Abstract
Yi 202 block has experienced multiple phases of tectonic movement, resulting in extensive development of natural fractures in the region. Actual drilling indicates that the presence of these natural fractures poses great challenges to drilling operations. However, the wells in this area are ultra-deep, and many sets of complex and difficult-to-drill formations are encountered in top-down drilling, which brings significant challenges to the drilling engineering. Based on triaxial and shear experiments, transverse isotropy theory and weak surface criterion, and integrating the mechanical damage under stress unloading and hydration in drilling wells, a well wall stability model under the synergistic effect of stress unloading and hydration has been established, and an analysis of the factors influencing the stability of the well wall of the formation has been carried out. The study shows that stress unloading occurs after drilling. The greater the confining pressure and axial stress, the more significant the shale strength weakening caused by stress unloading, and the magnitude of stress unloading is positively correlated with the degree of mechanical strength weakening. The more developed the laminations are, the easier it is to damage along the laminations. When stress unloading occurs, the weak structural surface acts synergistically with stress unloading, and the higher the degree of shale strength weakening is, the worse the mechanical stability is and the higher the risk of well wall instability. External fluids are prone to intrude along the bedding and promote shale hydration, making the shale mechanically less stable and reducing the well wall's stability. The effect of stress unloading on shale formation collapse pressure mainly occurs in the pre-drilling stage, and the effect of hydration damage on well wall instability mainly occurs in the late drilling stage. Beddings, stress unloading, and hydration affect the well wall stability together. The shale well wall stability model under stress unloading-hydration synergistic effect considers the influence of the three comprehensively. The prediction results align with the drilling actuality with good reliability after the field application test.
