页岩气水平井压裂与生产套管变形机理及其控制机制研究

To find the mechanism of abnormal deformation of horizontal shale gas well casing during fracturing and producing, according to the characteristics of trajectory and casing procedures as well as shale gas reservoirs, laws of conservation of quality, energy, momentum and the first law of thermodynamics will be used to establish equations and definite conditions, considering the correlations between pressure, temperature and physical parameters of well fluids as well as the longitudinal heat transfer and unsteady radial heat transfer. Than, circulative iteration inversion method will be used to analyze the temperature fields and pressure fields of well bore fluids and annulus as well as bounding rocks. Also analyzed will be the annular pressure down and buildup of trap fluids due to expansion or compression induced by temperature changing. The mechanical models of curved casings loaded by non-uniform external pressure and longitudinal force, bending moment, shear as well as internal pressure will be found. The mechanism and criteria burst strength and collapse strength of curved casings exerted by the above complicated loads will be studied by small parameter perturbation method and FLAC-3D. Based on study and analyses mentioned above, the factors, especially the major factors resulting in the abnormal deformation of horizontal shale gas well casing will be found out. That will be guide to optimize well design and choose fracturing and producing parameters. The models and methods developed and applied in this project may also be used for similar heat transfer engineering and buckling of revolutionary cylinders.

针对页岩气水平井压裂和生产套管异常变形现象，根据井身结构及储层特点，考虑流体温度、压力与流体物性参数的耦合，运用质量、动量、能量守恒原理及热力学第一定律，根据井筒流体纵向传热和井筒对地层非稳态传热特点，分别建立方程并给出边界条件，采用递推法循环迭代，得到井筒及套管与井眼环空和围岩的温度场与压力场；分析束缚空间内流体因温度降低收缩导致的压力降低和因温度升高膨胀导致的压力升高；考虑弯矩、剪切、轴向力共同作用，建立非均匀外压下弯曲套管横向屈曲变形微分方程，用试函数法和小参数摄动法建立套管的稳定性判断准则，得到临界载荷；结合FLAC-3D分析和实验验证，分析页岩气水平井套管挤毁变形和破裂变形机理；探寻套管变形主控因素与主控机制，为水平井优化设计及压裂与生产参数优化控制提供指导；提高液体注入与气井生产过程中井筒和环空压力场与温度场分析的科学性，丰富套管及类似圆筒结构非均匀外压横向屈曲变形分析方法。

页岩气水平井现场压裂过程和后续排液、生产过程中，经常出现套管变形，后续压裂泵送桥塞等工具难以通过或后续生产井下工具难以下入，而常规的套管力学分析方法又难以解释页岩气水平井套管的异常变形，现场难以选择套管和优化施工参数，制约了页岩气的压裂规模、压裂分级和安全生产。为此，提出了“页岩气水平井压裂与生产套管变形机理及控制机制研究”项目：根据页岩气水平井井眼轨迹和井身结构及储层特点，考虑页岩气水平井压裂排量大、泵压高、泵入液量大的特点，考虑页岩气产出过程中的相变，考虑流体温度、压力与流体物性参数的耦合，运用质量、动量、能量守恒定律及热力学第一定律，根据井筒流体纵向传热和井筒对地层非稳态传热特点，分别建立了压力温度耦合方程并给出定解条件，采用递推法循环迭代，得到了井筒及套管与井眼环空和围岩的温度场与压力场；分析了压裂与生产过程中套管内流体的压力场及温度场，分析了套管与井眼环空及围岩的温度场及套管的围压载荷，分析了套管外束缚空间内流体收缩或膨胀导致的环空压力降低或环空压力升高；考虑弯矩、剪切、轴向力共同作用，分析了非均匀外压下弯曲扁化套管的挤毁变形机理，分析了非均匀外压和内压作用下弯曲扁化套管的破裂变形机理；结合FLAC-3D分析和实验验证，分析了页岩气水平井压裂与生产套管变形的影响因素、主控因素与控制机制。研究表明：异常压力、井眼弯曲附加弯矩及剪切、大排量压裂注入使围岩岩石力学参数发生变化及地应力重新展布是复合载荷作用下套管变形的主要原因。通过查明套管变形主控因素与主控机制，据此从井眼轨迹设计、井身结构设计、套管选型、压裂与生产参数优化等方面提出页岩气水平井压裂与生产套管变形控制措施，可以为类似致密储层水平井压裂及生产参数控制提供理论指导，提高了气井注入与生产过程中的井筒和环空压力场与温度场分析的科学性，也丰富了套管及类似圆筒结构非均匀外压挤毁变形分析方法。