基于Hoek-Brown准则的煤层钻孔失稳破坏类型数值模拟研究

Numerical simulation of instability and failure types of coalbed borehole based on Hoek-Brown criterion

高瓦斯松软煤层、软硬复合煤层和突出煤层深孔钻进是公认的世界性难题,已成为制约部分高瓦斯和突出矿井瓦斯治理效果的瓶颈。为深入探讨煤层钻孔失稳机制,引入Hoek-Brown强度准则及地质强度指标GSI,在考虑孔隙率与渗透率动态变化特性的前提下,在弹塑性本构方程中引入塑性流动因子,使瓦斯渗流场方程含有体应变、煤岩体变形场方程含有瓦斯压力项,进而建立了基于H-B准则的含瓦斯煤流固耦合动态模型;采用COMSOL软件对模型进行解算,探讨了不同侧压系数条件下不同煤体结构煤层钻孔周围塑性区分布规律,并对煤层钻孔失稳破坏类型进行了划分。研究表明:无论在何种侧压系数条件下,孔周塑性区分布范围皆随GSI值的增大而逐渐减小,2者遵循幂函数关系;塑性区分布形状因侧压系数、GSI值不同而差别较大;当GSI值较小时,在不同侧压系数条件下,塑性区分别呈扇形、椭圆、"X"状分布;当GSI值较大时,塑性区分别呈圆形、"V"状、"平底锅"状分布;煤层钻孔失稳破坏类型分为应力控制型破坏、结构面控制型破坏、软煤中的喷孔破坏3类,分别发生在高应力硬煤中(60≤GSI≤100)、强度中等的煤体中(30<GSI<60)以及软煤中(0≤GSI≤30),失稳破坏的主控因素分别为应力、结构面力学性质、煤体力学性质及瓦斯内能。

It is well known that the deep hole drilling in soft coal seams containing high pressure gas,hard and soft compound coal seams and outburst coal seams is a worldwide problem,which has become a bottleneck restricting the gas control effect in some high gas and outburst coal mines.In order to reveal the instability and failure mechanism of boreholes drilled in above-mentioned coal seams,based on the Hoek-Brown failure criterion,geological strength index(GSI),and the dynamic characteristics of porosity and permeability of coal containing gas,the plastic flow factor is introduced into the elastoplastic constitutive equations,so that the coal body strain is contained in the gas seepage field equation and the gas pressure is contained in the deformation field equation of coal and rock,and then the fluid-solid coupling dynamic model is established.The model is implemented by using COMSOL software.The distribution law of plastic zones around the boreholes with different coal structures under different side pressure coefficients(SPC)is discussed,and the instability and failure types of borehole in coal seam are classified.Research results indicate that the distribution ranges of the plastic zones around the boreholes decrease gradually with the increase of the GSI value under different SPC,and there is a typical power function relation between the radius of plastic zones and GSI values.However,the shapes of plastic zones vary according to different SPC and GSI values.When GSI value is smaller,the shapes are approximately shaped like fans,ellipses,and the letter"X"under different SPCs.Otherwise,the shapes are approximately shaped like circles,the letter"V",and pans.The failure types of borehole in coal seam can be divided into stress-controlled failure,structural plane-controlled failure,and coal and gas blowout in soft coal seams,which respectively occur in hard coal seams whose GSI values vary from 60 to 100,coal mass with moderate strength,whose GSI values vary from 30 to 60,and soft coal seams,whose GSI values vary from 0 to 30.The main controlling factors of above-mentioned failure are stress,mechanical properties of structural surface,mechanical properties of coal body and gas internal energy.