Theoretical analysis and engineering application of controllable shock wave technology for enhancing coalbed methane in soft and low‑permeability coal seams

Coalbed methane(CBM)is a significant factor in triggering coal and gas outburst disaster,while also serving as a clean fuel.With the increasing depth of mining operations,coal seams that exhibit high levels of gas content and low permeability have become increasingly prevalent.While controllable shockwave(CSW)technology has proven effective in enhancing CBM in laboratory settings,there is a lack of reports on its field applications in soft and low-permeability coal seams.This study establishes the governing equations for stress waves induced by CSW.Laplace numerical inversion was employed to analyse the dynamic response of the coal seam during CSW antireflection.Additionally,quantitative calculations were performed for the crushed zone,fracture zone,and effective CSW influence range,which guided the selection of field test parameters.The results of the field test unveiled a substantial improvement in the gas permeability coefficient,the average rate of pure methane flowrate,and the mean gas flowrate within a 10 m radius of the antireflection borehole.These enhancements were notable,showing increases of 3 times,13.72 times,and 11.48 times,respectively.Furthermore,the field test performed on the CSW antireflection gas extraction hole cluster demonstrated a noticeable improvement in CBM extraction.After antireflection,the maximum peak gas concentration and maximum peak pure methane flow reached 71.2%and 2.59 m^(3)/min,respectively.These findings will offer valuable guidance for the application of CSW antireflection technology in soft and low-permeability coal seams.

Numerical Study on an Applicable Underground Mining Method for Soft Extra-Thick Coal Seams in Thailand

The EGAT Mae Moh Mine is the largest open pit lignite mine in Thailand and it produces lignite about 16 million tons annually. In the near future, the pit limit of the mine will be reached and underground mine will then be developed through the open pit in the depth of 400 - 600 m from the surface. However, due to the challenges for underground mining such as poor geological conditions, extra thickness (20 - 30 m) of coal seams, and weak mechanical properties of coal seams and the surrounding rock, the success possibility of underground mining and an applicable underground mining method is being investigated at the present. The paper discusses the applicability of multi-slice bord-and-pillar method for the soft extra thick coal seams in the Mae Moh mine by means of numerical analyses using the 3D finite difference code “FLAC3D”.

Rockburst mechanism in soft coal seam within deep coal mines

A number of rockburst accidents occurring in soft coal seams have shown that the rockburst mechanism involved in soft coal seams is significantly different from that involved in hard coal seams. Therefore, the method used to evaluate rockburst in hard coal seams is not applicable to soft coal seams. This paper established an energy integral model for the rockburst-inducing area and a friction work calculation model for the plastic area. If the remaining energy after the coal seam is broken in the rockburstinducing area is greater than the friction work required for the coal to burst out, then a rockburst accident will occur. Mechanisms of ‘‘quaking without bursting" and ‘‘quaking and bursting" are clarified for soft coal seams and corresponding control measures are proposed as the optimization of roadway layouts and use of ‘‘three strong systems"(strong de-stressing, strong supporting, and strong monitoring).

Macro and micro grouting process and the influence mechanism of cracks in soft coal seam

Grouting is an important method to reinforce soft coal roadway,and the presence of primary cracks in the coal body has an important influence on the grouting effect.With the discrete element simulation method,the grouting process of the soft coal seam was simulated.The mechanism of primary cracks on grouting was revealed,while the influence of fracture characteristics and grouting pressure on the grouting effect was analyzed.The results demonstrated that grouting in the soft coal seam involves the stages of seepage,rapid splitting,slow splitting,and stability.Due to the presence of primary cracks,the grouting diffusion radius increased significantly.Under the slurry pressure,the tensile stress concentration was formed at the crack tip,and the slurry split the coal once the splitting pressure was reached.In addition,the distribution characteristics of fractures are found to have a great influence on the grouting effect.It is observed that smaller fracture spacing is associated with a larger slurry diffusion radius and thus easier penetration of the primary crack tips.The fracture angle affects the direction of fracture propagation.The secondary fracture formed by splitting is a tensile fracture,which is more likely to extend along the direction parallel to the maximum principal stress.Overall,these simulation results have guiding significance for the setting of reasonable spacing of grouting holes in the practice of grouting engineering.

Outburst control in soft and outburst prone coal seam using the waterjet slotting technique from modeling to field work

The paper discussed a comprehensive numerical simulation and field work by the usage of waterjet slotting tech- nique to prevent the occurrence of outbursts in soft and outburst prone coal seams. This was based on the geological and ge- omechanical conditions of Jinjiachong Colliery, Guizhou Province, associated with varied waterjet slotting parameters such as slotting penetration, slotting thickness and slotting distance along the length of borehole. Also, to understand the variation of internal stress of coal seams after waterjet slotting application, the internal stress levels were compared with and without slot- ting application, and the results indicate that the internal effective stress levels can be reduced to 70% and 45% for the vertical and horizontal stresses, respectively, and the gas concentration can be increased up to 5 times when the waterjet slotting is ap- plied.

Analysis of stability of coal pillars with multi-coal seam strip mining

Strip mining is one of the efficient measures to control surface subsidence and mining damage. However, the researches on the laws of the geological mining factors to upper and lower pillar's stability are still deficient in multi-coal seam strip mining at present. Based on the three dimension fast Lagrangian analysis of continua (short for FLAC3D) numerical simulation software, the laws of the stress increasing coefficient on the coal pillar and its stability were systematically studied for different depths, different mining widths, different interlayer spacings, different mining thicknesses, different properties of interstratified rock and the spacial relations of the upper and lower pillars in vertical alignment in multi-coal seam strip mining. The function relation between the stress increasing coefficient of upper and lower pillars with the mining depth, mining widths, interlayer spacing, mining thickness, property of interstratified rock and the spatial relationship were obtained.

Three-dimensional numerical simulation of methane drainage by high-level drill holes in a lower protective coal seam with a “U” type face

Different drill-hole positions may produce different drainage results in low protective coal seams.To investigate this possibility,a 3D stope model is established,which covers three kinds of drill holes.The FLUENT computational fluid mechanics software is used to solve the mass,momentum and species conservation equations of the model.The spatial distributions of oxygen and methane was obtained by calculations and the drainage results of different drill-hole positions were compared.The results show that,from top to bottom,methane dilution by oxygen weakens gradually from the intake to the return side,and methane tends to float;methane and oxygen distribute horizontally.The high-level crossing holes contribute to better methane drainage and a greater level of control.Around these holes,the methane density decreases dramatically and a "half circle"distribution is formed.The methane density decreases on the whole,but a proportion of the methane moves back to deep into the goaf.The research findings provide theoretical grounds for methane drainage.

Discussion of the indexes of nonoutburst coal seam upgrade and its corresponding critical values

In view of the measurement difficulties of indexes recommended by the 50Items Experience of Coal Mine Gas Prevention in the process of the nonoutburst coalseam upgrade, this paper took the No.8 coal seam of Huainan Mining Group as research object. Discussed the suitability of indexes and corresponding critical values, putforward method in determining the indexes and its critical values by analysis and investigation of the gas geological condition and the-spot tracking near position where anoutburst occurred combined with laboratory experiment, and established the indexesand its critical values of nonoutburst coal seam upgrade in No.8 coal seam of HuainanMining Group. The results show that it is suitable to take gas content and tectonic softcoal thickness easily to gain in routine production as primary upgrade indexes that itscritical values are 7.5 m^3/t and 0.8 m, respectively. In addition, takefvalue and Ap valueas auxiliary indexes.

A case study of multi-seam coal mine entry stability analysis with strength reduction method

In this paper,the advantage of using numerical models with the strength reduction method(SRM) to evaluate entry stability in complex multiple-seam conditions is demonstrated.A coal mine under variable topography from the Central Appalachian region is used as a case study.At this mine,unexpected roof conditions were encountered during development below previously mined panels.Stress mapping and observation of ground conditions were used to quantify the success of entry support systems in three room-and-pillar panels.Numerical model analyses were initially conducted to estimate the stresses induced by the multiple-seam mining at the locations of the affected entries.The SRM was used to quantify the stability factor of the supported roof of the entries at selected locations.The SRM-calculated stability factors were compared with observations made during the site visits,and the results demonstrate that the SRM adequately identifies the unexpected roof conditions in this complex case.It is concluded that the SRM can be used to effectively evaluate the likely success of roof supports and the stability condition of entries in coal mines.

Influence of Water Injection Fracturing on Crack Damage in Soft Coal and Application in Gas Extraction

In order to study the hypotonic and rheological particularity of “three soft” coal seam in west Henan, China, this paper explored the stress and damage characteristics of crack in coal under condition of water injection fracturing based on ABAQUS platform;The cohesive element in T-P damage evolution criterion was used to describe the approximately linear relationship between crack width and extending distance in soft coal. The simulation results show that stress evolution and crack damage in soft coal is a gradually developing process under condition of water injection fracturing. When the static pressure is 4 - 10 MPa, and the injection time is about 1 - 2 hours, the damage range of crack in soft coal can basically reach an ideal data of 80 - 100 m, and then greatly improve the hypotonic performance of “three soft” coal seam.

山东省宁汶煤田鑫安煤矿3煤层开采水害隐患分析

为了查明鑫安煤矿开采3煤层的水害隐患类型,分析水害对矿井的潜在威胁和空间分布特征,采用现场质询、资料收集、井下踏勘的方法,逐一分析了煤矿大气降水及地表水、第四系砂砾层、新近系砂砾层、白垩系砂岩、3煤层顶底板砂岩、三灰、十下灰、奥灰、老空积水对3煤层开采的影响,排查分析出影响3煤层开采的水害隐患类型为3煤层顶底板砂岩、奥灰水和老空积水;采用“采后导水断裂带内含水层总厚度值”和“突水系数法”对开采3煤层的顶板砂岩充水危险性和底板奥灰突水危险性进行等级划分。研究分析出开采3煤层水害隐患的空间分布特征为3煤顶板水害主要影响矿井中部、老空水主要影响矿井浅部、奥灰水主要影响矿井东部。

沁水盆地南部3号煤压裂曲线特征及施工建议

由于煤岩的非均质性强,储层物性差异大,其压裂施工曲线形态各异。通过对沁水盆地南部3号煤层200多口井的压裂施工曲线分析,总结出前置液阶段分为下降型、下降波动型、稳定型和下降稳定型,携砂液阶段分为下降型、下降稳定型、上升型、稳定型和波动型,测压降阶段分为压力下降较陡型和压力下降较缓型。同时,阐述了每种类型产生的原因。最后给出压裂施工相关建议。

沁南3~#煤与15~#煤显微煤岩组分对微裂隙的控制研究

利用光学显微镜、扫描电镜对沁水盆地南部10个煤矿的3#煤与15#煤样品进行观察与研究,探讨了微裂隙的特征及其与煤岩组分的关系。研究发现微裂隙在3#煤与15#煤中普遍存在,15#煤裂隙较3#煤发育,但15#煤中的裂隙被矿物充填较严重。微裂隙在镜质组内最为发育,内生裂隙和外生裂隙均有发育,均质镜质体中的裂隙最为密集,其次是基质镜质体,其他显微组分中裂隙发育较少甚至不发育;惰质组中的裂隙主要发育于丝质体中,主要为外生裂隙中的张性裂隙,内生裂隙在高煤阶煤的惰质组中不发育。煤岩组分对煤中微裂隙的控制主要体现在成煤作用过程中,包括不同组分在泥炭化作用中吸收的水再释放、组分结构与生气量的综合作用及力学性质3个方面。

象山矿井主采3^#煤层涌水规律及充水因素分析

为查清矿井的涌水机理,为矿井防治水提供依据,依据大量生产资料,采用相关分析及理论分析相结合的方法,总结了象山矿井及主采3#煤层采面涌水规律,分析了充水因素,开展了煤层顶板含水层充水危险性预测。结果表明,矿井涌水量随着开采条件的变化总体呈现增高的变化趋势,采面涌水量具有背斜区低,向斜区高,回采期间具有先小后大,再由大变小的规律;充水通道主要为采动产生的顶板导水裂隙,充水水源主要来自煤层顶板山西组底部(K4)砂岩含水层与下石盒子组底部砂岩含水层水,涌水量大小与采面所在构造部位、顶板上覆含水层的岩性厚度及煤产量大小有关。在考虑采煤工艺和开采强度相同、同一采区构造类型相同、充水含水层富水性平面差异不够明显及大气降水及地表径流补给不变的情况下,依据顶板导水裂隙带范围内上覆砂岩含水层的岩性与厚度大小对3#煤层顶板含水层充水危险性进行了预测。为避免回采过程顶板突然涌水,应在顶板含水层充水危险性严重区采取事先疏放水措施。

樊庄北部3号煤层现今应力场分布数值模拟

为了制定针对樊庄区块北部煤层气井区3号煤层的有效缝网改造措施,利用工区二维地震资料对石炭系一一二叠系的构造样式及断层特征进行解释,在此基础上,利用压裂法、成像测井及有限元数值模拟方法对山西组3号煤层现今应力场特征进行研究。结果表明,3号煤层主应力满足σ_H>σ_v>σ_h,各主应力随深度的增加而增加,σ_v、σ_H及σ_h的应力梯度分别为0.025 MPa/m、0.018 MPa/m及0.013 MPa/m。3号煤层的σ_H及σ_h均略高于其顶底板砂泥岩地层1.0~2.5 MPa,这种应力特征易造成压裂穿层,缝高难于控制;3号煤层σ_H-σ_h为2.0~6.0MPa,且随深度增加而增加,这是造成工区埋深相对较浅的煤层压裂效果较好、产气量相对较高的重要原因。研究区现今地应力方向在SN及NNE之间,与喜马拉雅中晚期构造挤压运动相关。有限元模拟显示,煤储层应力场平面分布特征受控于埋深、岩性、褶皱及断层等因素。3号煤层σ_H为15.6~21.0MPa;σ_h为12.5~16.0MPa,预测结果与实测值相符。

沁水盆地柿庄南3号煤压裂地质研究

长期以来,在煤层气水力压裂研究中多偏重于压裂工艺的研究,而系统考虑地质因素的影响力度不够,从而导致压裂施工效果不明显。基于目标区的钻井、测井和岩心分析测试等资料,分析了主煤层煤岩孔裂隙发育特征、渗透率等特征,柿庄南区块及邻区煤层渗透率平面上变化较大,一般0.01-0.3 m D,平均0.09 m D以上。基于煤岩渗透率和煤岩敏感性测试,确定了煤层渗透率敏感性的主要地质因素——随着地应力梯度的增加呈指数降低,煤储层渗透性受地应力影响明显,随着弹性模量的增大,岩石的可压缩性降低,渗透率受到的影响也越小。

北皂矿H2103工作面矿压显现规律研究

龙矿集团北皂矿H2103工作面是海域开采第二个工作面,在首采工作面开采以后,H2103工作面开采,开采范围扩大,开采相对充分,研究发现:其矿压显现的规律更符合海域下三软煤层开采的实际规律,对实现安全高效开采具有参考指导价值。

布尔台矿井3^(-1)煤层冲刷带特征及预测

通过井下地质观测研究,对布尔台矿井3-1煤层冲刷带砂体的岩性、赋存特征和几何形态进行了描述,阐明了其识别特征。在对该矿井煤系沉积相分析的基础上,利用3-1煤层顶板砂岩等厚线图对冲刷带进行了预测,分析了矿井煤层冲刷带的总体分布规律。

SDQ-73型深孔取样装置在松软高突煤层中的适用性研究

煤层瓦斯含量井下直接测定结果的准确性很大程度上取决于所取煤样的质量。针对松软高突煤层中取样问题的现状,展开了SDQ-73型深孔取样装置在松软高突煤层的适用性研究。通过在首山一矿二1煤层12050、12061工作面的井下试验,分别完成了SDQ-73取样适用性试验及孔口接粉取样试验,并对两种取样方法的测试结果进行对比分析,得出该深孔快速取样装置可实现在松软高突煤层中的深孔、定点、快速取样,且取样质量明显高于传统的孔口接粉取样,完全符合GB/T23250《煤层瓦斯含量井下直接测定方法》的取样要求。

双管定向钻进技术在祁南矿313工作面的应用

针对祁南矿313工作面碎软煤层中顺煤层长钻孔施工时,煤层倾角大和32#煤碎软的问题,利用双动力头钻机及配套钻具,开展钻杆、套管双管定向钻进工艺研究;通过关键设备选型、钻杆和钻杆钻头、套管和套管钻头钻具选配、风动定向钻进工艺参数优化以及提出钻杆在套管内大直径筛管护孔等方法及试验,有效提高顺煤层钻孔深度,实现顺煤层长钻孔轨迹可控,并提高了地层适应性。