Marine Technology Reporter Blogs - bits

In a Technical Paper from 2013, Slim Hbaieb and Michael Azar of Smith Bits, a Schlumberger Company explained the advantages of an innovative hybrid bit design that had been tested successfully in Brazil’s pre- salt formations. The main advantages were that the hybrid bit mitigated geological and formation uncertainties, and eliminated the need for several trips to replace damaged bits. Normal PDC (polycrystalline diamond compact) bits can drill pre-salt carbonate rocks effectively until the cutters are damaged by problematic chert inclusions. The hybrid bit is capable of similar ROP (rate of penetration) and run lengths as standard PDC and allows the operator to achieve a long run at high ROP when no silicate inclusions are present.

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.