Field-Based Mud Design Optimisation for Mitigating Time-Dependent Wellbore Instability in a Shale Formation in Gulf Region

ABSTRACT: It is estimated that time-dependent shale instability accounts for up to 20% of the wellbore instability problems in Field X, Gulf region which are related, at least in part, to incorrect mud salinity and mud type. While drilling the Shale X formation, severe time-dependent instability problems due to shale sloughing have led to invisible lost time associated with hard back-reaming and stuck pipe. However, conventional method of handling time-dependent shale instability in using offset well drilling experience and practice in the field has not been robust and effective in mitigating the problems. Hence, a study was conducted to deploy the field-based Optimal Mud Salinity Design Tool to eliminate time-dependent wellbore instability problems related to improper mud salinity design in the field. Petrophysical and chemical properties of the shale formation were characterised on sizeable cuttings. End of Well reports of the offset wells were reviewed, and numerous time-dependent wellbore instability-related drilling events in the shale formation were identified. The integrated back-analysis of the identified instability events showed the instability was attributed to insufficient salinity in the drilling fluid which resulted in pore pressure increase due to the mud pressure penetration mechanism not being counteracted by the chemical potential mechanism. The output from the back-analysis were subsequently used to develop two optimal mud design criteria for the shale formation based on breakout mud weight and mud weight used. The tolerable upper limit hole enlargement and lower limit chloride concentration required for optimal mud salinity design were determined based on the invisible lost time for offset wells together with chloride concentration used and associated hole enlargements. Following that, the optimal mud design criteria were used with input from mechanical wellbore stability analysis to develop optimal mud salinity design for drilling the shale formation with water-based mud in two planned wells. As the required mud activity is approximately 0.85 and as excessive high KCl concentration could be detrimental to some shales with generation of microfractures, combined KCl and NaCl salinities with KCl limited to 5% are recommended as optimal mud design for the two wells.