Abstract
Carbonates are often drilled with aggressive drill bit features such as sharper PDC cutter chamfers and lower backrakes of the cutters. Bit aggressiveness features can produce higher penetration rates through hard carbonates, but the potential for increased wear can be detrimental over long intervals.
A field in the Timor Sea, offshore Northern Australia, has an 1800m surface hole interval where the chosen drill bit design must be capable of drilling a long carbonate section, prior to intercepting a hard, interbedded section. After this section, there is another long carbonate formation before casing point. Sustaining any slight cutter edge dulling can dramatically affect the penetration rate potential in the interbedded section, and the bit's ability to drill the post interbed section.
Hybrid technology in 17.5" hole size was recommended for this field during a 2014 drilling campaign due to its increased durability through interbeds as compared to conventional PDC technology. This durability was seen in offset fields in the region, and had resulted in performance advantages and time savings.
The new hybrid bit was used on the first well with success, drilling the interval in one run, as compared to the 2-3 bit runs seen on prior wells in the region when using conventional PDC and TCI technology. With learnings from this first well, a design change was implemented which saw the hybrid TCI cones employ a sparser wedge heel instead of ovoid heels. These modifications saw minimal change in performance on the later wells. From the 2014 campaign results, a recommendation was made to increase the insert heel aggressiveness. Development and improvement in insert carbide grades during 2015-2016 led to the capability and introduction of increased integrity and aggressiveness with conic geometry inserts introduced to hybrid TCI cutting structures. New hybrid designs with the sharper and higher density insert cutting structure were used in offset fields and showed strong performance improvements.
During the most recent 2017 drilling campaign, the sharp conics and dense insert cutting structure layout of the new Hybrid product line was applied in the Timor Sea and mirrored the performance advantages seen on offsets. A previous interval best time of 73.2hrs in 17.5" hole was reduced to just 37.7hrs with a sharp and dense hybrid design and to just 26.3 hours with a 16" hybrid utilising the new design features, reducing time taken to drill the interval by 50% and 65%.
This paper illustrates the design history, lessons learnt, and differences in insert geometry and how this contributed to the improvement in penetration rate in a challenging application.
