Damage and Permeability Development in Coal During Unloading

One of the key issues in protective seam mining is the pressure relief and permeability improvement effect. In this paper, the results of X-ray CT scanning experiments and permeability experiments using reconstituted coal specimens subjected to the same stress path and the same effective confining pressure (confining pressure minus pore pressure) are combined using the stress–strain relationship to study the damage to reconstituted coal specimens and its influence on permeability during the unloading process. When the effective confining pressure (σ ₃ ? p) is unloaded from 8 to 6 MPa and the deviatoric stress increases, the damage variables will increase by 0.0351 and 0.084, respectively, compared with the unloading point under the fixed axial displacement with unloading confining pressure (FADUCP) and fixed deviatoric stress with unloading confining pressure (FDSUCP) stress paths. At the same time, the permeability increased by 1.7 and 16.7 %, respectively. Therefore, the damage variable and permeability increased notably little in this process. After the effective confining pressure is unloaded to approximately 5 MPa, together with the decrease in the deviatoric stress, the growth of the damage variable and permeability begins to accelerate. In addition, the relative decrease in the deviatoric stress with appearing damage cracks, and the relative increase in permeability with the same amount of effective confining pressure being unloaded, shows that the damage to specimens under the FDSUCP stress path is larger than that from the FADUCP stress path.     

受载含瓦斯原煤解吸规律实验研究

依托自行研发的含瓦斯煤热–流–固–力耦合吸附/解吸实验系统,测定了不同围压和孔隙压力组合条件下,煤样加载破坏过程中不同加载阶段的瓦斯解吸量,并对实验数据进行了拟合,分析了轴压、围压和孔隙压力对含瓦斯原煤解吸规律的影响。结果表明:在煤样整个加载破坏过程中,受载原煤瓦斯解吸量呈现先减小后增加的“V”型变化趋势,且最小出现在屈服强度阶段,最大出现在脆性破坏阶段;受载原煤瓦斯解吸量随围压的增大而减小,随吸附平衡压力的增大而增大。      

Anisotropy of gas permeability associated with cleat pattern in a coal seam of the Kushiro coalfield in Japan

An underground investigation has been performed in a subbituminous coal seam exhibiting a particular cleat pattern in the Kushiro coalfield, Japan. The coal cleat pattern shows some analogy to isolated straight joints, and is believed to have been formed during the late Tertiary period by a compressive tectonic stress, roughly in the east–west direction. Three cylindrical coal specimens representing the three orthogonal axes of the coal seam with respect to the bedding plane and its associated cleat were cored from a large block of coal. Gas permeabilities of the three coal specimens were measured under the same hydrostatic pressure conditions. Results clearly revealed anisotropy in permeability of the coal seam under relatively low confining pressures of less than about 12 MPa. The specimen cored parallel to both the bedding plane and cleat strike showed the highest permeability, even though the cleats were partly filled with calcite and clay minerals. The permeability in this direction was 2.5 times higher than perpendicular to the bedding plane, and 3 times higher than in the direction parallel to the bedding plane but perpendicular to the cleat strike. This suggests that the cleats play a greater role than bedding planes in controlling fluid flow in the coal seam. The permeability in the three orientations, however, converged to the same value at confining pressures above about 16 MPa. This may suggest that both cleats and bedding planes in a coal seam can close due to earth pressure if the coal seam is located below a certain depth. It further indicates that the traditional view that gas permeability is always greater parallel to the coal bedding than perpendicular to it should be reconsidered.     

The Effects of Temperature and Pressure on the Porosity Evolution of Flechtinger Sandstone

A porosity change influences the transport properties and the elastic moduli of rock while circulating water in a geothermal reservoir. The static and dynamic elastic moduli can be derived from the slope of stress–strain curves and velocity measurements, respectively. Consequently, the acoustic velocities were measured while performing hydrostatic drained tests. The effect of temperature on static and dynamic elastic moduli and porosity variations of Flechtinger sandstone was investigated in a wide range of confining pressure from 2 to 55 MPa. The experiments were carried out in a conventional triaxial system whereas the pore pressure remained constant, confining pressure was cycled, and temperature was increased step wise (25, 60, 90, 120, and 140 °C). The porosity variation was calculated by employing two different theories: poroelasticity and crack closure. The porosity variation and crack porosity were determined by the first derivative of stress–strain curves and the integral of the second derivative of stress–strain curves, respectively. The crack porosity analysis confirms the creation of new cracks at high temperatures. The porosity variation was increasing with an increase in temperature at low effective pressures and was decreasing with a rise in temperature at high effective pressures. Both compressional and shear wave velocities were increasing with increasing pressure due to progressive crack closure. Furthermore, the thermomechanical behavior of Flechtinger sandstone was characterized by an inversion effect where the sign of the temperature derivative of the drained bulk modulus changes.     

沁水盆地煤样静水压力下渗透率实验及模型分析

为研究沁水盆地煤样渗透率演化规律,构建了煤样渗透率测定的瞬态压力脉冲法实验装置,使用N2和CO2在实验室开展了3种试验条件的渗透率测定,应用Connell模型对实验结果进行分析,并讨论了模型预测值和实验值之间差别的原因。结果表明,（1）在恒定孔压变围压条件下渗透率随有效应力增大而减小;在等有效应力条件下,渗透率随孔压增大而减少;在恒定围压变孔压条件下,随孔压增大,渗透率呈先减小后变大的趋势。（2）运用Connell模型预测的恒定围压变孔压条件的渗透率值大于实验值,原因可能是由于裂隙压缩性系数和吸附应变系数存在估计误差。通过开展实验室渗透率实验和模型分析,对指导实验室内二氧化碳封存和气体驱替实验及其模拟研究具有借鉴意义。     

煤层气井合层开发层间干扰分析与合采方法探讨——以平顶山首山一矿为例

多煤层合层开发是提高煤层气井单井产量的关键技术,然而工程实践中大部分煤层合采存在层间干扰问题,致使合采产气量提升不明显。为了提高合层开发煤层气井的产气量与开发效率,以平顶山首山一矿煤层气合采四2煤层和二1煤层为例,基于煤层气赋存的地质条件,分析了合采层间干扰的影响因素及干扰规律,并提出了煤层合层开发层间干扰的控制方法。结果表明:造成四2煤层和二1煤层合层排采产量低的主要因素是储层压力梯度、临界解吸压力和渗透率。其中,两层煤的储层压力梯度分别为1.05 MPa/hm和0.519 MPa/hm;渗透率分别为0.25×10–3 μm2和1.4×10–5 μm2;临界解吸压力分别为1.16~1.69 MPa和0.40~0.46 MPa;另外,两煤层间距大,平均170 m左右。以上主要影响因素差异,造成两层煤合采时层间矛盾突出,干扰严重,总体产量低,井组煤层气开发效率低。基于现状问题,探索提出大间距多煤层大井眼双套管分层控制合采工艺方法,以实现两层煤分开控制达到合采产能叠加的目标,从而提高煤层气井合采产量和开发效果。研究认识将为平顶山及类似地质条件的矿区多煤层煤层气高效合层排采提供新的技术途径。      

煤体结构对煤层气吸附-解吸及产出特征的影响

煤的孔隙、物理化学结构差异对煤层气的吸附-解吸及产出特征有巨大影响。基于对不同煤体结构煤的孔隙、结构、力学性质的认识,利用现场实测资料,分析了煤体结构对煤层气产出的影响。结果表明:构造变形使煤的孔容和比表面积增大,吸附能力增强。含气量和损失量呈正相关关系;在含气量相同的情况下,逸散速率相对大小依次为:原生结构煤<碎裂煤<碎粒煤<糜棱煤。原生结构煤和碎裂煤的临界解吸压力大于糜棱煤。在0~45 min、45~95 min、95~185 min,平均解吸速率关系为:原生结构煤<碎裂煤<糜棱煤,而在185~485 min内,平均解吸速率关系反生改变,即:糜棱煤<原生结构煤<碎裂煤。在含气量大致相等时,原生结构煤和碎裂煤的解吸量及解吸时间明显大于糜棱煤。      

韩城地区煤储层孔渗应力敏感性及其差异

以韩城煤层气区块3号、5号和11号煤层为例,进行不同围压条件下的煤心孔渗实验,探讨了该区煤储层物性与应力之间的耦合关系,建立了相应的数学模型。结果表明,煤心孔渗随围压的增加而不断下降,渗透率应力伤害远强于孔隙度应力伤害,但各煤层的应力敏感性各不相同:在实验围压从4.14 MPa（600 psi）增加到12.42 MPa（1 800 psi）条件下,11号煤层孔渗应力敏感性最强,孔隙度应力伤害达76.5%,渗透率应力伤害达93.3%;3号煤层孔渗应力敏感性最弱,孔隙度应力伤害38.5%,渗透率应力伤害77.9%;5号煤层孔渗应力敏感性较强,孔隙度应力伤害约45%,渗透率应力伤害达83.9%。分析认为,裂隙发育状况是造成各煤层间孔渗应力敏感差异的主要原因。从实验数据的拟合情况看,幂函数模式比指数函数模式更能准确地获取测试围压范围内的孔渗内插值。      

Stress distribution characteristic analysis and control of coal and gas outburst disaster in a pressure-relief boundary area in protective layer mining

Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. Furthermore, the protective coal seam mining technology featured by economic efficiency has been proven to be the most effective and widely applied method for the prevention of coal and gas outburst disasters. However, the determinations of the protective area coal and gas outburst prevention in a pressure-relief boundary area are fundamental issues that research should be focused on. The technical method for determining stress distribution in pressure-relief boundary area during protective coal seam mining is put forward in this paper. The method is based on a stress-seepage coupled relationship within a gas-containing coal seam. The method includes complex lab experiments and on-site measurements at the Qingdong Coal Mine. The final data illustrate that the permeability and vertical stress in the pressure-relief boundary area of the coal sample form a negative exponential function relationship. Additionally, the permeability of the coal sample within the abovementioned area is significantly different compared with that located at the center of the pressure-relief area. In the pressure-relief boundary area, the gas pressure distribution gradient is 0.0375 MPa/m, while the vertical stress distribution gradient registers 0.56 MPa/m. Under this condition, coal and gas outburst disasters are prone to be triggered. Therefore, effective precautions against coal and gas outburst disasters can be put forward in accordance with stress distribution characteristics within the abovementioned “boundary area.”     

孔隙压力对煤岩基质解吸变形影响的试验研究

煤层气开采过程中,伴随着煤层气不断地吸附、解吸和渗流,煤体产生变形,极易导致煤和瓦斯突出事故。以晋城天地王坡煤矿为例,通过实验室内试验,模拟煤层气在复杂地层漫长的形成和逐渐开采过程,得到了孔隙压力与解吸量、应变的变化关系,并拟合得出其相应关系表达式,揭示了一些新的规律:(1)初期解吸速度较快,解吸量随时间的增长而不断增加,后期解吸速度减缓,解吸量逐渐趋于稳定;(2)孔隙压力与解吸量、应变呈现抛物线曲线关系,随孔隙压力的升高,吸附和膨胀变形占主导,其值均在增大;(3)存在最小孔隙压力值,随孔隙压力的增大,解吸时间增长,孔隙压力越小,吸附解吸规律越不明显,对于晋城天地王坡煤矿3#煤样,该值在1.0MPa左右;(4)不同加载方式对解吸量和变形量影响较大,先部分加载吸附后全部载荷解吸结果同比加全部载荷吸附解吸结果高13%～77%。试验结果可为煤层气(CBM)抽放安全和煤与瓦斯突出防治提供理论依据。     

突出危险煤渗透性变化的影响因素探讨

通过对突出危险煤渗透性试验研究,系统分析了不同围压、不同瓦斯压力和不同应力-应变状态条件下突出煤样的渗透特性,分别建立了突出危险煤的渗透性与围压、瓦斯压力和应力-应变等主要控制因素之间的定性和定量关系,探讨了不同载荷条件下突出危险煤渗透性的控制机制和变化规律。研究结果表明,载荷条件对突出危险煤的渗透性具有重要影响：(1) 在固定瓦斯压力条件下,突出危险煤样的渗透率随围压的增大而减小,且服从指数函数变化规律。(2) 在固定围压条件下,受Klinkenberg效应影响,渗透率与瓦斯压力之间大致呈“V”字型变化;Klikenberg效应发生在瓦斯压力p < 1 MPa的范围内。(3) 在三轴压缩下的应力-应变全过程中,不同载荷条件下突出危险煤样的渗透率-应变曲线变化趋势几乎一致,且都呈“V”字型走势;在微裂隙闭合和弹性变形阶段,煤样渗透率随应力增大而减小;进入屈服阶段后,渗透率达到最小值并在峰值强度到达之前完成反超过程;峰值强度之后渗透率持续增大直至试验结束;煤样渗透率反超后的变化要较反超前变化平缓。     

构造煤与原生结构煤中甲烷扩散性能差异性分析

原生结构煤受构造应力破坏后形成不同类型煤体结构煤,其孔隙结构、吸附/解吸、扩散和渗透能力等将发生变化,从而影响煤层气的赋存与产出.通过对沁水盆地赵庄井田3号煤层原生结构煤与构造煤样品进行甲烷等温吸附实验、低温液氮和二氧化碳吸附实验,分析了构造煤与原生结构煤的吸附?解吸性能及孔隙结构特征;应用单孔和双孔非稳态扩散模型,揭...     

含瓦斯煤岩卸围压声发射特性及能量特征分析

利用自主研发的含瓦斯煤岩热流固耦合三轴伺服渗流装置配合声发射监测系统对不同围压作用下含瓦斯煤岩进行了卸围压试验,试验结果表明:不同围压作用下含瓦斯煤岩声发射事件率、累积振铃计数与应力曲线具有较好的对应关系,振幅总体分布在[42,60]dB之间,随着振幅的增加,声发射事件率呈现出递减的趋势;含瓦斯煤岩失稳破坏时声发射事件率与围压呈线性关系,而声发射累积振铃计数与围压呈指数函数关系;不同围压下含瓦斯煤岩卸围压实验中轴向应力加载阶段和围压卸载阶段能量特征是不同的,随着围压的增大,煤岩加载阶段吸收的能量明显增大,卸载阶段释放的能量也相应的增大,加载阶段和卸载阶段能量变化与初始围压均呈对数关系。      

Evaluation of shear strength of cohesionless soil due to excess pore water pressure

The shear strength of cohesionless soil is reduced as the water pressure inside the pores of the soil mass increases. The mathematical relationship between the shear strength and the pore water pressure was derived using Mohr–Coulomb failure criteria as a function of the confining pressure and the effective angle of friction. Experimentally, a series of consolidated drained triaxial tests with back pore water pressure was run on samples of saturated uniform dense sand. The tests were conducted at different confining pressures in the range of 100–400 kPa with an increment of 100 kPa. At each level of confining pressure, the tests were repeated at different values of back pore water pressure in the range of 0–100 kPa with an increment of 25 kPa. For each test, the initial applied back pore water pressure was kept constant during the test for comparing the results at the same effective confining pressure. This study concludes that the mathematical relationship gives accurate results at any level of confining pressure and/or pore water pressure as a function of the effective angle of friction that can be evaluated using single consolidated drained triaxial test at zero back pore water pressure.     

煤层应力敏感性对煤层气产量预测的影响

大量现场实践和室内实验表明,煤层气藏为典型的应力敏感性气藏,随着煤层气的开采,储层孔隙压力降低,渗透率大幅度降低,引起产量下降。为了研究煤层应力敏感性与渗透率的关系,进行了应力敏感性实验,结果表明:煤层渗透率对于应力变化非常敏感,当有效围压从1MPa增加到12MPa时,其渗透率最大值只有初始的10%,即使围压恢复至1MPa,岩心渗透率损失率在54.98%~79.09%。由于煤层这种应力敏感性,其渗透率不再是常数,而是压力的函数,利用实验得到的动态渗透率与有效围压的关系,建立了考虑渗透率变化、带人工裂缝的预测煤层气产量的二维气、水两相数值模型,现场应用表明预测的产量与和实际产量吻合。     

History matching of enhanced coal bed methane laboratory core flood tests

Enhanced coalbed methane (ECBM) involves the injection of a gas, such as nitrogen or carbon dioxide, into the coal reservoir to displace the methane present. Potentially this strategy can offer greater recovery of the coal seam methane and higher rates of recovery due to pressure maintenance of the reservoir. While reservoir simulation forms an important part of the planning and assessment of ECBM, a key question is the accuracy of existing approaches to characterising and representing the gas migration process. Laboratory core flooding allows the gas displacement process to be investigated on intact coal core samples under conditions analogous to those in the reservoir. In this paper a series of enhanced drainage core floods are presented and history matched using an established coal seam gas reservoir simulator, SIMED II. The core floods were performed at two pore pressures, 2 MPa and 10 MPa, and involve either nitrogen or flue gas (90% nitrogen and 10% CO₂) flooding of core samples initially saturated with methane. At the end of the nitrogen floods the core flood was reversed by flooding with methane to investigate the potential for hysteresis in the gas displacement process. Prior to the core flooding an independent characterisation programme was performed on the core sample where the adsorption isotherm, swelling with gas adsorption, cleat compressibility and geomechanical properties were measured. This information was used in the history matching of the core floods; the properties adjusted in the history matching were related to the affect of sorption strain on coal permeability and the transfer of gas between cleat and matrix. Excellent agreement was obtained between simulated and observed gas rates, breakthrough times and total mass balances for the nitrogen/methane floods. It was found that a triple porosity model improved the agreement with observed gas migration over the standard dual porosity Warren-Root model. The Connell, Lu and Pan hydrostatic permeability model was used in the simulations and improved history match results by representing the contrast between pore and bulk sorption strains for the 10 MPa cases but this effect was not apparent for the 2 MPa cases. There were significant differences between the simulations and observations for CO₂ flow rates and mass balances for the flue gas core floods. A possible explanation for these results could be that there may be inaccuracy in the representation of mixed gas adsorption using the extended Langmuir adsorption model.     

贵州省岔河勘探区主采煤层瓦斯压力分析

分析岔河勘探区龙潭组主要煤层6号煤瓦斯压力的实测结果,及部分钻孔依据煤层瓦斯含量计算的瓦斯压力,得出该主采煤层瓦斯压力均大于煤层突出危险性指标的临界值0.74MPa,说明该区主煤层存在突出危险性。比较实测瓦斯压力与计算结果,认为计算结果偏小的原因与煤层瓦斯含量有关：一是在钻具提升过程中部分瓦斯逃逸,造成煤样解吸测定过程中累计解吸的气体量变小;二是气体损失存在误差;三是解吸化验时间过长,产生泄出现象,导致实测瓦斯含量偏小。     

Emission reduction technology and application research of surface borehole methane drainage in coal mining-influenced region

Windblown methane is an important gas resulting in atmospheric greenhouse effect. Therefore, reduction in windblown methane is one of the important measures to mitigate atmospheric greenhouse effect. In China, weak coal seam of low permeability is common in coal mines, so it is beset with difficulties to decrease the methane emission rate by means of gas drainage from the virgin coal seam, further to decrease the windblown mine gas. Utilizing the pressure relief and permeability and fluidity improvement effect in coal mining an approach to release methane emission through surface borehole was established, for example establishing a comprehensive deformation fracture model of surface borehole in extraction area based on quantitative rule of overlying rock movement in pit and forming a technology to select the surface borehole arrangement site in extraction area on the basis of deformation of bore frame structure and distribution characteristics of extraction flow field. And optimization technology of shape and structure of surface borehole in extraction area on the basis of ultimate stress analysis of surface borehole casing was given. The technology overcomes effectively the problem that surface borehole casing is vulnerable to premature fracture due to impact of strata movement on the surface borehole, and further increases the drainage result of the surface borehole. The technology has been test in China Shanxi Jincheng Sihe Coal Mine, achieving good results, including 12,000 Nm³/d pure methane drainage rate from single borehole, 85% methane concentration and 1.1 million Nm³ accumulative methane drainage, which demonstrate practicability and advanced performance of the technology.     

煤层脉动水力压裂动静态响应数值模型及求解

煤层脉动水力压裂是一项大范围提高煤层渗透率的新技术,但针对脉动应力波在实际地层的传播、扰动规律研究较少。采用交错网格数高阶有限差分法,结合完全匹配层和准静态围压加载两种边界条件,建立了含围压无限大弹性地层脉动水力压裂动静态响应数值模型,并研究了不同加载方式、频率、振幅、围压条件下煤层最小主应力峰值分布规律。研究表明：由于应力波传播及干涉效应,相同围压、振幅条件下脉动压裂增透扰动区域远大于准静态压裂;煤层增透面积随震源频率和振幅的增加而增大,振幅需要克服由围压控制的启动压力才能产生增透区;增透面积与地层主应力成反相关且最小主应力起主导作用,其由3.5 MPa下降至2.0 MPa时,增透面积最高可提升600%。模型可模拟准静态和脉动加载下煤层的力学响应,其结果可为优化脉动水力压裂工艺参数提供指导。     

空化水射流热效应影响煤体渗透率试验研究

对空化水射流空泡溃灭瞬间和空化噪声声场作用产生的热效应进行了分析,得到了热效应作用下的煤体渗透率表达式,并开展了不同空化条件(泵压、空化腔围压)下的煤体温度和瓦斯渗流试验。结果表明:空化腔围压一定时,温度随泵压的增大而升高;而泵压一定时,温度随空化腔围压的增大而减小。当热应力大于平均有效应力时,煤体热膨胀出现微变形,产生的张应力使裂隙变宽、数量增多;热效应使得游离瓦斯内能增加,分子自由程大大缩短,同时吸附瓦斯解吸使得煤基质发生收缩效应,导致微孔隙及微裂隙结构产生膨胀变形,从而影响煤体渗透率。试验得到渗透率随温度升高呈指数规律增大,与理论得到的渗透率表达式描述的规律相符。