The Role of Drilling Azimuth on Wellbore Geomechanical Instability in a Major Offshore Field

Abstract

Abstract Saudi Aramco workover re-entry operations within Khafji sand stringers are widespread across Zuluf field. These horizontal sidetracks maintain maximum reservoir contact and introduce substantial increases in oil production and ultimate recovery. Wellbore instability is a hot topic surrounding these operations due to the presence of geo- and hydro pressurized shale within the reservoir. The in-situ stress regime in this field is predominantly strike-slip faulting and the drilling azimuth represents a dominant stability component. The current study analyzes 58 laterals sidetracked across the Khafji formation across Zuluf field, and the effect of the drilling azimuth on wellbore stability. All laterals were cut across 7 in. casings and drilled as 6-1/8 in. open hole, with an average of 4,200 ft. in length. To accurately evaluate these data, the azimuth plane was divided into four angular regions, with respect to the maximum and minimum principal horizontal stresses: maximum stress (17 laterals), transitional maximum stress (12 laterals), transitional minimum stress (14 laterals) and minimum stress (15 laterals). Stuck pipe and severe tight-hole problems are mainly addressed for this purpose, as they are the main wellbore instability signs in this case. The descriptive correlational approach showed that nearly 63% of the total stuck pipe and lateral abandonment incidents occurring due to shale bedding and instability, were actually along the minimum horizontal stress. Furthermore, 53% of the laterals drilled along the minimum horizontal stress experienced wellbore shale instability with either stuck pipe or severe tight holes. Hence, the findings confirm the sensitivity of wellbore stability to the drilling azimuth in strike-slip regions. They also suggest a statistically significant correlation between laterals drilled along the minimum principal horizontal stress and wellbore instability problems across Khafji reservoir in Zuluf field. This paper will further highlight the descriptive correlational method, examine the observed wellbore instability problems, and compare the findings to previously conducted laboratory tests and geomechanical simulations.