Efficient Drill Bits for the Pre-salt
The expression "pre-salt" refers to an aggregation of rocks located offshore along large portions of the Brazilian and West African coasts, it forms a carbonate rock interval that ranges under an extensive layer of salt which, in certain areas of the Brazilian coast, can be as much as 2,000 meters thick. The total depth of these rocks formations, or the distance between the surface of the sea and the oil reservoirs under the salt layer, can be as much as 7,000 meters. Drilling in the pre-salt is extremely complex and may require multiple trips being made by the drill bit in order to drill short lengths due to different rock layers with varying degrees of hardness. Another challenge is that the salt is very corrosive and is also plastic, as you drill down, it tends to close the well walls. As you penetrate, the salt, which is also soluble, can start caving in on top of the drilling bit, potentially causing the drill string to be tied down.
During drilling, the salt can also exert tension on the drill pipes and block the well, this tendency for borehole deviation, increases the importance of precise directional control. Previous attempts to drill through these formations only achieved a penetration rate of four to six meters per hour, an exceedingly slow rate of penetration (ROP). The formation also prematurely wore out drill bits and caused tool failure, requiring multiple trips downhole. Petrobras wanted to increase the ROP and drill through these sections in just one run downhole and chose Halliburton to help overcome these challenges in two different wells.
Halliburton recommended using two customized bits to power through this hard rock in each well and optimized the bit design to help solve Petrobras’ challenges. Halliburton used its SPARTA™ rock mechanics analysis model, part of its on-site DatCIsm process, to optimize bit selections for this particular formation. The Halliburton analysis found the main challenge was in the carbonate section, which was very complex due to different layers with varying degrees of hardness. The top section consisted of very hard shale, marl, and sandstone, while the bottom section consisted of carbonates and volcanic rock. Halliburton used the SPARTA models to optimize the FM3653Z and the FXD74D bits to deal with these challenges, managing to customize the two drill bits to power through the carbonate formations of the pre-salt wells with low levels of vibrations in the downhole tools.
Halliburton set a new record by drilling one well with the 16-inch FM3653Z bit at a ROP of 10.4 meters per hour, saving Petrobras 16.5 days of drilling time and $8 million. In another well, the 8 ½ inch FXD74D bit drilled at a ROP of 7.2 meters per hour, saving 75 hours of drilling time. These custom drill bit designs nearly double the ROP in pre-salt formations.The structural designs increased the amount of diamond available to drill. The designs also reduced vibration and tool failure. With these customized bits, Petrobras finally managed to drill through both sections in just one trip downhole.
Halliburton FXD74D Drill Bip - courtesy of Halliburton