Impact of Drilling Mud Intrusion on Borehole Stability in Naturally Fractured Rocks: A Coupled DEM-DFN Investigation

Abstract

ABSTRACT Borehole instability in naturally fractured rocks poses significant challenges to drilling. However, the weakening of fracture strength due to mud intrusion was not considered in most existing borehole stability analyses, which may yield significant errors and misleading predictions. In addition, the fracture distribution was oversimplified. In this paper, the impacts of mud intrusion on borehole stability in fractured rocks under anisotropic stress states are investigated using a coupled DEM (distinct element method) and DFN (discrete fracture network) method. A simplified, but reasonably realistic approach is developed to take into account the weakening of fracture strength due to mud intrusion. The results show that mud intrusion and associated weakening of fracture strength and hydro-mechanical coupling effect cause more shear failure of natural fractures and rocks, explaining why the increase in mud weight worsens borehole stability during overbalanced drilling in fractured rocks. This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations based on the consideration of mud intrusion and its weakening of fracture strength. INTRODUCTION Borehole instability is a phenomenon that the borehole size and its structural integrity can not be maintained in the openhole interval. Maintaining a stable borehole is of great importance because related instability problems pose negative influences on the drilling schedule and costs, and even lead to the abandonment of the well (Chen et al., 2003; Zhang, 2013; Li et al., 2019). Various factors that affect borehole stability were investigated to explain the mechanism involved with wellbore collapse such as mud weight, in-situ stress, wellbore trajectory, rock properties, and failure criteria (Zhao et al., 2012; Mansourizadeh et al., 2016; Liu et al., 2021; Li et al., 2019; Zhang et al., 2020; Li et al., 2022; Aslannezhad et al., 2021). Both theoretical and field data have indicated that an appropriate increase in mud weight does benefit wellbore stability in continuous media (Salehi et al., 2010; Fan et al., 2020; Ibrahim, 2021; Motahari et al., 2022). However, compared with non-fracture formation, increasing mud weight may negatively impact wellbore stability in naturally fractured rocks (Santarelli et al., 1992; Yamamoto et al., 2002). It has been demonstrated that the role of drilling mud in maintaining borehole stability should be reconsidered when drilling in fractured rock formations. In addition, the mechanism of coupling effect of in-situ stress anisotropy, natural fractures, and drilling mud intrusion on borehole stability has not been fully understood.