The drilling of hard formations usually requires specific drilling systems to achieve an efficient rate of penetration. Due to the limited depth of cut, the solution often involves the use of high RPM drive systems such as turbines or high speed motors and a fixed cutter product is usually the drill bit of choice.

With increasing well depth the roller cone option is normally avoided due to the low penetration rates and limited rotating hours obtainable with these drill bits. Roller cone use invariably leads to multiple trips and a significant increase in the time and cost of the drilling operation, especially for deeply buried reservoirs where hard rocks are generally encountered and bit diameter is reduced at TD.

Unavoidably, in some applications roller cone bits continue to be used in order to obtain the quality cuttings necessary for the geologist to correctly assess the formation. This is typically the case when the reservoir interval is encountered and exact formation knowledge is critical.

A new drilling solution has been developed to address this situation and is presented in this paper.

The idea consists of a fixed cutter bit which leaves the center of the hole uncut. The lack of bit center leads to the creation of a core. This core is broken by the bit itself and ejected at the side through a specifically designed junk slot. The core is then carried to the surface along with the other cuttings. This process leads to very high quality cuttings for surface examination.

The improvement is especially important where normal cuttings quality is poor due to the use of turbines or high speed motors together with impregnated diamond bits.

Importantly, as the center part of the bit is uncut this new technology also provides a significant increase in the rate of penetration. In a fixed cutter bit the center is one of the most inefficient cutting areas as the rotational speed of the cutter is low to null. By removing the center, an increased ROP efficiency in the range of 15 to 25 percent can be obtained depending upon the bit diameter.

The field tests that are presented in the paper show quantifiable improvements in terms of geological sample quality, cutting efficiency and drilling cost reduction.

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