How to use previous experiences from offset wells?
Near field exploration is a valuable business. You can tie-in to existing infrastructure for production and you have lots of offsets to work on. Within 8 kilometres of the planned well location (61°33'46.09''N, 02°01'57.74''E), we find 8 wellbores without considering the latest already drilled since the last year (34/4-15 S, 34/4-15 A and 34/4-16 S) and the planned side-track 34/4-16 A.
The planned trajectory is expected to reach Utsira and Lark formations, at approximately 1,000 m depth. Shetland Gp would be reached later at around 2,000 m depth and Intra Draupne and Heather formations (Viking Gp) below 3,350 m. Then Brent Gp would be the main target, where it is planned to conduct the Drill Stem Test (DST).
Defining hole sizes and drilling mud program
The upper hole sections of 42”x36” are planned to be drilled with seawater and bentonite and drilled down to 457 m MD. Water-based drilling fluid (WBM - Glydril) will be used in the 26" section together with foam cementation to the seabed. A 17 ½” hole is drilled down to 1,983 m MD with an oil-based drilling mud (OBM). OBM is used for the 12 ¼” section down to 3,311 m MD, and the deepest hole section of 8 ½” is planned to be drilled with OBM down to a depth of 3,800 m MD. The nearby wellbores 34/4-8 and 33/6-2 provide valuable PPFG data to run a decent comparison and quick check.
The mud program is expected to involve values of around 1.03sg for the shallowest section (42” hole), 1.25sg approximately for 26” hole section, then from 1.30sg to 1.42sg for the 17 ½” hole and a range of 1.42sg to 1.52sg for the next 12 ¼” hole section. Finally, the last section will be drilled with 1.48sg OBM. Mud weight data available from the offset wells indicate these are good enough ranges for this design.
A better look at the wellbore sections
A standard pattern is followed in designing the sections for the Dugong Appraisal. That becomes clearer by looking at some of the offset wells, including those latest drilled by Neptune.
There are 36 "x 30" guide tubes to be run and foam cemented along their entire length. A 20” casing is set in the 26” hole with foam cementation to the seabed. Drilling cuttings and any excess cement are discharged from the rig using mud return to rig (RMR) without the use of risers. Followed by a 13 ⅝” casing in 17 ½” hole with top of cement (TOC) at 200 - 400 m above set depth, and 9 ⅝” casing for the 12 ¼” hole section, cemented 300 - 400 m above set depth. A 7” liner is considered for the last hole. In the case of a discovery, a well test (DST) will be performed and data collection will be carried out according to a separate program and then plugged back permanently.
How long does this project is expected to take?
The well is scheduled to be drilled in about 71 days. In the case of a dry well, the drilling time is estimated at 41 days and in the case of discoveries, a well test is carried out with a duration of approximately 30 days. The well will be permanently plugged back (NORSOK D-010, 2013).
Once all the main design parameters (those previously mentioned) are covered, it’s time to take a look at further operational details. In the picture below, you can see an initial check through the relevant calculations for the current design. Run several calculations with just one click and Ta-Da!, get a lot of plots and the risk report on the right side panel shows clearly what needs to be reviewed (:
Did you see the risks with red markers? Well, some casing properties need some attention. But, that could be for a next occasion (;
Best of luck!
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