Demands for reutilizing existing slots on the offshore platform are expected to rise to maintain oil and gas productivity by drilling and completing the well to different targets, whereas most of the slots are occupied in mature fields. Slot recovery operations are one of the solutions to recover occupied slots. Different slot recovery techniques have been executed in the Offshore Abu Dhabi field, which include cut & pull, milling, Casing Pulling Tool, and Conductor Deflector Tool techniques which are time-consuming and require high overpull to release the casings. The hollow bit is a slot recovery technique that enables the user to recover casing by the standard fishing tool (i.e., casing spear and overshot) without excessive overpull which inherits a risk of surface equipment damage and safety issues on the rig floor. Moreover, overcoming the strong cement bonding between a 30" conductor and 13-3/8" casing is difficult and is dependent on the cement condition. The casing is cut into short intervals to be pulled out which results in a long operation duration. Hollow bit removes the annulus cement between 30" conductor pipe and 13-3/8" casing by washing over the cement. Casing cuts can be minimized since the removal of cement from the annulus overcomes the cement condition uncertainties which results in a shorter slot recovery duration. This paper presents the 3 hollow bit field trials conducted in 2021-2022 in Offshore Abu Dhabi. Lessons learned accumulated from each trial were implemented in subsequent wells to improve the performance. Performance was increased by 60% compared to other slot recovery methods by reducing the operation duration from 20.8 days to 12.5 days. Best practices that can be implemented to similar application by hollow bit are also presented.

We present a root-cause analysis of severe lost circulation and creation of its risk map in an offshore oil field, Abu Dhabi. Lost circulation of more than 100 barrel per hour (BPH) has occurred in sixteen (16) boreholes through the carbonate reservoir. Four (4) out of them experienced total loss. The authors investigated spatial distribution of lost circulation and allowable maximum overbalance based on review of drilling operation in nearly two hundred (200) boreholes. The critical overbalance to reactivate the natural fractures for the tensile-opening or shear-slip failures was analyzed by using geomechanical model. The present study clarified that most of severe lost circulations occurred at the specific sub-layers of the reservoir. The core observation showed that lost circulation occurred in the intervals in which fragmented rocks (rubbles) and fractures were distributed together. The fracture stability analysis revealed that the conductive fractures interpreted by borehole imager were geomechanically stable under conventional overbalance applied in drilling through the reservoir sections. Namely, the planer fractures were geomechanically stable under the current in-situ stress condition. The study concluded that the predominant root-cause of severe lost circulations in the carbonate reservoir was cave-related rocks (cave facies) and excessive overbalance applied to reservoir pressure. The cave facies were supposed to be formed by flank-margin cave system and its collapse due to deposition of overburden formations. A risk map of lost circulation defined the five areas with its potential risk. The risk map indicated not only the risk level of lost circulation but also practical recommendations on the depth to set casing and allowable maximum overbalance. It functions as a practical guide for the design of boreholes in the ongoing drilling campaign.

The 3,000 ft long lateral holes drilled through water-injected area in the carbonate reservoir in the offshore Abu Dhabi have been forced to implement hard backreaming. The abnormal extra operational time has been taken due to poor performance in the operation to pull out a bottomhole assembly after drilling to the total depth. The study aims to analyze root-causes of the hard backreaming through the carbonate reservoir in the studied field. The speed of tripping-out was analyzed every stand of drill pipe by using time domain data of movement of traveling block. The correlations between the speed of tripping-out and rock characteristics such as porosity and constituent minerals in rocks were investigated. Hole shape was analyzed in the representative intervals of low trip-out speed using 16-sector caliper derived from azimuthal density logging. Stress concentration around the borehole wall was also analyzed using geomechanical model. The investigation revealed that hole shrinkage due to plastic deformation of the borehole wall was the most possible root-cause of the hard backreaming in the carbonate reservoir. Namely, BHA had to ream up deformed borehole wall in tripping-out. From the viewpoint of rock characteristics, the speed of tripping-out was found to be lower in the specific geologic layers with higher content of dolomite. This is because dolomite rocks cause larger resistance in reaming it in tripping-out since the strength of dolomite rocks is larger than that of limestone. Based on our findings, use of reamers on bit is found to be the better solution to improve the tripping-out performance in the problematic geologic layers instead of conventional operational attempts such as spotting of acid and use of high viscous fluids in hole cleaning. In addition, optimization of the design and position of reamers and stabilizers is essential to succeed in the future 10,000 ft long extended-reach wells in the studied oil field.