Technology and application of pressure relief and permeability increase by jointly drilling and slotting coal

Difficulties with soft coal seams having a high gas content and high stress levels can be addressed by a technology of pressure relief and permeability increase. Slotting the seam by auxiliary drilling with a water jet that breaks the coal and slots the coal seam during the process of retreat drilling achieves pressure relief and permeability increase. Improved efficiency of gas extraction from a field test, high gas coal seam was observed. Investigating the theory of pressure relief and permeability increase required analyzing the characteristics of the double power slotting process and the effects of coal pressure relief and permeability increase. The influence of confining pressure on coal physical properties was examined by using FLAC3D software code to simulate changes of coal stress within the tool destruction area. The double power joint drilling method was modeled. Field experiments were performed and the effects are analyzed. This research shows that there is an “islanding effect” in front of the joint double power drill and slotting equipment. The failure strength of the coal seam is substantially reduced within the tool destruction area. Drilling depths are increased by 72% and the diameter of the borehole is increased by 30%. The amount of powdered coal extracted from the drill head increases by 17 times when using the new method. A 30 day total flow measurement from the double power drilled and slotted bores showed that gas extraction increased by 1.3 times compared to the standard drilled bores. Gas concentrations increased from 30% to 60% and were more stable so the overall extraction efficiency increased by a factor of two times.     

Induced drill-spray during hydraulic slotting of a coal seam and its influence on gas extraction

Hydraulic slotting can induce drill spray in a gassy, low permeability coal seam. This then influences subsequent gas extraction. This paper describes the drill spray phenomenon from a mechanical perspective and analyzes the effects of water jet damage during slotting. A simulation of the stresses around the drill hole and slot was prepared using FLAC-3D code. It helps explain the induction of drill spray during hydraulic slotting. The stress concentration around the bore increases as the diameter of the hole increases. As the hole enlarges the variation in stress also increases, which introduces an instability into the coal. This allows easy breaking and removal of the coal. Destruction of the coal structure by the water jet is the major factor causing drill spray. Energy stored as either strain or gas pressure is released by the water jet and this causes the coal to fracture and be expelled from the hole. Field tests showed the effect on gas extraction after slotting with drill spray. The concentration of gas increases after drilling. Compared to conventional techniques, the hydraulic slotted bore gives a gas concentration three times higher and has an effective range twice as far. This makes the gas extraction process more efficient and allows reduced construction effort.     

超高压水力割缝强化抽采瓦斯技术研究

水力割缝是一种重要的强化瓦斯抽采增透技术,现已开始在低透气性突出煤层应用。为了进一步考察其实际效果,选取新集二矿1煤组220112工作面底抽巷实施了100 MPa超高压水力割缝试验。试验结果表明:割缝后,瓦斯抽采纯量平均0.77 m3/min,是未割缝钻孔的瓦斯抽采纯量(0.34 m~3/min)的2.26倍;1煤层组瓦斯抽采钻孔抽采30、60天的抽采有效半径为5 m、7.5 m,极限抽采半径为8 m,相比水力冲孔、未割缝钻孔抽采有效半径显著增加,超高压水力割缝强化抽采瓦斯技术具有广泛的应用前景。     

煤层群煤与瓦斯安全高效共采体系及应用

提出了高瓦斯煤层群煤与瓦斯安全高效共采的概念：在煤层群开采条件下，首先开采瓦斯含量低、无突出危险的首采煤层，利用其采动影响使处在其上部和下部的煤层卸压，煤层透气性成百倍地增加，从而形成高效的瓦斯抽采条件．同时进行的卸压瓦斯高效抽采既解决了由卸压煤层向首采煤层涌出瓦斯问题，保障首采煤层实现安全高效开采，又大幅度地降低了卸压煤层的瓦斯含量，消除了煤与瓦斯突出危险性，为在卸压煤层内实施快速掘进与高效采煤方法提供了安全保障，从而实现了瓦斯与煤炭两种资源的安全高效共采．文中介绍了针对不同卸压瓦斯流动特点的近程、中程和远程卸压瓦斯抽采方法及工程应用实践，最后对高瓦斯煤层群煤与瓦斯安全高效共采体系的应用前景进行了分析．     

Progress on the hydraulic measures for grid slotting and fracking to enhance coal seam permeability

A method of hydraulic grid slotting and hydraulic fracturing was proposed to enhance the permeability of low permeability coal seam in China. Micro-structural development and strength characteristics of coal were analysed to set up the failure criterion of coal containing water and gas, which could describe the destruction rule of coal containing gas under the hydraulic measures more accurately. Based on the theory of transient flow and fluid grid, the numerical calculation model of turbulence formed by high pressure oscillating water jet was used. With the high speed photography test, dynamic evolution and pulsation characteristics of water jet water analysed which laid a foundation for mechanism analysis of rock damage under water jet. Wave equation of oscillating water jet slotting was established and the mechanism of coal damage by the impact stress wave under oscillation jet was revealed. These provide a new method to study the mechanism of porosity and crack damage under high pressure jet.Fracture criterion by jet slotting was established and mechanism of crack development controlled by crack zone between slots was found. The fractures were induced to extend along pre-set direction,instead of being controlled by original stress field. The model of gas migration through coal seams after the hydraulic measures for grid slotting and fracking was established. The key technology and equipment for grid slotting and fracking with high-pressure oscillating jet were developed and applied to coal mines in Chongqing and Henan in China. The results show that the gas permeability of coal seam is enhanced by three orders of magnitude, efficiency of roadway excavation and mining is improved by more than 57%and the cost of gas control is reduced by 50%.     

Principle and engineering application of pressure relief gas drainage in low permeability outburst coal seam

With the increase in mining depth, the danger of coal and gas outbursts increases. In order to drain coal gas effectively and to eliminate the risk of coal and gas outbursts, we used a specific number of penetration boreholes for draining of pressure relief gas. Based on the principle of overlying strata movement, deformation and pressure relief, a good effect of gas drainage was obtained. The practice in the Panyi coal mine has shown that, after mining the Cllcoal seam as the protective layer, the relative expansion deformation value of the protected layer C13 reached 2.63%, The permeability coefficient increased 2880 times, the gas drainage rate of the C13 coal seam increased to more than 60%, the amount of gas was reduced from 13.0 to 5.2 m3/t and the gas pressure declined from 4.4 to 0.4 MPa, which caused the danger the outbursts in the coal seams to be eliminated. The result was that we achieved a safe and highly efficient mining operation of the C 13 coal seam.     

Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region

A pressure relief gas extraction technical model of a typical mining area is proposed based on coal and gas simultaneous extraction theory. Flac3 Dwas employed to model vertical stress and displacement contour plot characteristics of non-outburst coal seam(No. 4) on top of outburst coal seam(No. 2) along strike and incline directions. Field investigations were also conducted to verify the scientific nature of the simulation. The results demonstrate that gas pressure in No. 2 coal seam dropped to approximately 0.55 MPa in the pressure relief multi-coal seam. The highest expansion rate of the coal mine reached up to 2.58%.The pressure-relief angle was 76° along the incline direction and 60° along the strike direction. As the expansion rate and pressure-relief angle increased and the gas pressure decreased, a large amount of gas flowed into the gob of No. 4 from No. 2 coal seam and was later discharged through specific gas pipes,which eliminated No. 2 outburst risks. This study resulted in positive outcomes in that gas extraction time was reduced by 13.5 days, due to pressure relief, and drilling work load was reduced by 0.1161 m/t coal. This method ensures that gas is discharged from the outburst coal seam quickly and safely,demonstrating that the proposed technical model of pressure-relief gas extraction is effective in a multi-coal seam region.     

Thermo-hydro-mechanical coupled mathematical model for controlling the pre-mining coal seam gas extraction with slotted boreholes

Drainage influence radius is the basic parameter for borehole arrangement, while the effect of high pressure water jet slotting technology on borehole drainage influence radius has not been studied systematically. In this paper, a fully thermo-hydro-mechanical(THM) coupled model which represents the non-linear responses of gas extraction was implemented to demonstrate the reliability of this model through history data matching. Based on this model, the susceptibilities of gas extraction with single slotted borehole, including the permeability, the gas pressure, the temperature, the coal adsorption characteristics and the radius of slot, were quantified through a series of simulations. The simulation results revealed that increasing the permeability, initial gas pressure and temperature could develop the influence radius of single slotted borehole. This finite element model and its simulation results can improve the understanding of the coal-gas interactions of underground gas drainage and provide a scientific basis for the optimization of drainage systems.     

Guiding-controlling technology of coal seam hydraulic fracturing fractures extension

Aiming at the uncontrollable problem of extension direction of coal seam hydraulic fracturing, this study analyzed the course of fractures variation around the boreholes in process of hydraulic fracturing, and carried out the numerical simulations to investigate the effect of artificial predetermined fractures on stress distribution around fractured holes. The simulation results show that partial coal mass occurs relatively strong shear failure and forms weak surfaces, and then fractures extended along the desired direction while predetermined fractures changed stress distribution. Directional fracturing makes the fractures link up and the pressure on coal mass is relieved within fractured regions. Combining deep hole controlling blasting with hydraulic fracturing was proposed to realize the extension guiding-controlling technology of coal seam fractures. Industrial experiments prove that this technology can avoid local stress concentration and dramatically widen the pressure relief scope of deep hole controlling blasting. The permeability of fractured coal seam increased significantly, and gas extraction was greatly improved. Besides, regional pressure relief and permeability increase was achieved in this study.     

Switching mechanism and optimisation research on a pressure-attitude adaptive adjusting coal seam water jet slotter

To investigate the attitude-switching mechanisms of existing jet slotters, which integrate drilling, punching and slotting operations, and to improve its fracture ability, we used the power bond diagram theory to analyse the dynamic flow pressure, and force of slotters. A mathematical model was developed for the dynamic characteristics of slotter systems. Furthermore, to study the effect of the main characteristic parameters on the ability of the nozzle to erode sandstone, multi-orthogonal experiments were carried out. And the optimised slots were applied in later practical operations. The research results show that the inlet fluid passed through the time-varying orifice to generate pressure differential thrust, which overcame the spring force, pushed the valve core to open the side nozzle, and closed the rear cavity channel thereby realising the switch of the slotter attitude. An optimal plan was established to balance the diameter, depth, and volume of punching, and a rock-breaking plan was developed for the slotter. Subsequently, the optimised water jet slotter was practically used in coal seam gas drainage. Compared with conventional dense drilling, water jet slotting technology significantly improves the ability, efficiency, and effect of increasing the permeability of the coal seam.     

突出矿井深部新水平开采瓦斯综合治理模式研究

针对矿井浅部瓦斯治理模式已不能保障深部采区安全高效生产的现状,提出一种适宜矿井深部新水平开采的瓦斯综合治理模式.工作面消突采用底板岩巷穿层钻孔预抽煤巷条带瓦斯;底板岩巷布置“一巷多用”,在工作面回采工程中可兼做回风巷、尾抽巷、措施巷;回采工作面采用沿空留巷Y型通风综合治理瓦斯.其中,顺层钻孔预抽本煤层瓦斯,高位钻场顶板走向钻孔抽采裂隙带瓦斯,上隅角、尾巷埋管抽采采空区瓦斯,形成矿井三维立体瓦斯抽采体系.     

Directivity effect of unloading bedding coal induced fracture evolution and its application

In the coal mining process, the gas contained in the coalbed is one source of the most serious accident hazards. Stress releasing from the coal deposit is the main controlling factor that leads to such accidents.Based on the bedding of coal samples, the gas permeability well describes the evolution of fracture, so the paper carries out research on the permeability properties of coal under different unloading directions.The research obtains that when the stress unloading direction is perpendicular to bedding, more penetrating fractures and bedding fractures occur, and the permeability significantly increases. Although the axial stress reduced, the confining pressure makes the permeability of the bedding plane fracture exist under constant. The permeability obtained when the unloading direction was perpendicular to the bedding is 52 times larger than that when it is parallel to the bedding. The results show that the efficiency of gas drainage is impacted by the relative direction of gas drainage drilling in relation to the bedding orientations. The maximum amount of gas extraction when drilling is perpendicular to the bedding is 1.3 times than that when drilling is oblique to the bedding, and 1.75 times than that when drilling is parallel to the bedding.     

Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams

Multiple coal seams widely develop in the deep Chinese coal-bearing strata. Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe. When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams. The safety and economics of exploited firstmined coal seams are tremendously restricted. Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult. Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane(CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs. Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction. In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam. During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods(combination of underground and ground CBM extraction methods). A typical integrated reservoir reconstruction technology, ‘‘One well for triple use", was detailed introduced and successfully applied in the Luling coal mine. The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction.     

Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

A gas–solid coupling model involving coal seam deformation,gas diffusion and seepage,gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining.The research results indicate:(1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase,thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient,which will cause a decrease in gas pressure.(2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam,which can effectively reduce the coal and gas outburst risk.The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.     

高瓦斯综采工作面瓦斯涌出预测与立体防治技术研究

针对高瓦斯低渗透煤层工作面瓦斯抽采与灾害控制难题,以土城矿15311综采工作面为研究对象,首先,初步分析了工作面瓦斯涌出来源,运用分源预测法预测了其瓦斯涌出含量,接着针对性地在3#煤层运用了顺层钻孔、底抽巷穿层钻孔、高位钻场以及采空区埋管等多种抽采方法,并联合工作面配风提出了立体瓦斯防治技术。最后,通过施工底抽巷截留钻孔对底抽巷溢出瓦斯进行截留抽放,考察了抽采效果。结果表明:15311综采工作面瓦斯来源主要为3#煤层和下邻近层,瓦斯抽采总量为45.4 m3/min,瓦斯抽采率为85.33%,回风流中瓦斯浓度未超过1%,瓦斯抽采达标,有效地控制了工作面高瓦斯的涌出。     

低透气煤层预裂瓦斯运移数值模拟及抽采试验

针对高瓦斯低透气性煤层瓦斯抽采难问题,利用数值模拟软件RFPA^2D—Flow再现了采取煤层深孔爆破预裂后,瓦斯在煤层及爆生裂隙中的流动规律。研究结果表明,预裂圈内煤和岩石的孔隙率大大提高,煤层透气性显著增加,但当裂隙圈之间不相交时,瓦斯同样很难在完整的低透气性煤体中运移,因此只有当抽采瓦斯钻孔处在裂隙圈中才能高效抽采瓦斯。现场试验证实,低透气性煤层预裂后,有效导通裂隙增加,布置在裂隙圈内抽采瓦斯钻孔可以获得高效抽采瓦斯效果,从而降低煤与瓦斯突出危险性。     

Fracture evolution and pressure relief gas drainage from distant protected coal seams under an extremely thick key stratum

When an extremely thick rock bed exists above a protected coal seam in the bending zone given the condition of a mining protective seam, this extremely thick rock bed controls the movement of the entire overlying stratum. This extremely thick rock bed, called a "main key stratum", will not subside nor break for a long time, causing lower fractures and bed separations not to close and gas can migrate to the bed separation areas along the fractures. These bed separations become gas enrichment areas. By analyzing the rule of fracture evolution and gas migration under the main key stratum after the deep protective coal seam has been mined, we propose a new gas drainage method which uses bore holes, drilled through rock and coal seams at great depths for draining pressure relief gas. In this method, the bores are located at a high level suction roadway (we can also drill them in the drilling field located high in an air gateway). Given the practice in the Halzi mine, the gas drainage rate can reach 73% in the middie coal group, with a gas drainage radius over 100 m.     

保护层开采卸压增透效应研究及其定量表征方法探索（研究生论坛稿件）

保护层开采是高突矿井瓦斯治理地主要方式及开采方法,保护层开采的卸压增透效果当前无法量化表征的,缺乏评价体系。以平板流体模型增透率理论为基础,结合平煤十矿地质条件,通过离散单元法建立保护层开采数值模型,获得保护层开采不同距离时采空区下方的增透率图谱及被保护层的应力分布。研究结果表明：保护层开采过程中,采面前方15 m处支撑压力会出现应力峰值,随着采面的推进,增透率与现场瓦斯流量呈现正相关关系;在采空区下方,被保护层卸压以及增透效应明显,且卸压区范围与增透率分布集中区域基本一致,均随着采空区范围的扩大而增大。该研究成果证实了增透率理论可以作为保护层开采卸压增透指标,对现场保护层回采设计、瓦斯抽采设计范围的确定提供了更加科学的依据。     

保护层开采上覆煤岩变形移动及瓦斯抽采效果

根据潘三矿东四采区实际开采条件和回采工艺,运用数值模拟和现场试验相结合的方法,分析了近水平煤层保护层开采过程中被保护层应力、变形量、透气性系数在保护层开采过程中的演化机制。结果表明：保护层开采过程中,被保护层存在未受影响区（原始应力区）、增压区、过渡卸压区、稳定卸压区和重新压实区,被保护层边界区域附近过渡卸压区内的透气性系数为原始透气性系数的30倍左右,煤体应力下降,产生了一定的膨胀变形,大大提高了瓦斯抽采效果。     

煤层注水促抽瓦斯及其影响因素的数值模拟

为有效预防煤矿瓦斯灾害,获取煤层注水促抽瓦斯的合理参数,以常村煤矿2103工作面为例,依据多相渗流理论,采用Fluent软件的VOF模型及多孔介质模型耦合求解,对煤层注水促抽瓦斯技术及其影响因素进行数值模拟,并将模拟结果应用于现场,对比分析数值模拟与现场实测数据,二者基本吻合.研究结果表明:煤层瓦斯含量以注水孔为中心径向逐步降低,以抽采孔为中心径向逐步升高;注水前抽采阶段,随着抽采时间的增加,抽采范围逐渐增大,抽采孔瓦斯流量先快速下降,后逐步缓慢降低;注水促抽阶段,随着注水时间的增加,注水范围逐渐增大,注水流量逐步降低,煤层瓦斯含量缓慢升高,抽采孔瓦斯流量逐渐增加;注水后抽采阶段,随着抽采时间的增加,压力水覆盖范围持续增大,煤层瓦斯含量逐渐降低,抽采孔瓦斯流量逐渐减小.注水时机、注水时间、注水压力、注水方式、布置方式及钻孔间距是影响煤层注水促抽瓦斯效果的6个主要因素.瓦斯正常抽采20 d后,按照一注一抽方式及5 m间距布置注抽钻孔,在8 MPa煤层注水压力下间歇注水10 d,煤层注水促抽瓦斯效果较好.