To develop gas/condensate reserves in Dorra field offshore Kuwait, the operator is setting a long string of large OD casing to facilitate short-radius horizontal drilling techniques in the 17-in hole section. The procedure has proved challenging for existing technologies and has led to the customization of tools for specific areas and in extreme cases on a well-by-well basis.
To standardize operations, engineers studied offset data and conducted a lithology/formation analysis. The investigation indicated the lithology is PDC drillable from surface down to 1200ft and that reaching the Tayarat dolomite at 4000ft with acceptable ROP was possible with current PDC technology. However while drilling the first two wells, three different PDCs failed to achieve the target depth in-spite of steadily increasing blade count (6,7,8) and diamond volume in critical wear areas. The lack of durability resulted in one/two extra trips for a new PDC and/or rollercone to finish the hole section. The operator required a new approach to reduce CPF.
A rock strength analysis was preformed to select the best TCI insert shape and gauge/heel configuration. An initial trial with a standard 17-in rollercone (GS18VEJ3) on rotary BHA proved successful. Although the new strategy required two bits to reach section TD, it reduced CPF by 10% compared to the best PDC/TCI performance. After analyzing dull bit condition and TCI cutting structure degradation, several additional challenges were identified as performance limiters.
A new design was produced that included a precision sealed bearing system and new cutting structure geometry and insert shapes. The new design (MGS18VC) was run on a motor BHA in the next two wells with outstanding results increasing average footage and ROP by 24% and 8% respectively compared to average PDC/TCI drilled wells. The customized bit reduced CPF by an additional 8.5% for a total cost savings of $100,000USD/well.