【作者】 李玉伟；

【导师】 艾池；

【作者基本信息】 东北石油大学 ， 石油与天然气工程， 2014， 博士

【摘要】 水力压裂是目前煤层气开发的主要技术手段，通过压裂能够在煤层内部形成复杂裂缝网络，并在井筒周围形成大面积的压力下降，使煤层气解吸表面积增大，保证煤层气迅速并相对持久地泄放。煤岩发育大量的割理裂隙等结构弱面，具有强度低、弹性模量低、泊松比高、易碎等性质，使煤岩与非煤岩石力学特性存在较大差别，应先对煤岩的力学特性开展研究，在掌握煤岩力学特性的基础上研究煤储层的压裂起裂力学机理。论文研究首先对试验区取样煤进行割理特征的宏观和细观测量，包括割理密度、长度和宽度等指标的测定，评价试验区煤岩的割理发育状况。其次，开展取样煤岩力学特性室内试验研究，通过试验测定不同取心方向煤样的抗拉和抗剪切强度、单轴和三轴抗压强度以及弹性模量和泊松比等重要力学参数，试验表明不同取心方向煤岩力学参数差异明显，存在明显各向异性特征。根据弹性力学理论，建立了煤岩等效弹性模量和泊松比计算模型，应用模型计算并讨论了煤岩等效弹性模量和泊松比与煤岩基质、面割理和端割理力学参数间的关系，当取心方向垂直面割理时，煤岩弹性模量取决于煤岩基质弹性模量和面割理法向刚度及间距的大小，而泊松比仅与煤岩基质弹性模量和泊松比、面割理法向刚度和间距有关；取心方向平行面割理时，煤岩弹性模量取决于煤岩基质弹性模量和端割理法向刚度与间距的大小，泊松比则由煤岩基质弹性模量和泊松比、端割理法向刚度和间距的大小所决定。考虑煤岩独特的力学特性，建立了正交各向异性煤储层井壁围岩应力分布的计算模型；基于岩石破裂的最大拉应力准则，推导了煤层裸眼完井水力压裂起裂压力计算公式；研究表明煤层裸眼完井井壁周向应力与煤岩基质和割理力学特征密切相关，煤岩基质弹性模量和泊松比、面割理与水平最大主应力方向夹角、面割理分布密度以及端割理分布密度均影响井壁应力分布状况，并且面割理与水平最大主应力方向相同时，压裂最容易起裂，面割理间距增加，裸眼井压裂所需的起裂压力增大。最后，应用有限元方法模拟了不同射孔参数、不同地应力状况以及煤岩不同力学特性条件下正交各向异性煤层射孔完井的水力压裂起裂压力，得出了射孔方位、射孔半径和射孔深度对起裂压力的影响；分析了不同方向地层主应力变化对射孔完井起裂压力的影响规律；分别讨论了煤岩基质弹性模量和泊松比、面割理与水平最大主应力方向夹角及面割理间距对起裂压力的影响。论文提出将具有割理弱面的煤岩非连续介质等效为连续介质研究，并通过理论模型计算描述了煤岩等效连续介质的力学参数变化规律，在此基础上建立了煤层水力压裂的起裂压力计算模型，论文成果为今后煤岩复杂割理裂隙系统下的水力压裂机理研究提供了新的思路和方向。更多还原

【Abstract】 The main technical means for developing coalbed methane is hydraulic fracturing, it canform complex fracture in coalbed, make the pressure drop in a large area around the wellbore,increase the surface area for coalbed methane desorption, and guarantee coalbed methanerelease quickly and relatively permanently. There are lots of weak structure surface in coalbedsuch as cleats and cracks, which have the characters of low strength, low Young’s modulus,high Poisson’s ratio and frangibility, it make difference in rock mechanics properties betweencoal and rock. Studying fractures cracking mechanism must be basing on mastering coalbedmechanical characteristics.In this paper, firstly the macroscopic and mesoscopic measurement of the coalbed cleatscharacters were conducted, which included cleats density, length and width, and the cleatdevelopment status in the test area was evaluated.Secondly, the indoor experiments for measuring the coalbed mechanics characteristicswere carried out and the measured parameters included tensile and shear strength, uniaxialand triaxial compressive strength and Young’s modulus, Poisson’s ratio in different directionof coring. It can be concluded from the experiments that the coalbed mechanics parameterswere difference in different coring direction, existing obvious anisotropic characteristics.The equivalent Young’s modulus, Poisson’s ratio calculating model of the coalbed wasestablished by elastic mechanics theory. The relationship between equivalent Young’smodulus, Poisson’s ratio and coalbed matrix, face cleats, butt cleats mechanical parameterswas calculated by using the model. When the coring direction is vertical to the face cleats, theYoung’s modulus is depending on coalbed matrix Young’s modulus, the normal stiffness offace cleats and the size of the spacing. And the Poisson’s ratio is only related to coalbedmatrix Young’s modulus, Poisson’s ratio, the normal stiffness of face cleats and the size of thespacing. When the coring direction is parallel to the face cleats, the Young’s modulus isdepending on coalbed matrix Young’s modulus, the normal stiffness of butt cleats and the sizeof the spacing. And the Poisson’s ratio is determined by coalbed matrix Young’s modulus,Poisson’s ratio, the normal stiffness of butt cleats and the size of the spacing.The borehole wall surrounding rock stress distribution calculating model for orthotropiccoalbed was established considering the unique mechanical properties of coalbed. Thefractures cracking pressure calculating formula in fracturing for coalbed open holecompletion was deduced basing on the maximum tensile stress failure criterion. By studying, open hole completion borehole wall circumferential stress was closely related to the coalbedcleats mechanics characteristics. The stress distribution status was affected by the anglebetween the face cleats and horizontal maximum principal stress, face cleats distributiondensity, butt cleats distribution density and the connected rate of butt cleats. When thedirection of face cleats was the same as maximum principal stress direction, the fractureswere easily to crack. With the increasing of face cleats spacing, the fractures crackingpressure needed for open hole completion was increasing.Finally, the fractures cracking pressure calculating model for perforation completion inorthotropic coalbed was simulated by finite element method with different perforationparameters, ground stress status, and coalbed mechanics parameters, and the effects ofperforation azimuth, radius and depth on cracking pressure was obtained. The effects ofprincipal stress variation in different direction on perforation completion cracking pressurewere analyzed. And the effects of coalbed matrix Young’s modulus, Poisson’s ratio, the anglebetween face cleats and horizontal maximum principal stress and face cleats spacing onfractures cracking pressure were discussed.The coalbed is discontinuous medium, which has the weak surface with cleats, and it isequivalent to continuum when studying in the paper. The mechanics parameters variationregulations of coalbed equivalent continuum were described by theoretical model, and onwhich the fractures cracking pressure calculating model in fracturing was established. Theachievements of the paper provide new thought and direction for studying hydraulicfracturing mechanism under coalbed complicated cleats system.更多还原