Understanding Shale Petrophysics Helps in Drilling Stable Boreholes: Case Studies

Abstract

Abstract Drilling stable boreholes in shale formations can be a very challenging task when water-based drilling fluids are used. The interactions between water phase in drilling fluids with shale rocks cause different drilling problems such as shale swelling and dispersion. The extent to which a shale sample swells or disperses depends on the clay content of the shale. Some types of clay have more swelling tendency while others have more dispersion tendency. Therefore, identifying the type of clay present in a shale rock is very important for developing drilling fluids capable of mitigating shale drilling problems. Gama ray data is a powerful tool to predict the clay mineralogy, determine shale reactivity and, as a result, predict the rock response when contacted by drilling fluids. Using petrophysical properties to characterize shale can help determine the appropriate drilling fluid and shale inhibitor to be used. The spectral gamma ray log has been used to determine the proportion of the radiation coming from potassium, uranium and thorium. The ratio of K/Th is utilized to identify clay mineralogy for different shale formations. In addition, formation density log data was utilized with neutron log to give more accurate information about lithology. The objective of this study is to discuss how petrophysics data of shale formations can be utilized to improve and optimize drilling practices to achieve the ultimate goal of enhanced wellbore stability. Depending on clay content, different shale formations have different responses when exposed to drilling fluids and, therefore, it is of high importance to characterize the shale formation to develop the appropriate drilling fluid.