Coal Seam Drilling Research Articles

Optimization and Practice of a High-Strength Acoustic Wave Indirect Penetration Enhancement Scheme for the Drilling of Structural Coal Seams

The structural coal seam drilling process often faces challenges such as shallow drilling depth, low hole formation rate, and the presence of blind areas in gas control. To address these issues, this study proposes a novel high-strength acoustic penetration approach and optimization design method under in situ conditions. Field tests were conducted at the Yunnan Bailongshan Coal Mine and Huainan Xieqiao Coal Mine to evaluate the effectiveness of this technique. The results demonstrate that the coal seam or its roof can act as an acoustic energy converter to generate high-intensity acoustic waves that penetrate the coal seam, and the field test results confirm the efficacy of this method in increasing gas extraction. This study proposes a novel ‘hole replaces seam’ technique, optimizing the extraction process and reducing the risk of explosions and providing a more efficient and safer method for gas control in structural coal seams. Accordingly, a new technical method for replacing the bottom (top) extraction lane is proposed.

Investigation of Coal Structure and Its Differential Pore–Fracture Response Mechanisms in the Changning Block

The deep coal seams in the southern Sichuan region contain abundant coalbed methane resources. Determining the characteristics and distribution patterns of coal structures in this study area, and analyzing their impact on pore and fracture structures within coal reservoirs, holds substantial theoretical and practical significance for advancing coal structure characterization methods and the efficient development of deep coalbed methane resources. This paper quantitatively characterizes coal structures through coal core observations utilizing the Geological Strength Index (GSI) and integrates logging responses from different coal structures to develop a quantitative coal structure characterization model based on logging curves. This model predicts the spatial distribution of coal structures, while nitrogen adsorption data are used to analyze the development of pores and fractures in different coal structures, providing a quantitative theoretical basis for accurately characterizing deep coal seam features. Results indicate that density, gamma, acoustic, and caliper logging are particularly sensitive to coal structure variations and that performing multiple linear regression on logging data significantly enhances the accuracy of coal structure identification. According to the model proposed in this paper, primary-fragmented structures dominate the main coal seams in the study area, followed by fragmented structures. Micropores and small pores predominantly contribute to the volume and specific surface area of the coal samples, with both pore volume and specific surface area increasing alongside the degree of coal fragmentation. Additionally, the fragmentation of coal structures generates more micropores, enhancing pore volume and suggesting that tectonic coal has a greater adsorption capacity. This study combines theoretical analysis with experimental findings to construct a coal structure characterization model for deep coal seams, refining the limitations of logging techniques in accurately representing deep coal structures. This research provides theoretical and practical value for coal seam drilling, fracturing, and reservoir evaluation in the southern Sichuan region.

Research on trajectory measurement strategy while drilling based on screen lowering process

Abstract In view of the soft coal seam drilling construction of small and medium-sized rotary drilling rigs in coal mines, the soft coal seam is usually used to support the hole wall due to the loose coal quality. The traditional wireless storage measurement trajectory instrument has a large outer diameter, which cannot realize the measurement while drilling in the drilling construction while the screen is lowered. In this paper, a reduced storage borehole trajectory measuring instrument is proposed, and the outer diameter and length of the probe tube are improved to be 20 mm and 692 mm. A fixed support structure for the probe pipe is designed, which can be applied to the drill pipe with an outer diameter of φ≥73mm, which is convenient for the measurement of the trajectory while drilling of the soft coal seam in the construction of the screen pipe. The industrial test shows that the measurement accuracy of the inclinometer is ± 0.2° in azimuth and plus or minus 0.1° in inclination, which realizes the process requirements of screen lowering without redrilling measurement.

Mechanism and application of drill pipe bending induced borehole collapse in soft coal seam drilling

AbstractGas extraction drilling is a necessary measure for managing gas hazards. For soft coal seams where gas extraction drilling holes are prone to collapse, it is believed that drill rod disturbance is the main cause of hole collapse. This study proposes a research approach to reduce wall stress by optimizing the drill rod structure. Through theoretical analysis, numerical simulation, and industrial tests, a stress model for the drill rod inside the hole was established, and a wall stress equation was derived. The effects of various parameters on wall stress were analyzed. The study suggests optimizing the drill rod structure to reduce the disturbance‐induced wall stress. SolidWorks was used for drilling stress simulation, and a four‐winged concave groove drill rod was developed. After strength verification, comparative industrial tests were conducted. The research results show that as the line density increases, the wall stress of the drilling hole increases. As the length of the suspended section increases, the wall stress initially decreases and then increases. With increasing drilling thrust, wall stress increases linearly, and the growth rate is greater with a larger diameter difference between the drill hole and the drill rod. Numerical simulation results indicate that the critical point maximum stress at the hole entrance, the critical point maximum stress at the hole bottom, and the average stress at the bottom section of the four‐winged concave groove drill rod with a concavity of 5 are significantly reduced compared to those of circular and grooved drill rods. Industrial test results show that using the four‐winged concave groove drill rod significantly reduces the extent of hole collapse. This study provides a reference for addressing the issue of hole collapse in gas extraction drilling for soft coal seams.

Dynamic gas emission during coal seam drilling under the thermo-hydro-mechanical coupling effect: A theoretical model and numerical simulations

Dynamic gas emission during coal seam drilling under the thermo-hydro-mechanical coupling effect: A theoretical model and numerical simulations

Theoretical study on the critical index of rock burst stress monitoring in coal seam drilling

Rock burst disasters severely restrict the safe and efficient mining of coal. The fundamental cause of their occurrence is the concentration of stress within the coal mass. Stress monitoring in coal seam drilling is widely used as an effective method for rock burst monitoring. However, how to scientifically and reasonably set the critical values of early warning indicators that match the conditions of each mine has always been a key issue restricting the accurate prediction of rock burst by the drilling stress method. This paper adopts a method combining theoretical analysis and field practice to conduct research on the critical values of drilling stress early warning indicators. Based on perturbation response instability theory, a mechanical model for the occurrence of impact ground pressure has been established. Based on the instability theory of disturbance response, a mechanical model for the occurrence of impact ground pressure has been established, leading to the derivation of the expression for the near-field critical stress of impact ground pressure events. The theoretical formula for the critical value of drilling stress early warning indicators was obtained based on the difference between the critical stress of rock burst occurrence and the actual stress in the roadway. This formula includes the mechanical parameters of the coal mass and its propensity for rock burst, roadway support stress, mining depth, stress concentration coefficient, and the initial installation pressure of the stress gauge. They can be determined by the geological and mining technical conditions of each mine. This theoretical formula breaks the uniformity of the critical values for stress warning indicators in various mine drill holes, allowing each mine to scientifically determine its critical value based on its own conditions. This theoretical method has been applied to a high-stress mine in Shanxi, China, and the critical values of drilling stress early warning indicators were obtained. When the monitored stress exceeded the critical value, dynamic phenomena of anchor rod and cable fractures occurred in the roadway roof. The distribution of microseismic events also shifted towards the warning area, and the microseismic monitoring indicators reached the warning values. This confirmed the engineering feasibility of the critical values for drilling stress early warning indicators determined by the theoretical method.

An investigation into mining coal losses at Klipspruit Colliery

Klipspruit Colliery is experiencing coal losses, which leads to a negative impact on coal production and subsequently loss in revenue. This study aimed to identify the possible causes of coal losses in the No. 4 upper and lower seams. The objectives were to identify potential sources of coal losses through a literature survey, to identify areas where the coal losses occurred, to determine factors affecting coal losses, and to identify the most cost-effective methods to reduce coal losses. Highwall losses, top-of-coal losses, and coal edge losses were identified as the major areas of coal loss at Klipspruit. These losses were mostly attributed to the drilling and blasting practises of the pre-split, interburden waste, and the coal seams. Recommendations include re-visiting the current pre-split design and the tolerances adopted for the interburden and coal seam drilling and blasting design. It is furthermore recommended that Klipspruit implement a continuous improvement plan for the drilling and blasting and refresher training for the blasting crew and operators.

Optimization Analysis of Key Parameters of Gas Drainage Drilling in Coal Seams Based on Screw Drills

Optimization Analysis of Key Parameters of Gas Drainage Drilling in Coal Seams Based on Screw Drills

Quantitative Study of the Weakening Effect of Drilling on the Physical and Mechanical Properties of Coal-Rock Materials.

Coal seam drilling is a simple, economical, and effective measure commonly used to prevent and control rock burst. Following rock burst, coal exhibits significant dynamic characteristics under high strain-rate loading. Our purpose was to determine the physical processes associated with impact damage to drilled coal rock, and its mitigation mechanism. An impact test was carried out on prefabricated borehole coal specimens, and the impulse signals of the incident and transmission rods were monitored. The crack initiation, expansion, and penetration of coal specimens were video-recorded to determine the mechanical properties, crack expansion, damage modes, fragmentation, and energy dissipation characteristics of coal specimens containing different boreholes. The dynamic compressive strength of the coal specimens was significantly weakened by boreholes under high strain-rate loading; the dynamic compressive strength and the dynamic modulus of elasticity of coal rock showed a decreasing trend, with increasing numbers of boreholes and a rising and decreasing trend with increasing borehole spacing; the number and spacing of boreholes appeared to be design parameters that could weaken coal-rock material under high strain-rate loading; during the loading of coal and rock, initial cracks appeared and expanded in the tensile stress zone of the borehole side, while secondary cracks, which appeared perpendicular to the main crack, expanded and connected, destroying the specimen. As the number of boreholes increased, the fractal dimension (D) and transmission energy decreased, while the reflection energy increased. As the borehole spacing was increased, D decreased while the reflective energy ratio decreased and increased, and the transmissive energy ratio increased and decreased. Drilling under high strain modifies the mechanical properties of impact damaged coal rock.

Discrete Element Simulation of Fracture Evolution around a Borehole for Gas Extraction in Coal Seams.

The stability analysis of underground caverns, tunnels, and boreholes is of great significance to underground engineering. In order to study the stress change of the coal around a hole and the evolution law of fractures during the coal seam drilling process, the discrete element simulation of the coal seam drilling process was carried out by using the particle flow code (PFC2D). The results show that during the drilling process, under the influence of confining pressure and drilling disturbance, the coal stress field around the hole and the development of fractures around the hole have the characteristics of zoning and dynamic evolution. In the axial direction of the borehole, it is divided into front and rear areas, and in the vertical axial direction, it is divided into the drilling disturbance zone and the confining pressure main control zone. During the drilling process, the direction of the maximum principal stress in the front zone gradually changes from the vertical hole axis to the direction parallel to the hole axis, and tension fractures are mainly developed along the drilling direction. In the rear zone, the principal stress direction tends to be stable and the principal stress value undergoes dynamic changes, and a large number of vertical hole axis tension fractures are developed. The drilling disturbance zone appears near the hole wall and has an important influence on the stability of the hole wall, while the confining pressure main control zone determines the antireflection effect around the hole and the influence radius of the hole. This work helps the understanding of the damage range and failure characteristics of the surrounding rock during the drilling process and has great significance for the guidance of drilling design.

Blasting Law of Liquid CO2 Phase Change in Coal Mine Based on Numerical Simulation.

In order to obtain the best CO2 blasting drilling efficiency and improve the gas permeability coefficient of the coal seam in the mining area, a research method of liquid CO2 phase change blasting in the coal seam based on electrochemical numerical simulation is proposed. In this paper, the electrochemical numerical simulation of coalbed methane caused by liquid CO2 phase change blasting is studied through theoretical analysis, the antireflection mechanism of liquid carbon dioxide gas explosion on broken coal seam is obtained, and the initial fracture length of coal body caused by gas explosion stress wave is deduced by the mathematical model. A method for improving CBM desorption with nonmechanical coal was proposed, namely, a method for electrochemically strengthening CBM desorption. A comprehensive theory and application system of liquid carbon dioxide phase change gas explosion and anti-reflection technology are established by combining theoretical analysis, experimental research, and numerical simulation with field industrial comparative experiment. According to the gas tracing method, the precise measurement of the working face shows that the impact radius of the collision caused by the explosion of the liquid carbon dioxide phase change gas is 2 m. Gas emissions increased the emissions of coal seam drilling in the affected areas by 4 to 8 times, and the gas emission attenuation coefficient decreased from 0.76 times to 0.93 times. The carbon dioxide phase change gas cracking theory and antireflection theory and their application technology system are systematically studied. The research shows that the liquid carbon dioxide phase transfer gas blasting and infiltration technology is not limited by the geological conditions of the coal seam and has the characteristics of high efficiency and inherent climate. In view of the wide area of coal mines and the harsh geological conditions of coal seams, carbon dioxide phase change gas-induced cracking antireflection technology has application prospects and expandability.

The Influence of a Drilled Hole on the Deformation Characteristics and Burst Proneness of Coal Samples

Abstract Borehole drilling in a coal seam is an efficient way to relieve ground stress and prevent coal burst. The deformational behavior and failure mechanism of a Φ50mm×L100mm coal sample with a 2–4 mm diameter drilling hole were studied under standard burst proneness laboratory testing. The results show that with the increase in borehole diameter, the uniaxial compressive strength (RC), impact energy index (KE), and elastic energy index (WET) decrease, and the dynamic failure time (DT) is prolonged. The overall burst proneness of the seam changes from strong to weak for a 4 mm hole sample. A high speed camera and acoustic emission (AE) monitor were used to study the deformation procedure and failure mode of the samples. It is found that cracks are propagated around the drilled hole at the initial stage of the loading, and the AE event and energy are weakened around the peak load. This suggests that the hole may significantly reduce the brittleness of the sample. The numerical method is employed to provide further insights on the internal deformation characteristics; the effect of hole sizes with diameters of 2–10 mm is also discussed. This paper provides quantified analysis methodology, monitoring technology, and borehole optimization for pressure relief drilling and burst proneness reduction in high coal burst-prone seams.

CFD-DEM Simulation of Reverse Circulation Pneumatic Cuttings Removal during Coal Seam Drilling

To solve the problems that the borehole depth is shallow and the borehole formation rate is low during the gas drainage drilling in soft coal seam with current cuttings removal method, a new technology of reverse circulation pneumatic cuttings removal is put forward. The CFD-DEM coupling method is used to establish the simulation model of cuttings-air two-phase flow in drill pipe. The effects of the air velocity for cuttings removal and the mass flow rate of cuttings on the flow characteristics, cuttings removal effect and pressure drop of cuttings-gas two-phase flow are analysed. The results show that the drag force of drilling cuttings becomes larger with the increase of air velocity and the stratified flow characteristic is obvious. The drill cuttings migration ratio is positively correlated with the air velocity for cuttings removal and negatively correlated with the mass flow rate of cuttings. When the mass flow rate of cuttings is constant, the increase of air velocity for cuttings removal leads to the increase of pressure drop in the inner hole of drill pipe. When the air velocity of cuttings removal is constant, the mass flow rate of cuttings and the pressure drop in the inner hole of drill pipe increases. Therefore, the appropriate air velocity should be selected considering the energy consumption during cuttings removal.

Experimental study on the law of CO production in coal seam drilling

Experimental study on the law of CO production in coal seam drilling

Pressure drop characteristics of reverse circulation pneumatic cuttings removal during coal seam drilling.

To solve the problems that the borehole depth is shallow and the drilling efficiency is low during the gas drainage drilling in soft coal seam with current cuttings removal method, a new technology of reverse circulation pneumatic cuttings removal is proposed. The working principle of reverse circulation pneumatic cuttings removal is analyzed, and the kinetic equation of cuttings in the inner hole of the drill pipe is established. Through experiments, the pressure drop in the drill pipe is measured to reveal the effects of air velocity, cuttings mass flow rate, and cuttings particle size on the pressure drop in inner hole of the drill pipe. When the cuttings mass flow rate is constant, the pressure drop increases with the increase in air velocity. When the air velocity is constant, the pressure drop increases with the increase in cuttings mass flow rate. At low air velocity, the pressure drop of cuttings is primary. As the air velocity increases, the pressure drop ratio of cuttings decreases. Under the same conditions, the order of pressure drop with different particle size cuttings is coarse cuttings > medium cuttings > fine cuttings. Empirical equation of pressure drop coefficient of cuttings is established, which is in good agreement with the actual data.

Time Determination of Gas Extraction in Coal Seam Drilling

In order to determine the time of the gas extraction of the solid unstable coal seam, based on the characteristics of the change of gas concentration under the condition of continuous extraction of high negative pressure, the drilling gas parameters were tested on the spot and the gas concentration attenuation curve was drawn, and the law of attenuation of the drilling gas concentration was analyzed comprehensively using gas flow theory and geomechanics-related theory. The results show that the interval between the detection and drainage is 0 to 60d, the time of the extraction is 3d, the time of extraction is 60 to 120d, the time of the water testing interval is 7d, the extraction time is 120 to 180d, the time of the water testing interval is 15d, and the extraction time is 180 to 300d. The interval between the measurement of the flow is 30d.

Research Advances of Prevention and Control of Hydrogen Sulfide in Coal Mines.

Sudden emission and casualty accidents caused by abnormal enrichment of hydrogen sulfide (H2S) in coal mines are becoming frequent increasingly, causing major casualties and environmental pollution. Scholars in various countries have developed various measuring devices for hydrogen sulfide content in coal and rock formations and their calculation methods. The existing prevention and control technologies of H2S in coal mines were summarized in various countries. According to the distribution characteristics, occurrence modes, and emission forms of H2S in coal mines, the prevention and control technologies of H2S in coal-bearing strata, airflow in tunnel, and underground water body are mainly introduced. Analyzed the effects of different ventilation systems on prevention and control of H2S, which include conventional ventilation system, partial homotropal ventilation system, and full homotropal ventilation system. The methods used mainly include neutralization by injecting alkalizer through drilling in coal seams with high pressure, spraying alkalizer in tunnel, attenuation by increasing wind amount, changing the ventilation method, pumping, dredging, and blocking the water that contains H2S as well as comprehensive prevention and control method. The basic agents adopted mainly include sodium carbonate (the mass percentage concentration is about 0.5% ~ 3.0%) and sodium bicarbonate solution, and some basic solution is added by an additive, such as surfactant, Fenton reagent, sodium dodecyl benzene sulfonate, sodium hypochlorite, or chloramine-T. The treatment effect and the main problems of each prevention and control technology are analyzed, and a comprehensive method of prevention and control techniques of H2S in coal mines is proposed. According to current technological level as well as the cost, the effective prevention and control techniques of H2S should take the occurrence, distribution, and emission forms of H2S in coal mines as well as the content into consideration.

Analysis of drilling failure in soft and hard sandwiching of coal seam

Abstract The severe abrasion of drill bits will be confronted during drilling in soft and hard sandwiching of coal seam. Not only does this failure result in the increasing consumption of bits, but also it delays the construction period, significantly extending the duration of the delay, even causing borehole instability, resulting in a catastrophic buried well accident. In order to reveal the failure causes, the authors of this paper researched the wear properties of the tooth of bit. As the thermal damage is one of the main threats to drilling failure in soft and hard sandwiching, thermal related effects on the wear performance of polycrystalline diamond compacts (PCD) were studied by an Amsler friction and wear testing machine. Meanwhile, silica sands were added to the interface of wear couples throw a funnel instrument to meet the demand that wear conditions correspond more closely to those in actual coal seam drilling. The friction coefficient and wear rates was measured. The X-ray diffraction analysis was used to investigate the chemical composition and crystal structure of the PCDs soaked at different temperatures. The morphologies of the surface of PCDs were examined by metalloscope and scanning electron microscopy. The results show that the wear rate and wear coefficient all rose with the increase of wear load. So the drilling load over proper operation limits especially when the equipment stuck happened is the primary threat to bit failure. On the other hand, the wear coefficient is not sensitive to the soaking temperature when it is under 700 °C. But it decreases obviously when the temperature was over 800 °C, which seems to be corresponding to the graphitization of diamond and the oxidation of Co. This phenomenon indicates that the high temperature over 700 °C is the main cause of the drilling failure.

An elastoplastic model of collapse pressure for deep coal seam drilling based on Hoek–Brown criterion related to drilling fluid loss to reservoir

Abstract Too high borehole pressure can lead to the drilling fluid loss to the coalbed methane (CBM) reservoir, and then cause reservoir damage. In CBM rich regions of China, the drilling fluid loss caused by higher borehole pressure is a particularly prominent problem because the target coal seam is deep and soft. Therefore, selecting an appropriate drilling fluid density or borehole pressure is the most effective way to both avoid drilling fluid loss and maintain wellbore stability in these regions, which means that the collapse pressure must be calculated accurately enough. In this paper, the elasto-plastic characteristic of the deep coal seam under high confining pressures at X region in Qinshui basin is determined based on the conventional triaxial compression test, and the mechanical parameters related to solve the collapse pressure are obtained. Then, selecting the Hoek–Brown strength criterion as the limit equilibrium condition, an elasto-plastic model of collapse pressure for deep coal seam drilling is derived to relieve the problem of drilling fluid loss to formation, considering the radius of plastic zone, the geological strength index (GSI), the non-uniform in-situ stress coefficient and the unconfined compression strength of the intact rock. Taking CBM X–X1 well in X region as an example, the elasto-plastic results of the collapse pressure are calculated and compared with elastic results, and the relations between the collapse pressure and the influence factors are analyzed. The results show that the elasto-plastic results of the collapse pressure decrease by 6.64 % than the elastic results calculated by Hoek–Brown criterion when the radius of plastic zone is 1.5 times of borehole radius; the elasto-plastic results of the collapse pressure decrease with the increasing of the radius of plastic zone and the non-uniform stress coefficient; the collapse pressure presents the variation trend of the minus power function as the GSI increasing.

Polymer Drilling Fluid with Micron-Grade Cenosphere for Deep Coal Seam

Traditional shallow coal seam uses clean water, solid-free system, and foam system as drilling fluid, while they are not suitable for deep coal seam drilling due to mismatching density, insufficient bearing capacity, and poor reservoir protection effect. According to the existing problems of drilling fluid, micron-grade cenosphere with high bearing capacity and ultralow true density is selected as density regulator; it, together with polymer “XC + CMC” and some other auxiliary agents, is jointly used to build micron-grade polymer drilling fluid with cenosphere which is suitable for deep coal seam. Basic performance test shows that the drilling fluid has good rheological property, low filtration loss, good density adjustability, shear thinning, and thixotropy; besides, drilling fluid flow is in line with the power law rheological model. Compared with traditional drilling fluid, dispersion stability basically does not change within 26 h; settlement stability evaluated with two methods only shows a small amount of change; permeability recovery rate evaluated with Qinshui Basin deep coal seam core exceeds 80%. Polymer drilling fluid with cenosphere provides a new thought to solve the problem of drilling fluid density and pressure for deep coal seam drilling and also effectively improves the performance of reservoir protection ability.