Adsorption behavior of silica nanofluid on coal and its injection enhancement mechanism

Coal seam water injection has proven effective in mitigating coal and gas outburst disasters in mines. However, its drawbacks, including the poor wettability of coal seams and the susceptibility to filtration loss of injected water, have led to low construction efficiency and uneven control outcomes. In this study, we propose a novel approach to address these issues by utilizing a water-based silica nanofluid to alter surface wettability. The four-stage deposition process of nanoparticles on the coal surface is identified, and the time-varying behaviour of modified coal wettability is revealed under the influence of key parameters, such as particle concentration. These findings provide a foundation for the application of nanofluids in modifying the wettability of reservoirs

Editorial: Advances and applications of artificial intelligence and numerical simulation in risk emergency management and treatment

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Effect of calcium lignosulfonate on the deformation and failure characteristics of cementing stone and its modification mechanism

In the coalbed methane extraction of surface wells in the mining area, the mining of coal seam in working face will cause a significant disturbance to overlying rock layer, which in turn will lead to the deformation and damage of surface wells. The coalbed methane extraction of surface wells in the mining-disturbed areas cannot be effectively performed. Cementing technology can effectively elongate the life span of mining-disturbed coalbed methane surface wells. As a critical factor of the cementing quality of surface wells, the early strength of the cement needs to meet the high cementing requirements. Cement stone is formed after cement consolidation. The mechanical properties of cement stone are of great importance to maintain the stability of mining-disturbed coalbed methane surface wells. It is an effective way to improve the cementing effect by adding dispersants to enhance the mechanical properties of cement stone. There are few researches on the deformation and failure characteristics of calcium lignosulfonate modified cement under the influence of mining disturbance. Uniaxial compression tests of calcium lignosulfonate modified cement with different mass fractions under cyclic loading were carried out to investigate the effect of calcium lignosulfonate on the deformation and failure characteristics of cement stone in mining-disturbed coalbed methane surface wells and its modification mechanism. The results show that the P-wave velocity and peak stress of cement stone increase first and then decrease with the increase of calcium lignosulfonate mass fraction, while the total acoustic emission ringing counts of cement decreases first and then increases with the increase of calcium lignosulfonate mass fraction. With the increase of the uniaxial cycle steps, the deformation modulus of cement stone shows a strengthening phenomenon, and the initial loading and unloading cycle has the most significant strengthening effect on the deformation modulus. With the addition of calcium lignosulfonate, the modified cement stone shows a tensile-dominated → shear-dominated → tensile-dominated combined damage modes. Besides, the fractal dimension of modified cement stone shows a trend of decrease before increase, indicating that the addition of the appropriate amount of calcium lignosulfonate can effectively improve the damage resistance of cement stone under uniaxial cyclic loading. With the increase of the mass fraction of calcium lignosulfonate, the pores between cement hydration products show a trend of decrease first and then increase. The addition of an appropriate amount of calcium lignosulfonate can promote the formation of abundant C−S−H gel and ettringite in cement, and the precipitates interweave on the surface of cement particles, which can significantly improve the peak stress of cement stone, and play a positive role in improving the mechanical properties of cement stone. Additionally, the porosity of cement stone decreases, which leads to the increase in the longitudinal wave velocity of cement stone and the decrease of cumulative acoustic emission ringing counts of the cement stone during cyclic loading. However, when the calcium lignosulfonate is excessively added, the air entraining and electrical repulsion of calcium lignosulfonate play a dominant role in the hydration process, which will introduce more bubbles, resulting in the occurrence of the gap between the cement particles, and the inhibition of the early formation of C−S−H gel and ettringite, which has a negative impact on the mechanical properties of cement stone. Furthermore, the porosity of cement stone increases, which leads to the decrease of the longitudinal wave velocity of cement stone and the increase of the cumulative acoustic emission ringing count of cement stone in the process of cyclic loading. Therefore, the influence of calcium lignosulfonate on the mechanical properties of cement stone has a double effect

Influence of coupled effect among flaw parameters on strength characteristic of precracked specimen: Application of response surface methodology and fractal method

Hydraulic slotting is an effective method for enhanced coalbed methane (ECBM) recovery, and it has been widely employed in China. Although there have been many studies of this technique, the influence of slot parameters on the strength characteristic of the coal, which is an important factor that affects the permeability enhancement effect, has rarely been studied. Thus, only limited information is available regarding the pressure relief and permeability enhancement mechanisms of this technique. In the current study, the influence of flaw parameters on the compressive strength of a precracked sample under biaxial compression is discussed. The results indicate that an increase in the flaw length and width has a negative effect on the compressive strength, whereas an increase in the flaw inclination angle has a positive effect on the compressive strength. The results of the response surface methodology (RSM) indicate that the interactions among the flaw parameters have a significant influence on the compressive strength. The propagation patterns of cracks are quantified by the fractal dimension, which is used to explore the mechanism of compressive strength variation with changes in the flaw parameters. The study results indicate that the variation in the flaw parameters changes the propagation pattern of cracks, resulting in different compressive strengths. In addition, an opposite variation trend of the compressive strength and fractal dimension with flaw parameters is also observed. The research results are expected to guide the field application of hydraulic slotting

Novel integrated techniques of drilling-slotting-separation-sealing for enhanced coal bed methane recovery in underground coal mines

Coal bed Methane (CBM), a primary component of natural gas, is a relatively clean source of energy. Nevertheless, the impact of considerable coal mine methane emission on climate change in China has gained an increasing attention as coal production has powered the country's economic development. It is well-known that coal bed methane is a typical greenhouse gas, the greenhouse effect index of which is 30 times larger than that of carbon dioxide. Besides, gas disasters such as gas explosive and outburst, etc. pose a great threat to the safety of miners. Therefore, measures must be taken to capture coal mine methane before mining. This helps to enhance safety during mining and extract an environmentally friendly gas as well. However, as a majority of coal seams in China have low-permeability, it is difficult to achieve efficient methane drainage. Enhancing coal permeability is a good choice for high-efficiency drainage of coal mine methane. In this paper, a modified coal-methane co-exploitation model was established and a combination of drilling–slotting-separation–sealing was proposed to enhance coal permeability and CBM recovery. Firstly, rapid drilling assisted by water-jet and significant permeability enhancement via pressure relief were investigated, guiding the fracture network formation around borehole for high efficient gas flow. Secondly, based on the principle of swirl separation, the coal–water–gas separation instrument was developed to eliminate the risk of gas accumulation during slotting and reduce the gas emission from the ventilation air. Thirdly, to improve the performance of sealing material, we developed a novel cement-based composite sealing material based on the microcapsule technique. Additionally, a novel sealing–isolation combination technique was also proposed. Results of field test indicate that gas concentration in slotted boreholes is 1.05–1.91 times higher than that in conventional boreholes. Thus, the proposed novel integrated techniques achieve the goal of high-efficiency coal bed methane recovery

A prediction model for the slot depth of high pressure water jet

Coal mines in China are characterized by low permeability, and hydraulic slotting is an effective technique to increase the gas permeability of coal. However, the slot depth cannot be accurately determined, which results in the layout parameters of the slotted borehole being empirically designed. In this paper, based on the law of momentum conservation, the velocity distribution law at the cross section of the water jet was derived. Based on the linear relationship between water jet impact power and the erosion volume, the law of pressure attenuation of water jet and Mohr–Coulomb criterion, changes of the slot depth with time and the influence range of water jet slot is confirmed. The maximum slot depth prediction model is also established. The results of field test show that the maximum error between the theoretical calculation value and the measured value is less than 10%, which better validates the prediction model of the slot depth. Keywords: Gas drainage, Hydraulic slotting, Impact power, Slot dept

Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered

Seepage and Damage Evolution Characteristics of Gas-Bearing Coal under Different Cyclic Loading–Unloading Stress Paths

The mechanical properties and seepage characteristics of gas-bearing coal evolve with changes in the loading pattern, which could reveal the evolution of permeability in a protected coal seam and allow gas extraction engineering work to be designed by using the effect of mining multiple protective seams. Tests on gas seepage in raw coal under three paths (stepped-cyclic, stepped-increasing-cyclic, and crossed-cyclic loading and unloading) were carried out with a seepage tester under triaxial stress conditions. The permeability was subjected to the dual influence of stress and damage accumulation. After being subjected to stress unloading and loading, the permeability of coal samples gradually decreased and the permeability did not increase before the stress exceeded the yield stage of the coal samples. The mining-enhanced permeability of the coal samples in the loading stage showed a three-phase increase with the growth of stress and the number of cycles and exhibited an N-shaped increase under the stepped-cyclic loading while it linearly increased under the other two paths in the unloading stage. With the increase of peak stress and the accumulation of damage in coal samples, the sensitivity of the permeability of coal samples to stress gradually declined. The relationship between the damage variable and the number of cycles conformed to the Boltzmann function

Influence of nanosized magnesia on the hydration of borehole-sealing cements prepared using different methods

Abstract Gas drainage is an effective technology for gas control in coal mines. A high borehole-sealing quality is the fundamental precondition for efficient gas drainage. The expansibilities of cement pastes used in borehole-sealing processes are critical for the borehole-sealing effect. Nanosized magnesia expansive agents are used to improve the expansibilities of cement pastes and improve the borehole-sealing effect. Nuclear magnetic resonance spectrometry and scanning electron microscopy were adopted to study the effects of nanosized magnesia on the hydration of borehole-sealing cements used with different preparation methods. The results showed that an increase in the mass fraction of the nanosized magnesia promoted cement hydration, and the mass fraction was positively correlated with the promotion effect. The use of different preparation methods did not change the water-phase distribution in the cement. When using the wet-mixing preparation method, nanosized magnesia promoted the induction, acceleration, and deceleration periods of hydration; when using the dry-mixing preparation method, the nanosized magnesia promoted the induction period of cement hydration, and the promotion effect was less obvious than that seen when using the wet-mixing method. When using the wet-mixing preparation method, the nanosized magnesia was uniformly dispersed, thus enlarging the surface area of the reaction, which provided more nucleation sites for the hydration products of the cement and therefore accelerated the hydration reaction. When using the dry-mixing preparation method, the nanosized magnesia powders were dispersed nonuniformly and aggregated. Under these conditions, only a few nanosized magnesia particles on the surfaces of the aggregated clusters took part in hydration, so only a small number of nucleation sites were provided for the hydration products of cement. This led to inconsistent hydration of cement pastes prepared using the dry-mixing method. The surface porosity of the cement prepared with the wet-mixing preparation method first decreased and then increased with increases in the mass fraction of the nanosized magnesia. The cement surface exhibited compact hydration products and few pores, and the surface was relatively smooth. In comparison, the surface porosity of the cement prepared using the dry-mixing method fluctuated with increasing mass fraction of the nanosized magnesia, resulting in a rough cement surface and microfractures on some surfaces. The two preparation methods both reduced the surface porosity of the cement. The wet-mixing preparation was more effective and consistent in improving the compactness of the cement than the dry-mixing preparation. These results provide important guidance on the addition of nanosized magnesia in borehole-sealing engineering and the selection of cement preparation methods, and they also lay a solid foundation for realizing safe and efficient gas drainage