Figure 1. ROP improvement curves are generated using data from experimental tests conducted with an impregnated MCB at the Paris School of Mines drilling centre. (Image courtesy of Diamant Drilling Services)

Extreme drilling conditions characterized by high hydrostatic pressure (over 20,000 psi) and temperature (over 300°F or 148°C), combined with hard formations (high confined compressive strength — 40,000+ psi) require specific adaptation of the well design and in the equipment selection.

Low ROP and lack of quality cuttings

These conditions lead to low overall rates of penetration (ROP), which means low depth of cut. As a consequence, the average size of cuttings recovered is extremely low (0.020 to 0.20 mm) making them irrelevant for geological identification of the formations. This occurs with either impregnated type drill bits or even polycrystalline diamond compact bits in applications at these low ROPs. If downhole conditions (high pressure/ high temperature or HP/HT) exceed the measurement while drilling or logging while drilling specifications, accessing this critical information will necessarily require an expensive coring operation.

The micro-coring bit approach

In order to solve these problems, TOTAL S.A. and Diamant Drilling Services developed a drill bit that generates micro-cores of formations during conventional drilling operations.

The micro-core is generated in the center of the bit, where the cutting structure is interrupted. The ?-core, advancing to its nominal size, reaches a PDC stop. A lateral force is applied on the ?-core, inducing the shearing. The ?-core is led to the annular via an evacuation area. This area is larger and deeper than a conventional hydraulic waterway. Keeping this transit area open prevents of any risk of balling to the cutting structure.

The micro-coring bit (MCB) design can be adapted to any type of fixed cutter structure. The dimension of the core will be defined according to the bit diameter, the bottomhole assembly design and the application requirements, starting typically from 20 mm for a 6-in. bit up to 40 mm for a 22-in. bit.

Generation of the core will be mainly governed by the rock strength and the drilling parameters applied during the operations. A ductile cutting mechanism has to be maintained at the bit/rock interface, requiring close weight-on-bit control to enhance recovery of the rock fragments at surface.

ROP improvement

The “No Center Bit” effect of the MCB saves a significant part of the energy used in the cutting action of the bit. Applying this available energy to the reduced cutting structure allows the production of a higher depth of cut, increasing the cutting efficiency and overall ROP of the bit. The ROP improvement ratio compared to identical drill bits with a closed cutting structure will be directly related to the micro-core size and bit diameter (Figure 1).

Field applications

To date, four runs in two field applications have been made with a MCB since the concept was experimentally proven. The first application successfully validated the feasibility of the concept. An intact micro-core was recovered from 13,489 ft (4,096 m) depth. In the second application, the bit drilled 361 ft (110 m) between 12,861.5 and 13,488 ft (3,920 and 4,111 m) in hard limestone, hard and abrasive sandstone, siltstone and shale layers. The MCB produced recognizable rock fragments (10 mm) in comparison with the usual powder (rock flour) generated by impregnated cutting structures. Outstanding ROP improvements were reported in comparison with the offset performance in similar conditions.

The micro-coring bit is run as any conventional drill bit but offers a dual benefit by generating micro-cores of formation and by increasing the cutting efficiency of the bit. This results in the operational gains of improved geological knowledge and reduced drilling costs.