fracture model coupled with the multi-field effects and with power law length distribution of fractures in coal seam is established based on the fractal theory for porous media. In this work, we study the influences of the power law exponent [Formula: see text] of fracture length and the ratio [Formula: see text] of the minimum to maximum fracture lengths on the permeability of coal seam and the evolution mechanism of permeability with the structural and mechanical parameters of coal seam. It is found that the permeability of coal seam is inversely proportional to [Formula: see text], directly proportional to [Formula: see text], and to Langmuir volume constant and Langmuir volume strain constant. Compared with other factors, the power law component [Formula: see text] of fractures has the most significant effect on the coal seam permeability.

Case Studies of Comprehensive Gas Control Method during Fully Mechanized Caving of Low-Permeability Ultrathick Coal Seams

Slicing fully mechanized caving mining is a standard high-efficiency mining method for ultrathick coal seams. However, the effectiveness of gas control has accentuated the difficulty in fully mechanized top coal caving of low-permeability ultrathick coal seams. This study focused on mining the No. 9-15 coal in Liuhuanggou Coal Mine, Xinjiang Province, China. To this aim, the results of theoretical analyses and field tests were combined to explore a comprehensive gas control method for fully mechanized caving of low-permeability ultrathick coal seams. The No. (9-15)06 panel was a top-slicing panel of the No. 9-15 coal with a mining height of 9 m. Gas analysis results revealed that gas emissions in the No. (9-15)06 panel are mainly sourced from the coal wall, caving top coal, goaf, and neighboring coal seams. Based on gas source separation, a comprehensive gas control method was proposed. The proposed method was based on the combination of gas predrainage alongside the coal seam, high-position drilling on the top, preburial of pipes in the goaf, and pressure-balancing ventilation. The permeability and gas predrainage were enhanced by hydraulic fracturing in low-permeability coal seams. According to the characterizations of coal seam and crustal stress distribution, the arrangement of the boreholes and backward-segmented fracturing technology were designed. From the field results, the coal seam presented a remarkable prefracturing under hydraulic fracturing. Besides, the mean gas predrainage from the boreholes was enhanced by four times compared to the prehydraulic fracturing state. Finally, using the proposed comprehensive control method based on the gas sources, field tests were performed in the No. (9-15)06 panel. The measured results demonstrated that gas concentration in the return airflow is fluctuated within a range of 0.05% to 0.35%. The proposed gas control method can provide an insightful reference for other similar projects.

Study of the influencing factors of the liquid CO2 phase change fracturing effect in coal seams.

To study the influence of different factors on the cracking effect of the liquid CO2 phase transition, the mechanics of coal rock crack extension based on liquid CO2 phase change blast loading were studied. Through the application of simulation software to analyze the influence of coal seam physical parameters (in situ stress, gas pressure, modulus of elasticity and strength of coal) and blasting parameters (fracturing pore size and peak pressure of detonation)on the effect of liquid CO2 phase change cracking, the simulation results showed that the cracking effect of liquid CO2 phase change was positively correlated with the changes in gas pressure, elastic modulus, fracture hole diameter and peak vent pressure, negatively correlated with the variation in situ stress and compressive strength, and nearly independent of the tensile strength. In addition, by using Gray correlation analysis to analyze the influence degree of six main factors on the cracking effect, the calculation results showed that the effect of blasting parameters was greater than that of physical parameters. The main controlling factor that affected the blasting effect was the peak pressure of blasting release. By conducting comparative engineering trials with different blasting parameters, the test results showed that the crack effect of the coal seam was positively correlated with the change in fracture hole diameter and peak venting pressure, which was consistent with the results obtained from the simulation. The experimental results and simulation results for the effective radius of coal seam fracturing were basically consistent, with the error between the two types of results falling below 10%. Therefore, the reliability of the blasting numerical model was verified. In summary, the research results provide theoretical guidance for applying and promoting liquid CO2 fracturing technology in coal mines.

Anisotropy of the effective porosity and stress sensitivity of coal permeability considering natural fractures

The distribution of natural fractures in coal seams and their effect on the anisotropy of the stress sensitivity of permeability are key issues that are commonly encountered in the simultaneous extraction of coal and gas. To fully understand the law of anisotropy of the effective porosity and stress sensitivity of coal permeability, a modified permeability model and the corresponding anisotropic evaluation index of the stress sensitivity were established considering the influence of the natural fracture distribution of coal. Based on anisotropic coal samples obtained from the Tashan coal mine, the effective porosity and permeability in three orthogonal directions under different stress conditions were systematically evaluated, and the anisotropic evolution law of the stress sensitivity of the permeability was quantitatively analyzed. The results show that the natural fracture system of coal exhibits complex heterogeneity, and the compressibility of the fractures in different directions is different, resulting in significant anisotropic permeability and stress sensitivity. The effective porosity sensitivity of the coal permeability exhibits a power relation with the effective porosity, and a higher effective porosity corresponds to a more notable anisotropy of the effective porosity sensitivity. Under a low stress, the coal stress sensitivity in the direction perpendicular to the bedding planes is the largest. When the stress increases to 19 MPa, the dominant direction of the stress sensitivity changes, and the direction parallel to the face cleats experiences the maximum stress sensitivity of permeability at a high stress level. The power index of the correlation between the coal permeability and effective porosity in different directions of the coal samples in the Tashan coal mine ranges from 2.42 to 9.18, and a constant value of 3 is not observed. This result can be attributed to the influence of the pore size, fracture width and difference in the pore–fracture distribution.

Dilation behavior and acoustic emission response to water/CO2 injection-induced shear fracturing in coal seams

The low permeability of coal seams significantly restrains the efficient extraction of coal-seam methane. Hydroshearing treatment is suitable for enhancing the permeability of coal seams because of their low mechanical strength and unique fracture–matrix structure. The shear dilation behavior associated with the increase in the void volume in porous media is critical for the evaluation of the permeability enhancement during hydroshearing. In this study, we performed conventional triaxial shear experiments to induce purely fluid injection-induced shear failure (hydroshearing) in coal specimens. The axial loading and mean stress unloading experiments were compared to investigate the effects of confinement and fluid injection on the dilation behavior. The experimental results and quantitative inelastic deformation analysis indicated the following: 1) The significant stress drop in the stress–strain curves and shear localization in computed-tomography images indicated the brittle failure of the coal specimens under the hydroshearing condition. 2) The dilation behavior of the coal specimens was sensitive to effective confinement. Mean stress unloading and hydroshearing improved the dilatancy factor from a negative or low value under axial loading to 0.3–1.0. With a suitable injection fluid (CO2) and rate (slow), the dilatancy factor could be further enhanced to 1.7–3.0.3) Owing to the significantly heterogeneous fluid-pressure distribution in coal, the acoustic emission (AE) system detected massive randomly localized micro-breakages, which could not be revealed by global deformation prior to the formation of the shear band in the coal specimens. Therefore, AE monitoring is essential for the hydroshearing experiments on coal to verify the reliability of global deformation measurements. 4) With slow CO2 injection, the fluid-pressure distribution became less heterogeneous. Moreover, the microfractures induced by CO2 sorption can enhance the brittleness and dilation behavior of coal specimens.

Prediction of velocity, gas content from neural network modeling and estimation of coal bed permeability from image log in coal bed methane reservoirs: Case study of South Karanpura Coalfield, India

P-wave velocity and gas content of major coal seams are predicted from three wells from the South Karanpura coalfield, India. Multilayered feed-forward neural network (MLFN) model is developed for prediction P-wave velocity from two wells using three input log parameters: gamma ray, resistivity and bulk density. The model is tested on coal seams: Sayal, Balkudra, Banasgarah and Argada of three wells namely; A, B and C with satisfactory goodness of fit (R2 = 0.77). The ratio of P-wave and S-wave velocities (Vp/Vs) ranges from 1.68 to 3.10 in the coal seams of this field. Cleat density using image log is obtained from four major coal seams namely; Balkudra, Kurse, Hathidari, Banasgarah varying from 1/m to 11/m. Ash content varies from 15 to 51% whereas gas content of major coal seams varies from 2.30 to 4.4 10−3 kg/m3. There is a good correlation between ash content and Vp/Vs of coal seams under the study area. MLFN model for prediction of gas content of the above-mentioned coal seams from two well is trained with input parameters such as: Vp/Vs, bulk density, cleat density and ash content. The model estimates gas content of other coal seams: Argada from well A, Hathidari from well B and Balkudra, Kurse, Banasgarah from well C with satisfactory R2 = 0.77. Maximum horizontal stress orientation is observed from azimuthal shear wave anisotropy from cross multipole array acoustic (XMAC) log for a well A. It varies from N110° to N115° in these coal seams. Permeability of coal seams are computed from X-tended Range Microresistivity Imager (XRMI) tool from a well C. Permeability of coal is varying from 0.5 md in Saunda seam to 17.29 md in Banasgarah seam. The estimated permeability matches well with the pre-fracture permeability of these seams. It is observed that increase in cleat density enhances coal seam fracture permeability in coal seam.

Measuring system to investigate geo- and gas-dynamic processes in hydraulic fracturing of coal seams

A measuring system has been developed for a test bench allowing the study of gas-dynamic processes in large-sized rock samples when performing hydraulic fracturing of a coal seam in a borehole model. This system provides collection of the data on pressure in a hydraulic fracturing device and loading of pneumatic jacks which create compressive stresses in the borehole zone, it also records acoustic emission during crack formation. An additional capability is implemented to measure the flow rate of gas supplied to sample through the holes on its side wall and pumped from the borehole.

Mechanism of Single Proppant Pressure Embedded in Coal Seam Fracture

The strength and rigidity of proppant and coal are quite different. Proppant is easily embedded in coal, resulting in the decrease of fracture aperture and conductivity. The mechanism of proppant embedded in the coal fracture surface needs further study. In this paper, through theoretical analysis and experimental research of single proppant embedded, the proppant embedded law is analyzed from macro and micro aspects. The damage factor of coal surface is introduced, the damage model of the whole process of single proppant pressure embedded is established, and the mechanism of coal fracture surface embedded pit and cracks is revealed. The results show that, from the center of the contact surface to the outside, the surface failure forms of coal fracture are pressure failure, tensile-shear failure, and tensile failure. The proppant is embedded in the coal fracture surface to form four areas: central compaction area, layered fracture area, fracture development area, and original state area. During the energy release of proppant embedded in coal, the energy of proppant is released first and the energy of coal is released later. Greater than 3 times the contact radius, the influence of single proppant on the coal fracture surface can be ignored.

Experimental study of deformations and failures of the coal wall in a longwall working face

The deformations and failures of coal wall in front of a working face cause serious mining difficulties. This paper presents the results of the physical model experiments that were conducted to study the deformation and failure process of coal seam in front of a longwall working face. In order to simulate the formation process of the bearing pressure on the roof rock seam, the special loading device was designed. The stepped loadings were applied on the top side of a sample to simulate the non-uniform loading on the coal seam. The proposed integrity coefficient was used to evaluate the failures of the coal seam. Results show that in the advancing process of a working face, the coal seam undergoes a complex fracturing process and the fracturing of coal seam is closely related to the distance from the coal wall. When the coal body is quite close to the wall of a working face, different types of fracture surfaces appear. The fracture surfaces near the front face of coal wall are tensile while the fracture surfaces away from the front face of coal wall are shear and tensile or shear. These results are useful for understanding the deformation and failure of coal wall.

Coupling Technology of Deep-Hole Presplitting Blasting and Hydraulic Fracturing Enhance Permeability Technology in Low-Permeability and Gas Outburst Coal Seam: A Case Study in the No. 8 Mine of Pingdingshan, China

The current study aims to analyze the principles of integrated technology of explosion to tackle the problems of coal seam high gas content and pressure, developed faults, complex structure, low coal seam permeability, and high outburst risk. Based on this, we found through numerical simulation that as the inclination of the coal seam increases, the risk of coal and gas outburst increases during the tunneling process. Therefore, it is necessary to take measures to reduce the risk of coal and gas outburst. We conducted field engineering experiments. Our results show that the synergistic antireflection technology of hydraulic fracturing and deep-hole presplitting blasting has a significant antireflection effect in low-permeability coal seams. After implementing this technology, the distribution of coal moisture content was relatively uniform and improved the influence range of direction and tendency. Following 52 days of extraction, the average extraction concentration was 2.9 times that of the coal seam gas extraction concentration under the original technology. The average scalar volume of single hole gas extraction was increased by 7.7 times. Through field tests, the purpose of pressure relief and permeability enhancement in low-permeability coal seams was achieved. Moreover, the effect of gas drainage and treatment in low-permeability coal seams was improved, and the applicability, effectiveness, and safety of underground hydraulic fracturing and antireflection technology in low-permeability coal seams were verified. The new technique is promising for preventing and controlling gas hazards in the future.

Experimental Study on Stress Transfer and Fracture Law During the Hydraulic Fracturing of Coal Seams

Hydraulic fracturing is a key technology for increasing the permeability of coal seams and improving the extraction effect of coalbed methane. Exploring the parameters that represent the infiltration effect is an essential part of the optimization process of hydraulic fracturing. In this work, based on a similarity principle design, a hydraulic fracturing test was carried out on large raw coal samples using an indoor hydraulic fracturing physical simulation test system. Based on the distribution of primary cracks in the coal samples, the law of crack extension during hydraulic fracturing was studied. The stress variations before and after hydraulic fracturing of the coal seams were monitored, and the stress transfer laws during confining pressure loading and hydraulic fracturing were investigated. The experiment showed that the new fissures in the fracturing process are more likely to extend to the primary fissure development area. When the applied coal body stress exceeds the strength of the coal sample body, the internal cracks in the coal sample are completely penetrated. During this complete penetration, the energy accumulated by pressure water injection is continuously released, and the count detected by the acoustic emission increases sharply. The coal is mainly subjected to compressive stress during the confining pressure loading process, whereas the coal body is mainly subjected to a shear stress during the hydraulic fracturing process. Both the stresses can be transferred through the coal samples. When the stress increase during hydraulic fracturing is greater than the confining load, the overall effect is due to different stresses. As the cracks continue to expand and extend during hydraulic fracturing, the stress transfer effect becomes weaker. Through the exploration of the law of crack extension and stress transmission, this experiment showed that large-scale physical simulation experiments in a laboratory setting can help effectively simulate on-site hydraulic fracturing conditions. The test method and results reported herein provide a reference and basis for the design and optimization of on-site hydraulic fracturing parameters.

Preparation and performance analysis of bisamido-based cationic surfactant fracturing fluid for coal seam water injection

Coal seam water injection fracturing can achieve the gap formation of the coal seam and increase the porosity of the coal seam. The water injection fracturing fluid wetting the coal body can wet the coal body in advance, reducing the generation of coal dust during coal mining, as well as play a preventive role in many coal mine accidents and optimize the safety state of the coal mine. To further improve the water injection efficiency of the coal seam. In this paper, erucic acid, N,N-dimethyl-1,3-propanediamine and epichlorohydrin were used as raw materials to synthesize a bisamido cationic surfactant under acidic conditions, and mixed with a KCl solution to prepare a clean fracturing fluid that can be used in the water wetting of a coal seam. The functional groups and microstructure changes of the product are observed by FTIR and SEM. Related performance experiments show that the product has the advantages of good shear stability viscosity, low filtration loss, good gel breaking performance and strong wettability. The analysis of the cleanability shows that the gel breaking liquid has a low residue content and good compatibility. Therefore, the product is an ideal clean fracturing fluid for water injection fracturing of coal seam.

Effects of different conductive ions on pore-structure evolution of medium- and high-rank coal bodies induced by electric pulses

Electric pulse fracturing technology is a new type of coal seam fracturing and permeability increasing technology based on high-pressure shock wave technology. Compared with conventional technology, electric pulse fracturing has the advantages of a higher energy efficiency, less environmental pollution, and a shorter fracturing time, but having the disadvantages of rapid energy loss and a small fracture radius. To reduce the energy loss of electric pulses and increase the fracture radius, the experimental system of high-voltage electric pulse was employed in this study. The effects of NaCl, CaCl2, and AlCl3 conductive ions on the evolution of the pore structure induced by electric pulses were examined for medium- and high-rank coal bodies. The experimental results indicated the adsorption capacity of coal for ions was positively correlated with the valence of the cations and the soaking time, and the amount of ions adsorbed was larger when the solution was more alkaline. Amounts of ions adsorbed onto the coal surface decreased in the following order Al3+ > Ca2+ > Na+. After the treatment, the conductivity of the coal was significantly improved, making the plasma channel in the coal body more fully developed in the process of high-voltage electric pulse impaction, thus reducing significantly the breakdown voltage of the coal samples. Additionally, tthe total pore volume, total specific surface area, porosity, and average pore diameter of the treated coal increased and that the percentages of macropores and mesopores increased, indicating that the pore structure of coal samples is significantly improved.