复合加卸载条件下含瓦斯煤渗流特性及其应用研究

发布时间：2018-07-06 18:41

  本文选题：复合加卸载 + 有效应力　； 参考：《中国矿业大学(北京)》2015年博士论文

【摘要】：瓦斯抽采作为矿井瓦斯治理的基本技术手段之一,在突出矿井、高瓦斯矿井以及瓦斯矿井的深部采区发挥着至关重要的作用。煤层受采动影响后,煤层应力重新分布,煤体局部发生损伤甚至破坏变形,煤体孔隙一裂隙结构和渗透能力发生改变,孔隙率和渗透性的改变不仅会导致瓦斯渗流速度和孔隙压力分布的变化,而且会引起煤层应力和位移的改变。对于水力扩孔后的抽采钻孔而言,其周围煤体中同样存在着采动应力场和瓦斯渗流场的耦和效应,水力扩孔形成的钻孔开挖效应对煤体渗透性的影响比较明显。为了深入探讨水力扩孔开挖效应对钻孔周围煤体渗透率的影响,本文以本煤层水力扩孔工艺技术为工程背景,开展复合加卸载条件下含瓦斯煤样渗流特性试验,建立抽采钻孔周围煤体流固耦合模型,进行普通抽采钻孔和水力扩孔钻孔周围煤体应力场分析及瓦斯渗流规律数值模拟,最后运用体现工作面瓦斯抽采效果的煤层残余瓦斯含量和钻孔瓦斯流量等数据,与数值模拟结果进行对比验证。首先,基于钻孔周围煤体切向应力由原岩应力先升高至峰值强度而后减小至残余强度、径向应力由原岩应力逐步递减直至破坏卸荷的演变特征,以受载含瓦斯煤体渗流特性试验装置为实验平台,开展了常规三轴加载和复合加卸载应力路径下含瓦斯煤样渗流特性试验,研究了围压、轴压、应力路径、孔隙压力与煤样渗透率之间的定性定量关系。常规三轴加载应力路径为固定围压的同时,持续增加轴压直至煤样进入残余强度阶段；复合加卸载应力路径为持续增加轴压的同时,减小围压直至煤样进入残余强度阶段,尤其是复合加卸载应力路径与钻孔周围煤体应力变化过程极为相似。常规三轴加载含瓦斯煤样和复合加卸载含瓦斯煤样的全应力-应变曲线均可以分为煤样压密阶段、线弹性阶段、屈服阶段、峰后软化阶段和残余强度阶段等五个阶段；在固定轴压和围压的条件下,常规三轴加载煤样渗透率随着孔隙压力的升高呈现先减小而后增大的“V”字形变化规律；在固定轴压和孔隙压力的条件下,常规三轴加载煤样渗透率随着围压的增大而呈现指数函数规律性下降；在屈服强度前后,常规三轴加载煤样体积应变与渗透率均呈指数函数关系变化。复合加卸载含瓦斯煤样的峰值强度较常规三轴加载含瓦斯煤样的峰值强度更低；复合加卸载煤样峰值强度时的轴向变形均小于常规三轴加载煤样峰值强度时的轴向变形,复合加卸载煤样峰值强度时的径向变形均大于常规三轴加载煤样峰值强度时的径向变形；随着轴向应变的持续增大,复合加卸载煤样渗透率呈现先减小后增大的变化规律；复合加卸载条件下卸载围压引起煤样屈服后渗透率的增加量较常规三轴加载煤样屈服后渗透率的增加量更大；卸载起始轴压的改变可引起加卸载阶段煤样渗透率差异性演变；卸载起始时刻煤样渗透率随着卸载起始围压的增大呈现指数函数规律性下降；相同的卸载起始轴压和卸载起始围压时,随着孔隙压力的升高,复合加卸载煤样峰值强度呈线性降低,从而导致煤样失稳破坏的加速出现,进而缩短了煤样渗透率发生剧烈改变的时间。接着,开展了复合加卸载应力路径下煤样孔隙率测定实验,在分析复合加卸载煤样有效应力作用机制的基础上,从孔隙率的基本定义出发,同时考虑煤样吸附膨胀变形、孔隙气体压缩变形和热膨胀变形等影响因素,建立复合加卸载条件下煤样孔隙率演化模型；对比国内外各类渗透率演化模型,以立方定律和受载煤体应力-应变本构方程为桥梁,建立复合加卸载条件下煤样渗透率演化模型；考虑瓦斯流动克林伯格效应、瓦斯吸附-解吸-扩散过程的传质特征、煤体的吸附膨胀效应等影响因素,建立复合加卸载含瓦斯煤体非线性渗流场方程；通过对钻孔周围煤体弹塑性分析,求解得到钻孔周围煤体应力场、位移场的解析表达式；通过耦合变量,实现应力场、变形场、渗流场等多物理场耦合,并最终实现钻孔周围含瓦斯煤体流固耦合模型的建立。其中,不同卸载起始轴压条件下,复合加卸载煤样的孔隙率演变规律表现为沿着加卸载应力路径的逐步推进,煤样内孔隙率均呈现先减小而后增大的变化规律,但平均有效应力与煤体孔隙率的关系曲线却存在较大差异；复合加卸载煤样在外界应力和孔隙压力的共同作用下,煤样在不同的受载阶段均受到有效应力的约束,区别之处在于不同受载阶段时三类有效应力各自所占的比重存在着差异；对于复合加卸载煤样而言,在压密阶段和线弹性阶段有α→φ,在屈服阶段和峰后软化阶段有φ≤α≤φd,在残余强度阶段有φd≤α≤φc；复合加卸载煤样孔隙率演化方程及渗透率演化方程可以分别表述为然后,以复合加卸载应力路径下含瓦斯煤样的轴向应力-轴向应变-渗透率演变规律为理论基础,结合钻孔开挖后周围煤体应力重分布规律,分析了抽采钻孔周围煤体的渗透率演变特征,将抽采钻孔周围煤体视为黏弹塑性介质,从而将抽采钻孔周围煤体从钻孔孔壁到煤体深处依次划分为残余强度区、塑性软化区、黏弹性区和原始应力区,其中残余强度区和塑性软化区内的煤体经历过峰值应力的作用,共同组成了极限应力区,该区域的范围大小决定了钻孔抽采效果的好坏；建立了考虑煤体流变、剪切扩容和塑性软化特性的钻孔周围煤体黏弹塑性模型,并推导了抽采钻孔周围不同区域内煤体应力-应变解；在考虑克林伯格效应、瓦斯吸附-解吸-扩散过程的传质特征、煤体吸附膨胀效应和孔隙率、渗透率动态演变规律等影响因素的基础之上,建立了包括钻孔周围煤体内瓦斯流动非线性渗流方程,渗透率演变方程和钻孔周围不同区域煤体的切向应力-径向应力-体积应变方程等方程的抽采钻孔周围含瓦斯煤体流固耦合模型；运用数值模拟软件COMSOL,引入钻孔周围含瓦斯煤体流固耦合模型,以工作面瓦斯抽采为背景,分析了普通抽采钻孔周围煤体应力、应变变化规律和抽采后煤层瓦斯含量、煤层渗透率变化规律及影响因素。其中,沿着远离钻孔方向,钻孔周围煤体切向应力整体上表现为先增高至峰值、而后下降至原岩应力的演变特征,钻孔周围煤体径向应力则表现为逐渐升高至原岩应力的演变规律；相同应力环境和煤层赋存条件下,随着钻孔孔洞半径的增加,钻孔卸压范围不断增大；不同抽采时刻条件下,钻孔周围煤体渗透率沿着远离钻孔方向大致呈现先减小而后又有所恢复的规律；随着抽采时间的延长,钻孔周围一定范围内煤体残余瓦斯含量均呈现逐渐下降的趋势；此外,模拟得到钻孔抽采30天时的有效影响半径为2.77m,与现场实测值差别较小最后,针对水力扩孔工艺技术的特点,选取试验矿井典型工作面,对水力扩孔技术应用效果进行现场考察,重点考察工作面水力扩孔后钻孔瓦斯抽采效果,建立顺层扩孔钻孔开挖模型,借助钻孔周围含瓦斯煤体流固耦合模型,分析了顺层扩孔钻孔周围煤体应力分布规律和瓦斯渗透率变化规律,并和普通抽采钻孔周围煤体应力分布规律和瓦斯渗透率变化规律进行了对比,同时利用现场考察结果验证数值模拟所得到的扩孔钻孔周围煤体渗透率演变规律的正确性。其中,钻孔扩孔完成之后,扩孔钻孔周围煤体的径向应力峰值有所降低,钻孔周围煤体所受切向峰值应力向远离钻孔的方向转移；水力扩孔技术的实施扩大了抽采钻孔的有效卸压范围,普通抽采孔和扩孔钻孔在抽采一段时间后,其周围煤层残余瓦斯含量均发生不同程度的下降,但扩孔钻孔较普通抽采孔的下降程度更大、下降范围更广；扩孔钻孔瓦斯抽采量相对于普通抽采钻孔无明显提高,但抽采钻孔经水力扩孔之后,钻孔瓦斯流量衰减系数减小,钻孔瓦斯有效抽采时间增长；水力扩孔影响范围内钻孔月瓦斯抽采量相比未经扩孔影响的普通抽采钻孔有显著提高。本论文的创新点主要体现在：(1)依据钻孔开挖效应对钻孔周围煤体应力变化的影响,设计了复合加卸载条件下含瓦斯煤渗流特性实验,实验得到了卸载起始轴压、卸载起始围压、孔隙压力和应力路径对煤体渗透率演变的影响规律；(2)设计了复合加卸载应力路径下煤样孔隙率测定实验,并建立了考虑煤样有效应力作用机制、煤样吸附膨胀变形、孔隙气体压缩变形和热膨胀变形等因素的复合加卸载下煤样孔隙率及渗透率演化模型：(3)在考虑克林伯格效应和瓦斯吸附-解吸-扩散过程的传质特征、煤体吸附膨胀效应、热膨胀效应、孔隙气体压缩效应等影响因素的基础上,建立了抽采钻孔周围含瓦斯煤体非线性瓦斯渗流控制方程,通过抽采钻孔周围煤体瓦斯流动规律数值模拟结果和现场实测抽采钻孔周围煤层残余瓦斯含量的对比分析,对非线性瓦斯渗流控制方程的正确性进行了验证。
[Abstract]:In order to study the influence of coal seam stress redistribution on the permeability of coal body around the borehole , the influence of gas seepage velocity and gas seepage field on the permeability of coal body is studied .
The stress path of composite loading and unloading is to increase the axial pressure continuously , the confining pressure is reduced until the coal sample enters the residual strength stage , especially the stress change process of composite loading and unloading stress path is similar to that of the coal body stress change process around the borehole .
Under the condition of fixed axial pressure and confining pressure , the normal triaxial loading coal sample permeability decreases with the increase of pore pressure and then increases the variation rule of " V " shape .
Under the condition of fixed axial pressure and pore pressure , the normal three - axis loading coal - like permeability decreases with the increase of confining pressure and the regularity of exponential function decreases ;
Before and after the yield strength , the volumetric strain and permeability of the conventional triaxial loading coal sample varied with exponential function . The peak intensity of the composite loading and unloading gas - containing coal sample was lower than that of the conventional triaxial loading gas - containing coal sample .
the axial deformation of the composite loading and unloading coal sample peak intensity is less than that of the conventional triaxial loading coal sample peak strength , and the radial deformation when the peak intensity of the composite loading and unloading coal sample is greater than that of the conventional triaxial loading coal sample peak strength ;
With the sustained increase of axial strain , the permeability of composite loading and unloading coal sample decreased first and then increased .
Under the condition of compound loading and unloading , the increase of permeability after loading coal sample under the condition of unloading confining pressure is much larger than that of the conventional triaxial loading coal sample yield .
The variation of coal sample permeability in the loading and unloading stage can be caused by the change of unloading initial axial pressure .
The coal sample permeability decreases with the increase of unloading initial confining pressure and the regularity of exponential function decreases .
On the basis of analyzing the effective stress action mechanism of the composite loading and unloading coal sample , the coal sample porosity evolution model is established based on the analysis of the effective stress action mechanism of the composite loading and unloading coal sample , the influence factors of the coal sample adsorption expansion deformation , the pore gas compression deformation and the thermal expansion deformation are taken into consideration , and the coal sample porosity evolution model is established under the condition of composite loading and unloading .
The permeability evolution model of coal sample under the condition of complex loading and unloading is established by using cubic law and the stress - strain constitutive equation of the coal - loaded coal body as the bridge in comparison with the various models of permeability evolution at home and abroad .
Considering the influence factors such as the mass transfer characteristics of the gas flow Klingberg effect , the gas adsorption - desorption - diffusion process , the adsorption expansion effect of the coal body and the like , a nonlinear seepage field equation of the gas - containing coal body is established and unloaded ;
Through the elastic - plastic analysis of the coal body around the borehole , the analytical expression of the stress field and displacement field of the coal body around the borehole is obtained .
The coupling variables are coupled to realize the coupling of multiple physical fields such as stress field , deformation field , seepage field and so on .
Under the combined action of external stress and pore pressure , the coal samples are subject to effective stress at different loading stages . The difference lies in the difference of the specific gravity of three types of effective stress at different loading stages .
For composite loading and unloading coal samples , there is 伪 鈫，

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