Borehole stability in naturally fractured rocks with drilling mud intrusion and associated fracture strength weakening: A coupled DFN-DEM approach

Abstract

Borehole instability in naturally fractured rocks poses significant challenges to drilling. Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure (Pw) and pore pressure (Pp) during drilling, which may cause wellbore instability. However, the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses, which may yield significant errors and misleading predictions. In addition, only limited factors were analyzed, and the fracture distribution was oversimplified. In this paper, the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM (distinct element method) and DFN (discrete fracture network) method. It provides estimates of the effect of fracture strength weakening, wellbore pressure, in situ stresses, and sealing efficiency on borehole stability. The results show that mud intrusion and weakening of fracture strength can damage the borehole. This is demonstrated by the large displacement around the borehole, shear displacement on natural fractures, and the generation of fracture at shear limit. Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure, which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations. A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore. Moreover, the effect of sealing natural fractures on maintaining borehole stability is verified in this study, and the increase in sealing efficiency reduces the radial invasion distance of drilling mud. This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations, which can consider the discrete fracture network, mud intrusion, and associated weakening of fracture strength. The information provided by the numerical approach (e.g. displacement around the borehole, shear displacement on fracture, and fracture at shear limit) is helpful for managing wellbore stability and designing wellbore-strengthening operations.