The aims of the campaign were:
- Zero damage to formation, thereby enhancing oil production from the wells
- Reduction of NPT due to mud losses
- Reservoir assessment while drilling
- Improved well placement in the reservoir section through real-time analysis.
Drilling was initiated using base oil which was replaced by produced fluid as drilling proceeded. Average ECD of 4.1 MWE resulted during drilling of the first well, further reduced to 3.9 MWE in the second well. A lower completion was run and packer was set in flowing conditions in both wells.
ONGC said designing an under-balanced drilling process in which the produced fluid was to be stored and circulated for drilling on an offshore rig proved to be a major task, with each individual activity planned and executed following intensive deliberations.
A Hazard Identification (HAZID) analysis was performed using an independent facilitator. During the subsequent Hazard Operability (HAZOP) analysis, the team discussed and prepared process flow diagrams, process and instrumentation diagrams, valve numbering diagrams, and other detailed procedures.
Drilling up to the landing point in the three wells was completed by March 17. Assembly of the UBD equipment rig had started 11 days earlier while the rig was preparing the wells for UBD.
After initial commissioning, UBD in the first well, HK#2H, started on April 6. Drilling was initiated with 100 psi of under balance by injecting nitrogen in a concentric annulus of 9⅝-in. x 7-in. tieback casing, while pumping base oil as drilling fluid.
Using experience from the first well, equipment rig-up on the second well, HK#9H, was completed in 5.5 days. The well started producing crude while drilling a 6-in. drain hole in under balanced condition.
During a test the well flowed at an average rate of 1,683 b/d. Drilling was then terminated, followed by real-time analysis of the reservoir fluid and LWD logs.
Among the highlights of the campaign, according to ONGC, were:
1.) Real time monitoring of reservoir behavior while placing the well improved decision making
2.) Well HK#9H produced well fluid while drilling a 6-in. drain hole
3.) A complete package of RSS-LWD was run in UBD conditions
4.) Lower completion ERD packer set in under balance condition
5.) During drilling with UBD technology about 4,699 bbl of crude was exported to the platform
6.) UBD specific wellhead assembly was designed in consultation with M/s BHEL
Results, ONGC added, open the way for future simultaneous drilling and assessment of depleted reservoirs.
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