Abstract

Al-Khafji Joint Operations is developing the offshore Khafji Field in the X-Neutral Zone by drilling horizontal wells. These wells encounter shale formations namely Ahmadi Shale and Wara Shale before reaching the pay zones, and Intermediate Shale between KB-1 and KB-2 Sands. While drilling these shale formations, severe time-dependent wellbore instability problems due to shale sloughing were experienced. These problems have led to several stuck pipes and sidetracks.

A drilling fluid design optimization study was previously conducted on planned horizontal wells in the field in order to evaluate the potential time-dependent wellbore stability mechanism(s) in Ahmadi Shale, Wara Shale and Intermediate Shale, to define appropriate mud type, salt type and concentration, and to develop solution and strategy to mitigate and/or manage the wellbore instability problems.

End of Well reports of selected offset wells were reviewed, and numerous time-dependent and mechanical wellbore instability-related drilling events in Ahmadi Shale, Wara Shale and Intermediate Shale were identified. Both mud pressure penetration and chemical potential mechanisms were considered in the back-analysis of the identified time-dependent wellbore instability in the shale formations. The analysis showed that the instability was attributed to insufficient salinity in the drilling fluid which resulted in pore pressure increase due to the mud pressure penetration mechanism being not adequately counteracted by the chemical potential mechanism. As a consequence, the pore pressure increased near the wellbore wall which reduced the effective mud support and led to unstable wellbore condition.

The impact of time-dependent pore pressure change on wellbore instability in the offset wells from back-analysis of the events was used to develop two optimum drilling fluid design criteria for Ahmadi Shale, Wara Shale and Intermediate Shale. Based on the design criteria together with input from mechanical wellbore stability analysis, recommendations on optimum drilling fluid design were developed for drilling Ahmadi Shale and Wara Shale for a planned horizontal sidetrack in the field. The sidetrack was drilled and completed successfully without any drilling problems by adopting the recommendations of the study and close interaction between the field, operations and onshore specialist teams. Several bit runs and wiper trips were conducted smoothly during the drilling period.

Introduction and Objectives

The Khafji Field is located in the Arabian Gulf about 40 km offshore Al-Khafji (Figure 1). The field extends about 20 km NE-SW by 8 km NW-SE, in the shape of an asymmetrical domal structure with steeper formation dip in the western flank. The structure is believed to have been generated by a NE-SW trending wrench fault with vertical throw at basement.

Al-Khafji Joint Operations (referred to KJO, hereafter) is developing the Khafji Field in the X-Neutral Zone by drilling horizontal wells. These wells encounter shale formations namely Ahmadi Shale and Wara Shale before reaching the pay zones, and Intermediate Shale between KB-1 and KB-2 Sands. While drilling these shale formations, severe time-dependent wellbore instability problems due to shale sloughing were experienced. These problems have led to several stuck pipes and sidetracks.

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