Maintaining the stability of deviated and horizontal wells: Effects of mechanical, chemical and thermal phenomena on well designs

Abstract

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 three-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. The cooling of 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 programmes 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.

A Non-Isothermal Constitutive Model for Chemically Active Elastoplastic Rocks

“…9 it can be seen that the reduction in hoop stress is more pronounced for lower values of chemical exponent. It has been experimentally shown that different types of ionic solutions modify the shale's mechanical properties differently (Al-Bazali et al 2008b). For example, CaCl 2 hardens clays more than NaCl.…”

Time-Dependent Measurements of Shale’s Compressive Strength When Contacting Ionic Solutions

“…Alteration of the compressive strength of shale has generated much interest over the past decades, and several researchers tried to understand the reasons and effects of it on the overall integrity of shale to provide methods and techniques for safe shale drilling. − While these studies presented great results and analysis and shed light on the causes and consequences of the alteration of compressive strength of shale on the stability of drilled wellbore, these studies did not monitor the development of shale’s compressive strength alteration (reduction or enhancement) as a function of time. Their recommendations were based on the final state of shale’s compressive strength measurements.…”