Laboratory Workflow for Shale Drilling-Fluid Optimization and the Effects of Shale Preservation on Measurements—Widespread Applications in Major UAE Fields

ABSTRACT: Wellbore instability can cost millions of dollars lost due to stuck pipe and loss in hole and non-productive time. This known geomechanics phenomenon can be avoided using proper understanding of the rock mechanical behavior and optimized mud weight to keep the borehole stable. However, drilling through shale can be more complicated due to secondary mechanism of wellbore instability due to osmotic effect between mud system and shale formations. This type of instability normally worsens with time and known as time dependent wellbore instability (Subbiah et al., 2018). Therefore, selection of the drilling fluid to counter rock-fluid interaction together with optimum mud weight able to improve and minimize the wellbore instability risk. Inadequate mud weight and non-optimum drilling fluid chemical composition can lead to wellbore cavings and breakouts, which have been observed in the study area. This scenario becomes more critical when the well inclination angle increases where plane of weakness failure can readily impact mud weight and composition selection. This paper highlights a new novel laboratory workflow for assessment of drilling fluid composition on shale reactivity. This allows optimal selection of drilling fluids to counter time-dependent shale reaction, a common problem encountered with Early Cretaceous Shale formation in the region. In addition, the importance of adequate shale preservation and the use of mineral oil to prevent property altering fluid loss is highlighted in this study. 1. INTRODUCTION Drilling high-angle development wells in shale has been always a challenge due to nature of the anisotropy and lamination. This can worsen by time dependent on wellbore instability where the instabilities in the formations are largely attributed to the increase in pore pressure in the formations with time due to both mud-pressure penetration and chemical potential mechanism (Khan et al., 2016; Subbiah et al., 2018; Tan et al., 1996). Thus minimizing the drilling risk considering both mechanical and chemical aspect of shale is important for overall success for any drilling program. This can be achived by designing optimum safe and stable mud weight as well as optimum drilling fluid suited/or customized for specific shales.