Study on effect of in-situ stress ratio and discontinuities orientation on borehole stability in heavily fractured rocks using discrete element method

Abstract

Abstract Borehole instabilities pose significant challenges in drilling and completion operations, specifically in the areas where pre-existing fractures are intersected by the borehole. In-situ stresses play a vital role in failure mechanism around an excavation. In addition, discontinuities increase the probability of instability. Therefore analyses of effect of in-situ stress in discontinuous media have significant importance in identifying efficient drilling methodologies. Numerical investigation on the behaviour of an unsupported vertical cylindrical borehole in heavily fractured rock mass is presented in this study. Discrete Element Model (DEM) based code Universal Distinct Element Code (UDEC) is used as the simulation tool. With taking into account the in-situ stress conditions in Cooper basin, South Australia, an unsupported borehole of 0.15 m radius in the centre of the model was simulated comprising of two fracture sets. The vertical stress applied correlates with the 1.5 km depth of the Cooper basin. The effect of fracture orientation and in-situ horizontal stress ratio (σH/σh) on the stability of the rock mass around the borehole was investigated. It has been shown that the induced stresses due to excavation lead to the development of a yielded zone around the borehole. Borehole stability criteria relevant to the extent of yielded zone and maximum displacement around the borehole were introduced into stability analysis. Results show that when the in-situ stress ratio increases the rock blocks at borehole wall tend to move towards the centre of borehole, consequently yielded zone around the borehole increases. Similarly, the fracture orientation changes the angle of borehole fracture intersection which aids in displacement increase as well as the location of block detachment. Furthermore the change in fracture orientation highly influences the formation of yielded zone.