超深层孔隙-裂缝型致密砂岩地层的井壁失稳热流固耦合机理研究

The existing model regards the fractures as weak plane and thus the thermo-hydro-mechanical coupling theory of single-porosity porous medium is adopt to study the mechanism of wellbore instability of fractured tight sand in ultra-deep formation. However, the coupling effect between the matrix block and fracture system is ignored. Under the framework of the daul-poroelastic theory, it is conceptualized as two separate and overlapping continua: matrix block and fractures, and each has its own mechanical deformation characteristics and physical parameters (dual-porosity and dual-permeability) in this study. Combining the phenomenological method (micromechanical approach associated with experimental analysis) with the system macroscopic approach (utilizing the conservation equations including mass, momentum and energy and entropy production theory to establish different forms (including local thermal equilibrium(LTE) and local thermal non-equilibrium (LTNE) theory) of Clausius-Duhem inequality for fractured porous medium) to establish the governing equation for the behavior of fractured porous medium considering the thermo-hydro-mechanical coupling. When the plane strain assumption holds, one derive the three-dimensional numerical and analytical solutions about the field variables around arbitrary deviated borehole under non-hydrostatic stress field in the fractured porous formation. Through the comparison between the new model and the existing model, it clarifies the mechanism of wellbore instability considering thermo-hydro-mechanical coupling for fractured tight sand in ultra-deep formation and proposes the engineering solutions.

针对超深层孔隙-裂缝型致密地层的井壁失稳机理，现有模型将孔隙-裂缝型致密砂岩中的裂缝视为弱面，采用单重孔隙介质的热流固耦合理论进行研究，忽略了基岩块体与裂缝系统间的相互耦合作用。本项目以双重孔隙弹性理论为基础，将孔隙-裂缝型致密砂岩概念化为独立但重叠的两个连续性介质（基岩块体和裂缝系统具有各自的力学变形特征和物性参数（即双孔隙度双渗透率型））。采用唯象方法（实验微观力学分析）和系统宏观方法（应用质量、动量和能量守恒方程及熵增原理，建立不同形式（局部热平衡（LTE）和局部热非平衡（LTNE）理论）的克劳修斯-杜亨不等式）相结合，用以构建孔隙-裂缝介质热流固耦合行为的控制方程。基于该控制方程，推导出平面应变条件下，孔隙-裂缝型地层内钻任意斜井眼井周围岩场变量的三维数值解和解析解。通过新模型与已有模型对比分析，阐述超深层孔隙-裂缝型致密砂岩地层的井壁失稳热流固耦合机理，提出工程解决对策。

以双重孔隙弹性理论为基础，将孔隙-裂缝型致密砂岩概念化为独立但重叠的两个连续性介质(基质系统和裂缝系统具有各自的力学变形特征和物性参数(即双孔隙度双渗透率型))。采宏观热动力学方法(应用质量守恒、动量守恒和能量守恒方程及熵增原理)，建立不同形式(局部热平衡(LTE) 和局部热非平衡(LTNE)理论)的克劳修斯-杜亨不等式，进而确定孔隙-裂缝介质热流固耦合行为的控制方程的基本框架。结合唯象方法确定出双重孔隙热弹性耦合模型本构方程的孔隙热弹性参数。基于平面应变条件，推导出孔隙-裂缝型地层钻任意斜井眼的井周围岩场变量的三维解析解。研究表明，超深层孔隙-裂缝型致密砂岩地层的井壁失稳机理为热流固耦合作用导致安全钻进密度窗口具有时间相关性。随井眼钻开后间隔时间增加，破裂压力减小而坍塌压力增加致使安全密度窗口逐渐变窄。裂缝发育程度与安全密度窗口大小呈正相关性。该项研究量化了裂缝尺寸的封堵比例与安全密度窗口的关系，对钻前合理设计“含时间相关性的安全密度窗口”提供了理论依据，为改善超深层孔隙-裂缝型致密砂岩地层井眼稳定问题提供了新方法和新思路。