Customized bits boost pre-salt rates of penetration

Customized bits enabled Petrobras to successfully drill hard carbonates of pre-salt reservoirs and hard and interbedded post-salt formations of two different offshore wells, while at the same time lowering vibrations, increasing rates of penetration, and contributing to zero tool failures during drilling. Halliburton's Roberto Zambrana explains how it was done.

Successful economic development of Brazils pre-salt reservoirs requires a broad range of technologies and operational capabilities. Reservoirs in this domain are complex, heterogeneouslayered carbonates, which make accurate reservoir characterization very challenging. Drilling is difficult with low rates of penetration (ROP) and a tendency for borehole deviation, elevating the importance of precise directional control.

While drilling offshore wells in southern Brazil, Petrobras encountered several challenges. The operator wanted to save money and rig time while drilling through very hard formations into the pre-salt reservoirs. Previous attempts to drill through these hard formations achieved an ROP of just 2-3m/h. The formation also prematurely wore out drill bits, causing tool failure and requiring

FM3653Z (left) and FXD74D, the two Halliburton bits customized for Brazils hard carbonates.

In previous wells in the region, drill bits were changed out two to three times to drill just 200m downhole, increasing both rig time and the associated cost significantly, so Petrobras was looking for a more efficient solution. In this deepwater application, where offset costs reached more than $26,000/m, Petrobras needed bits capable of drilling the entire section into the pre-salt reservoirs of two wells, an 81/2in and 16in hole section, with higher ROP. Average rates of field histories were 2.0m/h and 6.3m/h, respectively.

Petrobras turned to Halliburton. Using the SPARTA rock mechanics analysis model part of Halliburton's on-site Design at the Customer Interface [DatCI] process to optimize bit selections, the formations main challenges were pinpointed and two of the companys drill bits customized to power through the hard pre-salt reservoir formations with low levels of vibration in the downhole tools.

Roberto Zambrana is a senior technical advisor and application design evaluation specialist for Halliburton's Drill Bits & Services business line in Maca, Brazil. An SPE member, he has a petroleum engineering degree from Universidad Autónoma Gabriel Rene Moreno in Santa Cruz, Bolivia. He joined Halliburton in 1998 as a services coordinator in Bolivia and moved into his current position in 2004.

The amount of diamond available was increased by designing a dual-row cutting structure on the company's 81/2in FX Series bit and a single-row structure with high cutting density on the 16in FM Series bit. The customized bits also featured new blade geometry specifically designed to increase stability, mitigating lateral and torsional vibration problems. This force-balanced optimization, incorporating X3 Series cutter technology, produced the optimal blade and cutter configuration for the targeted environment. The X3 Series cutters combine abrasion and impact resistance with greater thermal stability, enabling the bit to withstand friction heat while limiting heat buildup. Advanced computational fluid dynamics (CFD) analyses were also used to provide hydraulic optimization that maintained cleaning and cooling of these cutting structures.

Petrobras also used Halliburton's Geo-Pilot XL robust rotary steering system that supports extreme drilling conditions, while Halliburton optimized the entire bottomhole assembly with its MaxBHA software.

These combined efforts resulted in faster drilling and more durable bits than were employed for the previous offset wells. The 81/2in FX Series bit drilled the 81/2in section with an ROP of 7.2m/h, saving $1.5 million and 75 hours of rig time. The 16in FM Series bit drilled a 1594m section in a single run at 10.4m/h. The run set a new record and saved Petrobras 16.5 days of drilling time and more than $8 million. Petrobras was so impressed with these results that it is now using the Halliburton on-site DatCI process to develop bits for use in other wells.