Published: 05/16/2013
Published: 05/16/2013
An operator attempting to drill and set casing for vertical borehole sections in northern Iraq was experiencing a number of formation-related challenges.
Beyond its sticky characteristics, the formation's instability was causing boreholes to collapse, and the boreholes that were completed were out of gauge. Additionally, wellbore pressure was causing severe mud losses. Although employing an underbalanced drilling (UBD) method using a nitrogen-charged mud system to counter this last condition, the operator was still forced to spend an inordinate amount of time running and setting casing. Before attempting to drill a 615-m, 14-in vertical section and set 13.375-in casing in this challenging formation, the operator sought a more efficient drilling assembly.
To increase wellbore quality and casing efficiency, Schlumberger recommended using the 14250 Rhino XS hydraulically expandable reamer to underream the 14-in pilot-hole section. To ensure proper activation and performance of the Rhino XS reamer in a formation that produced such challenging wellbore pressure, it was essential that the reamer's hydraulic-up nozzles were correctly selected before attempting to enlarge the borehole below the casing ID restriction. Precise nozzle setup would require running the Rhino XS reamer with the selected BHA in simulations that matched the formation and drilling conditions previously experienced by the operator.
To fully optimize the operator's underbalanced drilling approach, HART software for hydraulic optimization was used to run simulations for the operator's application. During these virtual runs, an equivalent flow rate and average fluid density of the two-phase nitrogen mud system was introduced that matched the operator's underbalanced drilling method. To further increase drilling performance, an i-DRILL engineered drilling system design analysis was conducted to provide the operator with optimal drilling parameters for the application.
For the first time, an underbalanced underreaming run was successfully made in a well with a two-phase nitrogen mud system. In one run, the operator underreamed the 615-m section, enlarging it from 14 in to 17 in. Compared with previous reaming efforts in this formation, the operator was able to set the 13.375-in casing more efficiently, saving approximately 3 rig days.
Challenge: Underream a 14-in, 615-m pilot-hole section, enlarging it to 17 in through an unstable formation known to cause sticking in one trip.
Solution: Run a 14250 Rhino XS hydraulically expandable reamer to increase wellbore quality and casing running efficiency.
Results: Underreamed the 615-m section in one trip, enlarging it from 14 in to 17 in and enabling the operator to set the 13.375-in casing more efficiently; saved approximately three rig days, marking the first time an underbalanced underreaming was performed in a well with a two-phase nitrogen mud system.
