Wellbore instability, particularly in shale formations, is regarded as a major challenge in drilling operations. Many factors, such as rock properties, in-situ stresses, chemical interactions between shale and drilling fluids, and thermal effects, should be considered in well trajectory designs and drilling fluid formulations in order to mitigate wellbore instability related problems.
A comprehensive study of wellbore stability in shale formations that takes into account the 3-dimensional earth stresses around the wellbore as well as chemical and thermal effects is presented in this work. The effects of borehole configuration (e.g. inclination and azimuth), rock properties (e.g. strength, Young's modulus, membrane efficiency and permeability), temperature, and drilling fluid properties (e.g. mud density and chemical concentrations) on wellbore stability in shale formations have been investigated.
Results from this study indicate that for low permeability shales, chemical interactions between the shale and water-based fluids play an important role. Not only is the activity of the water important but the diffusion of ions is also a significant factor for saline fluids. Cooling drilling fluids is found to be beneficial in preventing compressive failure. However, decreasing the mud temperature can be detrimental since it reduces the fracturing pressure of the formation, which can result in lost circulation problems. The magnitude of thermal effects depends on shale properties, earth stresses and wellbore orientation and deviation.
Conditions are identified when chemical and thermal effects play a significant role in determining the mud-weight-window when designing drilling programs for horizontal and deviated wells. The results presented in this paper will help in reducing the risks associated with wellbore instability and thereby lowering the overall non-productive times and drilling costs.
With the advances of drilling technologies, highly deviated, horizontal, and extended-reach wells are becoming routine operations [1–4]. However, one of the critical problems that have hampered lowering the cost of drilling is wellbore instability. Curing wellbore instability can be complicated and costly as a result of associated nonproductive time and escalating mud costs. Nevertheless, proper well design including well trajectory optimization and drilling fluid formulation may help prevent the problem and avoid the complex process of curing stuck pipe, lost circulation caused by wellbore collapse and breakdown. Due to the unique properties of shales, such as low permeability, lamination and chemically reactive clay , a comprehensive approach including mechanical, chemical, and thermal considerations must be undertaken when combating wellbore instability.
Based on the analysis of in-situ stresses and rock properties, Moos et al. (1998) put forward a method to optimize well trajectories . Awal et al. (2001) found that the optimal trajectory can be vertical, directional, or horizontal, depending on whether the region is tectonically relaxed or active and also whether the prevailing in-situ stress is normal, overthrust/reverse, or strike-slip . When a vertical well is drilled into a normally stressed formation (sv > sH >sh), such as the one done in the Ula Field, a stable wellbore is realized. However, under a strike-slip stress regime (sH > sv >sh), directional and horizontal wells are more stable than vertical ones . Russell et al. (2003), in analyzing the Tullich Field data , recommended that the wellbore should not be drilled parallel to the maximum horizontal stress (sH) in order to minimize wellbore instability incidences.