基于水化进程的泥岩地层井壁坍塌机理研究

Mudstone formation is one of the main strata encountered in drilling engineering, wellbore instability caused by hydration of mudstone is always one of the difficult engineering problems to be solved. Most of the existing researches on the wellbore instability caused by the hydration of mudstone only consider the influence of the total water content on the rock strength and hydration strain of mudstone from the macroscopic level, and fail to fully reveal the mechanism of action of the microscopic hydration process of mudstone on the wellbore instability. In this project, the type and quantity of absorbed water was determined using thermosgravimetry-differential scanning calorimetry (TG-DSC) method. A theoretical model for quantitatively characterizing the microscopic hydration process of mudstone by using the content of absorbed combined water was established. The variation law of hydration strain and total water content with absorbed combined water content in microscopic hydration process was defined. Based on the results of non-destructive rock-electricity tests and rock mechanics experiments, the relationship between the microscopic hydration process of mudstone and the macroscopic rock mechanics parameters was constructed. The prediction model of wellbore collapse pressure for mudstone stratum considering hydration process is established, the collapse instability mechanism of mudstone stratum is studied, the main controlling factors of mudstone stratum instability are clarified, and the engineering and technical countermeasures for preventing mudstone stratum wellbore collapse are explored. The research results enrich the theory of wellbore instability in mudstone formation and have important guiding significance for the safe, economic and efficient development of the oil and gas resources.

泥岩地层是油气井钻探过程中钻遇的主要地层之一，因泥岩水化作用导致的井壁失稳问题一直是亟待解决的工程技术难题之一。现有关于泥岩水化作用引发的井壁失稳研究大都仅从宏观层面考虑总含水量对泥岩强度和水化应变的影响，未能完全揭示泥岩的微观水化进程及其对井壁失稳的作用机理。本项目拟以同步热重-差示扫描量热分析法为基础，定量测量泥岩水化进程中的吸附水类型和含量，建立利用结合水含量定量表征泥岩水化进程的理论模型，明确水化进程中水化应变和总含水量随结合水含量的变化规律；基于岩芯无损岩电测试结果和岩石力学实验结果，构建泥岩微观水化进程与宏观岩石力学特性参数之间的关系；建立考虑水化进程作用的泥岩地层井壁坍塌压力预测模型，研究泥岩地层坍塌失稳机理，明确泥岩地层失稳主控因素，探索预防泥岩地层井壁失稳的工程技术对策。研究成果丰富了泥岩地层井壁失稳理论，对油气资源的安全经济高效开发具有重要的指导意义。

泥页岩井壁失稳一直是钻井工程中一个复杂的世界性难题，通常认为泥页岩地层失稳是因为钻井液滤液进入地层与泥页岩中的黏土矿物发生化学作用导致井壁垮塌。对泥页岩地层的失稳研究从纯力学、钻井液化学研究，到力-化耦合研究，再到力学、化学、热力学等多场耦合研究。但这些研究大都是仅从宏观层面上分析泥页岩的失稳原因，不能从微观上揭示泥页岩失稳的机理。.为明确泥页岩的微观水化进程及其对井壁稳定性的影响规律，首先，通过XRD测试明确工区泥页岩的黏土成分，以同步热重-差示扫描量热分析法测量单一黏土矿物在水化进程中的吸附水类型和含量，定量表征黏土矿物水化进程中总含水量随结合水含量的变化规律，通过线性叠加的方式得到泥页岩水化进程定量表征模型；其次，基于岩芯无损岩电测试装置测试结果构建岩电参数与含水量关系，结合岩石力学实验，建立泥岩微观水化进程与宏观岩石力学特性参数之间的关系；再次，建立横观各向同性泥页岩水平井渗流模型，明确渗流附加应力和井筒含水量的分布；最终，建立泥页岩地层坍塌压力模型，明确失稳机理及其主控因素，探索形成防塌工程技术对策。主要认识有：.（1）伊利石、高岭石、绿泥石和工区泥页岩结合水及含水量关系模型分别为：ωib=2830ρ﹒ωi0.75∑ASi﹒ti，ωkb=13900ρ﹒ωk0.057∑ASi﹒ti，ωcb=2680ρ﹒ωc0.87∑ASi﹒ti和ωb=Xωi+Yωk+Zωc。.（2）岩石力学特性参数与电阻率或含水量之间的相关性：dE/dW=-0.5E1﹒E2/(ωb-ωi)0.5﹒e^(-E2﹒(ωb-ωi)0.5)，dμ/dW=1.289，C=C0-57.134﹒(ωb-ωi)，Φ=Φ0-192.513﹒(ωb-ωi)，E1=3.819×104，E2=-10.691。.（3）高渗透率方向渗流压力传递更快，径向渗流应力更小，周向渗流应力更大，轴向渗流应力不变；附加渗流应力随渗透率比值、储层厚度、泊松比增大而增大，随储层宽度增大而减小，孔隙压力的变化规律与附加渗流应力相反；地层含水饱和度和含水量随着钻井时间的增加而增加。.（4）当同时考虑水化和渗流作用时，泥页岩井壁坍塌压力存在坍塌周期；当钻井时间较短且不考虑水化，井壁稳定性好；当钻井时间长，考虑水化时坍塌压力高，井壁易塌。基于理论研究成果，结合现场，形成泥页岩地层防塌措施。