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.

渝南地区二叠系龙潭组C_(25)煤层煤层气基础地质条件分析

渝南地区二叠系龙潭组煤层气勘探开发处于起步阶段,为揭示该区主力煤层C_(25)的煤层气基础地质条件,基于区内煤田钻孔、煤层气钻井获取的地质资料和实验测试数据,对煤岩煤质、煤层孔渗、含气性等特征进行了分析。结果表明:研究区二叠系龙潭组C_(25)煤层具有“发育稳定、厚度相对大、孔渗低、变质程度高、含气量高”的特征。C_(25)煤层全区发育稳定,呈“北厚南薄”的趋势,北部一般大于1.5 m;煤岩煤质属于半亮型煤;有机组分中镜质组含量介于51.7%~72.2%;镜质组最大反射率整体介于1.8%~2.2%,变质程度处于高成熟—过成熟阶段;煤岩孔渗较差,孔隙度介于3.46%~8.46%,渗透率大多低于0.01×10^(-3)µm^(2);含气性好,一般大于10.0 m^(3)/t;同时,煤层顶底板均为良好封隔层。结合綦煤1井、Y2井产气效果,认为渝南地区二叠系龙潭组C_(25)煤层具备较好的煤层气勘探开发基础地质条件。

高温和高压下CBM的爆炸极限

利用自主开发的实验装置,测定了20,60,100,150,200℃及常压,100,200和300 kPa初始条件下煤层气(CBM)的爆炸极限值.结果表明,随初始温度和压力的增加,爆炸极限上限变大,下限变小,爆炸极限范围变大,危险性增加;初始温度和压力对爆炸极限上限的影响大于对爆炸极限下限的影响.研究结论为CBM开发使用过程安全工艺参数的确定提供了实验依据.

高、低阶煤孔隙结构差异性及其对甲烷吸附特性的影响研究

采集沁水盆地高阶煤、二连盆地低阶煤,采用氮气、二氧化碳吸附法对煤基质纳米级吸附孔隙进行测试,分析高低阶煤的储气特征,从而确定影响高、低阶煤煤层气开发的关键因素。研究结果发现,高、低阶煤的比表面积相似,高阶煤的比表面积均值为215.33 m~2/g,可吸附甲烷49.53 cm~3/g;低阶煤的比表面积均值为212.17 m~2/g,可吸附甲烷48.96 cm~3/g。高、低阶煤储气主要为吸附气,占总储气量的99%以上,高阶煤储气主要是微孔吸附气,占总储气量的99.71%;低阶煤不同孔隙均有贡献,微孔最多,占比47.33%,其次是中孔,占比32.644%,大孔最少,占比19.69%。地层水侵入对低阶煤储气量影响大于高阶煤,平衡水条件下,高阶煤可储气量43.46 cm~3/g,低阶煤可储气量33.89 cm~3/g;受微孔特性影响,高阶煤储层储气量较高,但易储难逸,因此煤层气开采的关键问题是疏导微孔吸附气;受中、大孔及亲水性强影响,低阶煤储气量较低,且易储易散,勘察储气聚集区是低阶煤煤层气开发的首要工作。

分段多簇密切割压裂技术在淮南矿区煤层气抽采中的应用

针对淮南矿区碎软低渗煤层瓦斯抽采技术难题,以13-1煤层为例,在分析煤层及其顶底板概况的基础上,采用煤层顶板分段压裂水平井技术,充分利用水平井水平段,单段分3簇射孔,簇间距25 m,实现分段多簇密切割体积压裂改造。微地震监测结果表明,单段裂缝长度102.5~211.4 m,缝高17.3~27 m,裂缝主体向下延伸,实现了井筒与煤层的沟通。排水采气效果表明,最高产气量1519 m^(3)/d,取得了良好的产气效果。研究成果为淮南矿区煤层气地面预抽和瓦斯区域治理提供了工程借鉴。

沁水盆地深部软煤煤层气开发实践

我国煤层气开发主要集中在煤层深度小于1200 m的区域,埋深大于1500 m的深部煤层气开发未取得实质性突破。针对深部软煤井壁稳定差、压裂改造难的问题,对沁水盆地深部软煤煤层气的开发开展了一系列技术攻关。研究结果表明:钻井过程中使用“氯化钾聚合物+井眼强化剂”钻井液体系,可有效控制煤层相对井径扩大率。使用16~18 m 3/min超大排量、高液量、高砂量光套管压裂工艺,配合降阻剂来降低施工摩擦阻力,细砂、中砂、粗砂占总砂量体积比分别为15%、70%、15%,加砂量占总携砂液体积比自3%开始每次提高1%,可实现对深部软煤的充分改造,裂缝动态半长最大达到322.51 m,大幅提高了单井控制面积,排采产气效果良好。

“十四五”以来新疆煤炭地质工作进展及发展方向

“十四五”以来,新疆煤炭地质工作紧紧围绕煤炭和煤层气勘查开发、煤田灭火、煤矿山生态修复治理开展,取得一系列重要成果和进展,在保障国家能源资源安全、支撑生态文明建设和服务自治区经济社会高质量方面发挥了积极作用。开展煤炭勘查项目约50个,完成钻探进尺60余万米,提交查明煤炭资源量174×10^(8)t,新发现煤炭资源量5.4×10^(8)t,新发现煤炭潜力区2处,为自治区充分释放煤炭优质先进产能和大中型矿井建设提供资源保障;开展煤层气资源调查、勘查和开发项目14个,完成钻探进尺约4.5×10^(4)m,估算煤层气资源量234×10^(8)m^(3)。在阜康大黄山实现了平均单井产量突破,在尼勒克、库拜煤田初步实现了深部煤层气调查和勘查技术突破。立足多年煤炭勘查技术和成果积累优势,围绕煤炭地质服务市场充分发挥作用,大力开展煤田灭火工作,积极拓展采煤塌陷区治理工作,主动服务矿井安全生产。完成火区治理13处,治理火区面积552×10^(4)m^(2);开展煤矿隐蔽致灾因素普查、水文地质类型划分等地质技术服务50余个,为煤矿山安全生产提供了技术保障;塔城地区额敏县采煤沉陷区综合治理恢复耕地10000亩;不断加强科研工作,突破了深部煤层气钻井和大规模压裂技术,形成了一套较完整的火区勘查、治理、监测施工工艺技术体系,初步建立了新疆煤田地质综合信息数字平台。今后,新疆煤田地质将坚持地质找矿核心职能,加强地质科技创新,围绕煤炭、煤层气、战略性关键金属、非金属矿产大力开展找矿突破战略行动,不断提升地灾防治和生态修复治理能力,积极服务生态文明建设。

纳雍补作区块煤层气成藏控因及勘探方向

基于大量的煤田地质勘探资料,深入分析了研究区煤的变质程度、煤岩煤质特征、煤层顶底板封盖性、煤层埋深、构造和水文地质条件等煤层气成藏控制因素,采用体积法估算了研究区煤层气地质资源量,并进一步提出下一步勘探方向。结果表明:研究区水文地质和高致密的煤层顶底板对煤层气起到封闭控气作用;煤层埋深、煤岩煤质和煤的变质程度是造成各主采煤层含气量差异的主要因素;估算出研究区煤层气资源量为68.82×10^(8) m^(3),为中型气藏,另外优选出区块1为Ⅰ级甜点区,可作为研究区先期勘探开发区。

煤层气压缩机无级气量调节节能功能的实现

为解决传统煤层气往复式压缩机气量调节系统所存在的能耗大、运行效率低的问题,在对压缩机无级气量调节原理分析的基础上,对当前局部行程的无级气量调节系统所存在的问题进行总结,重点提出了无级气量调节系统液压站、蓄能器等设计思路。对所设计的无级气量调节系统的密封性能和执行机构响应性能进行测试。

Coalbed Methane-bearing Characteristics and Reservoir Physical Properties of Principal Target Areas in North China

The coalbed methane (CBM) resources in North China amounts up to 60% of total resources in China. North China is the most important CBM accumulation area in China. The coal beds of the Upper Paleozoic Taiyuan and Shanxi formations have a stable distribution. The coal reservoir of target areas such as Jincheng, Yanquan-Shouyang, Hancheng, Liulin, etc. have good CBM-bearing characteristics, high permeability and appropriate reservoir pressure, and these areas are the preferred target areas of CBM developing in China. The coal reservoirs of Wupu, Sanjiaobei, Lu'an, Xinmi, Anyang-Hebi, Jiaozuo, Xinggong and Huainan also have as good CBM-bearing characteristics, but the physical properties of coal reservoirs vary observably. So, further work should be taken to search for districts with high pressure, high permeability and good CBM-bearing characteristics. Crustal stresses have severe influence on the permeability of coal reservoirs in North China. From west to east, the crustal stress gradient increases, while the coal reservoirs permeability decreases.

Prediction of coal structure using particle size characteristics of coalbed methane well cuttings

Severely deformed coal seams barely deliver satisfactory gas production. This research was undertaken to develop a new method to predict the positions of deformed coals for a horizontal CBM well. Firstly, the drilling cuttings of different structure coals were collected from a coal mine and compared. In light of the varying cuttings characteristics for different structure coals, the coal structure of the horizontally drilled coal seam was predicted. And the feasibility of this prediction method was discussed. The result shows that exogenetic fractures have an important influence on the deformation of coal seams. The hardness coefficient of coal decreases with the deformation degree in the order of primary structural, cataclastic and fragmented coal. And the expanding-ratio of gas drainage holes and the average particle size of cuttings increase with the increase of the deformation degree. The particle size distribution of coal cuttings for the three types of coals is distinctive from each other. Based on the particle size distribution of cuttings from X-2 well in a coal seam, six sections of fragmented coal which are unsuitable for perforating are predicted. This method may benefit the optimization of perforation and fracturing of a horizontal CBM well in the study area.

New Progress on the Coal Fines Affecting the Development of Coalbed Methane

Objective The production of coal fines is very common in the development of coalbed methane（CBM）in the eastern margin of the Ordos Basin,China.A large amount of produced coal fines seriously affect the productivity of CBM wells（Wei Yingchun et al.,2013）.Therefore,the production problems of CBM wells caused by coal fines have attracted extensive attention.

Dependence of fluid flow on cleat aperture distribution and aperture–length scaling: a case study from Gondwana coal seams of Raniganj Formation, Eastern India

Dimensions and mutual relationships amongst fracture(cleat)parameters such as maximum aperture width,average aperture width,length,spacing etc.control the connectivity within a fracture network and the fluid flow in a coal seam as the matrix permeability,here,is negligible.In this paper,we document cleat size distributions and investigate length–aperture relationships from coals of Raniganj coalfield in Eastern India.This coalfield has a proven extractable reserve of six billion tons of coal and holds immense potential to be one of the largest coal bed methane fields serving India’s growing energy needs.Here,cleat length(L)correlates with corresponding maximum aperture width(Dmax)in a power-law function with an exponent of 0.84(DmaxαL0.84)instead of the commonly observed exponents of 1 or 0.5 applicable for other natural‘opening-mode’fractures.The conventional wisdom pertains that laminar fluid flow(Q)through an isolated,smooth-walled,parallel-plate fracture,embedded in an impermeable matrix,is directly proportional to the cube of its aperture width(b,equivalent to Davg;cubic law:Qαb3).This assumes a linear relationship between length and fracture aperture.However,the modified relationship between cleat length and average aperture width changes the cubic law applicable for Raniganj coal seam and now fluid flow correlates with aperture width in a power-law function with an exponent of 4.25(Qαb4.25)instead of 3(cube).Such simplifications will come handy for the modeling and estimation of fluid flow as it will reduce the effort of cleat length measurement which is anyway difficult and can be misleading due to the risk of undersampling.

Monte-Carlo Method for Coalbed Methane Resource Assessment in Key Coal Mining Areas of China

Monte-Carlo method is used for estimating coalbed methane （CBM） resources in key coal mining areas of China. Monte-Carlo method is shown to be superior to the traditional volumetric method with constant parameters in the calculation of CBM resources. The focus of the article is to introduce the main algorithm and the realization of functions estimated by Monte-Carlo method, including selection of parameters, determination of distribution function, generation of pseudo-random numbers, and evaluation of the parameters corresponding to pseudo-random numbers. A specified software on the basis of Monte-Carlo method is developed using Visual C＋＋ for the assessment of the CBM resources. A case study shows that calculation results using Monte-Carlo method have smaller error range in comparison with those using volumetric method.

The coal cleat system:A new approach to its study

After a general analysis regarding the concept of coal ＂cleat system＂, its genetic origin and practical applications to coalbed methane （CBM） commercial production and to C02 geological sequestration projects, the authors have developed a method to answer, quickly and accurately in accordance with the industrial practice and needs, the following yet unanswered questions： （1） how to define the spatial orientation of the different classes of cleats presented in a coal seam and （2） how to determine the frequency of their connectivites. The new available and presented techniques to answer these questions have a strong computer based tool （geographic information system, GIS）, able to build a complete georeferentiated database, which will allow to three-dimensionally locate the laboratory samples in the coalfield. It will also allow to better understand the coal cleat system and consequently to recognize the best pathways to gas flow through the coal seam. Such knowledge is considered crucial for understanding what is likely to be the most efficient opening of cleat network, then allowing the injection with the right spatial orientation, of pressurized fluids in order to directly drain the maximum amount of gas flow to a CBM exploitation well. The method is also applicable to the CO2 geological sequestration technologies and operations corresponding to the injection of CO2 sequestered from industrial plants in coal seams of abandoned coal mines or deep coal seams.

Characteristics of dissolved inorganic carbon in produced water from coalbed methane wells and its geological significance

Based on long-term dynamic tracing of dissolved inorganic carbon(DIC)and stable carbon isotope(δ13CDIC)in produced water from 20 coalbed methane(CBM)wells in western Guizhou,the spatial-temporal dynamic variations ofδ13CDIC of the GP well group produced in multi-layer commingled manner were analyzed,and the relationship between the value ofδ13CDIC and CBM productivity was examined.The produced water samples of typical wells in the GP well group were amplified and sequenced using 16S rDNA,and a geological response model ofδ13CDIC in produced water from CBM wells with multi-coal seams was put forward.The research shows that:δ13CDIC in produced water from medium-rank coal seams commonly show positive anomalies,the produced water contains more than 15 species of methanogens,and Methanobacterium is the dominant genus.The dominant methanogens sequence numbers in the produced water are positively correlated withδ13CDIC,and the positive anomaly of v is caused by reduction of methanogens,and especially hydrogenotrophic methanogens.Vertical segmentation of sedimentary facies and lithology in stratum with multi-coal seams will result in permeability and water cut segmentation,which will lead to the segmentation ofδ13CDIC and archaea community in produced water,so in the strata with better permeability and high water cut,theδ13CDIC of the produced water is abnormally enriched,and the dominant archaea is mainly Methanobacterium.In the strata with weak permeability and low water cut,theδ13CDIC of the produced water is small,and the microbial action is weak.The shallow layer close to the coal seam outcrop is likely to be affected by meteoric precipitation,so theδ13CDIC of the produced water is smaller.The geological response model ofδ13CDIC in produced water from multi-coal seams CBM wells in the medium-rank coal reveals the geological mechanism and microbial action mechanism of theδ13CDIC difference in the produced water from the multi-coal seams CBM wells.It also provides effective geochemical evidence for the superimposed fluid system controlled by sedimentary facies,and can also be used for the contribution analysis of the produced gas and water by the multi-layer CBM wells.

Resource types, formation, distribution and prospects of coal-measure gas

Coal-measure gas is the natural gas generated by coal, carbonaceous shale, and dark shale in coal-measure strata. It includes resources of continuous-type coalbed methane (CBM), shale gas and tight gas reservoirs, and trap-type coal-bearing gas reservoirs. Huge in resources, it is an important gas source in the natural gas industry. The formation and distribution characteristics of coal-measure gas in San Juan, Surat, West Siberia and Ordos basins are introduced in this paper. By reviewing the progress of exploration and development of coal-measure gas around the world, the coal-measure gas is confirmed as an important strategic option for gas supply. This understanding is mainly manifested in three aspects. First, globally, the Eurasian east-west coal-accumulation belt and North American north-south coal-accumulation belt are two major coal-accumulation areas in the world, and the Late Carboniferous–Permian, Jurassic and end of Late Cretaceous–Neogene are 3 main coal-accumulation periods. Second, continuous-type and trap-type are two main accumulation modes of coal-measure gas;it is proposed that the area with gas generation intensity of greater than 10×10^8 m^3/km^2 is essential for the formation of large coal-measure gas field, and the CBM generated by medium- to high-rank coal is usually enriched in syncline, while CBM generated by low-rank coal is likely to accumulate when the source rock and caprock are in good configuration. Third, it is predicted that coal-measure gas around the world has huge remaining resources, coal-measure gas outside source is concentrated in Central Asia-Russia, the United States, Canada and other countries/regions, while CBM inside source is largely concentrated in 12 countries. The production of coal-measure gas in China is expected to exceed 1000×10^8 m^3 by 2030, including (500–550)×10^8 m^3 conventional coal-measure gas,(400–450)×10^8 m^3 coal-measure tight gas, and (150–200)×10^8 m^3 CBM.

Time-varying diffusion characteristics of different gases in coal particles

Based on the analytical solution of gas diffusion in spherical coal particles with a constant diffusion coefficient, a calculation method of time-varying diffusion coefficient is proposed by constructing objective function. The time-varying diffusion behavior of methane, nitrogen and carbon dioxide in the coal particles was studied. The results show that with the increase of diffusion time, the diffusion coefficients of methane, nitrogen and carbon dioxide gas in the coal particles exhibit an attenuation characteristic, eventually approaching a limit value individually. The diffusion coefficient of carbon dioxide is larger than methane, and the diffusion coefficient of nitrogen is smallest. Significant phenomenon of limited diffusion was observed for coal of strong adsorption capability. Through the analysis of the diffusion coefficient of gases at different diffusion time, a mathematical model describing the time-varying diffusion characteristic of gases is obtained. The implementation of mixed gases to replace coal bed methane has a very important practical significance.