Experimental Study and 3D Numerical Analysis of Wellbore Instability in Naturally Fractured Formations

Abstract

Abstract Exploration and development of fractured formations is difficult, because complex conditions such as stuck pipe, wellbore collapse, and even the loss of borehole intervals occur frequently, causing huge economic losses and affecting the later drilling operations. However, most of the existing literature on wellbore stability analysis in fractured formations focuses on the coupled effects of fluid flow and deformation, and the impact of three-dimensional natural fracture characteristics (roughness, dip angle, etc.) on wellbore instability is not yet closely examined, which may yield significant errors and misleading predictions. This paper carried out a series of experiments to analyze the shale sample's fracture characteristics and shear strength. Then, a fully coupled 3D hydro-mechanical model was then constructed using the distinct element method (DEM) to investigate the wellbore instability. Finally, a comprehensive parametric study was performed to analyze the effects of characteristics of fractures (e.g., distribution density, surface roughness and associated shear strength, and dip angle) on wellbore stability under different in-situ stress states. The results show that the increase of the number of parallel weak surfaces results in a larger radial displacement around the wellbore. Fracture roughness poses an important impact on wellbore stability because larger fracture roughness is associated with higher rock shear strength. As the dip angle of the weak plane increases, the maximum wellbore displacement increases and then decreases, and the peak displacement occurs at the dip angle of 45°, which is more obvious in a normal fault stress regime. This research results provide a useful tool to understand and assess the effect of natural fracture characteristics on wellbore instability for drilling design in fractured formations.