An operator is delivering complex extended-reach drilling (ERD) wells from an offshore platform and experiencing up to three weeks of nonproductive time (NPT) to pull out of hole (POOH) bottomhole assembly (BHA). Multiple BHA’s have been lost in the 17 ½-in. hole section in faults, and packing-off in unstable formations. Initially the root cause was identified to be hole cleaning, in particular, in the "heel" of the section-avalanche zone-extending 400m below the 18 5/8-in. casing shoe. Cleaning an ERD high angle (>70°), large hole well is a well-known challenge. The initial solution was to reduce the hole size from 17 ½-in. to 16-in. while running the same casing string. This was done primarily to increase the annular velocity to improve hole cleaning. As we will see, as the Operator continued to analyze the problems, the root cause changed, as did the solutions, but 16-in. hole size was still the preferred hole size for the many benefits it brings. All risks associated with the change in hole size had to be understood and managed.

Solutions that were executed, included: trajectory optimization based on updated Geomechanics modelling, Extended Leak-off-tests, flat rheology drilling fluid, mud additives, cementing recipes, "fit for purpose" BHA design, Drilling/POOH/Casing running pre-job modeling and real time monitoring. All this helped to optimize the delivery of a "cemented-in-place 13 3/8-in." production casing for the most recent well in the Lunskoye gas field.

The optimized high angle 16-in. hole section was drilled from 1264m to 3806m Measured Depth (MD) (2542m length) at over 80° inclination. The 13 3/8-in. production casing was then run and cemented without any issues to the planned depth in this reduced hole size. Previously, the same size production casing was run on the project in 17 ½-in. sections. On this particular well the hole section was delivered 29 days ahead of the planned approved for expenditure (AFE) time of 55 days. Additional benefits gained by drilling a smaller hole size were savings of 67 m3 of cement and 110 m3 of drilling fluid. Two logistic boat trips were also saved due to fewer materials required. The method also increases the cuttings re-injection (CRI) capability and longevity, bringing additional long-term savings.

The successful drilling of this well shows the benefits and qualitative acceptance of downsizing the hole for this major Sakhalin offshore project. The well also provides a basis for the oil and gas industry population to consider the possibility and benefits of drilling a 16-in. extended reach hole size instead of a 17 ½-in. hole and cementing in place the 13 3/8-in. casing string.

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