Low penetration rates and a lack of quality cuttings are two of the main drilling challenges in drilling deep reservoirs. The extreme drilling conditions in these reservoirs are typically characterized by high hydrostatic pressures of more than 20,000 psi and temperatures of more than 149°C (300°F).

One of the primary operational issues of concern is the “hard to drill” behavior of the formations due to either the high intrinsic mechanical properties of the rock or from strengthening of the rock due to the high confining pressure. The confined compressive strength of the formations often exceeds 40,000 psi. The drilling of such hard formations usually requires specific drilling systems to achieve an efficient ROP. The best solution often involves the use of high-rpm drive systems such as turbines or high-speed motors, and a fixed cutter bit is usually the drill bit of choice.

A new drill bit solution has been jointly developed by DDS, a Tercel Oilfield Products company, and Total SA to address these problems. The MicroCore bit concept was designed with the goal of continuously generating microcores of the formation while drilling while simultaneously improving the cutting efficiency of the drill bit. Not only does this concept improve the quality of the cuttings recovered at the shakers while drilling, but it also improves the drilling performance of the bit when combined with application-specific design methodology.

Due to the limited depth of cut of fixed cutter bits in drilling these deep hard rock formations, real-time formation evaluation and analysis using cuttings are extremely difficult. The average size of the cuttings recovered at the shakers is extremely low (0.02 mm to 0.2 mm), making the cuttings irrelevant for geological identification of the formations. This occurs with all types of fixed cutter bits, including heavy-set polycrystalline diamond compact (PDC), diamond impregnated, and natural diamond fixed cutter bits. Geological identification of the formation through traditional mud logging becomes a much more complex task and could require an expensive coring operation to collect formation and geological information needed for rock identification and evaluation.

image- fixed cutter bit

A fixed cutter bit leaves the center of the hole uncut (1), which creates a core. This core is broken by the bit itself (2) and ejected at the side through a specifically designed junk slot (3). (Images courtesy of Tercel Oilfield Products)

How the bit works

The MicroCore bit concept consists of a fixed cutter bit that 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, which leads to high-quality cuttings for surface examination. The improvement is very important where normal cutting quality is poor due to use of turbines or high-speed motors combined with impregnated or natural diamond bits.

The microcores and microcore fragments generated by this bit concept differ from conventional cuttings as the texture is preserved from the shearing action of the cutting process. The cuttings produced are therefore much larger and undisturbed. The relevant petrophysical and geomechanical properties can be more easily evaluated, and these are more representative of the in situ formation properties. Cuttings of this type can be invaluable when used in conjunction with well-known formation evaluation services and technologies such as advanced cuttings characterization, fluid logging and analysis in real time, and digital rock physics.

The shearing mechanism that induces the breakage of microcores requires less energy than a conventional cutting process occurring in the center area of the drill bit. In a fixed cutter bit the center of the bit is one of the most inefficient cutting areas, as the cutting speed of the cutter is very low or none. Compared to a conventional drill bit design with a fully closed cutting structure, a significant part of the energy used in the center area can be saved.

Applying this available energy to the reduced cutting structure allows the production of a higher depth of cut for a given weight on bit, therefore increasing the cutting efficiency and overall ROP of the bit.

By removing the center, increased penetration rates can be obtained depending on the bit diameter. This increase is in the range of 15% to 25% in the laboratory when rock is drilled under atmospheric pressure condition. In field application the increase is higher, depending on the effect of the confining pressure, and has been observed at a value higher than 100% in low-ROP applications.

Application-specific designs can be produced for all types of fixed cutter bits, including heavy-set PDC, impregnated, hybrid, and natural diamond bits. The size of the microcore will range between 10 mm in diameter for smaller bit sizes of 6 in. up to 40 mm in diameter for 17½ in. and larger. Depending on the formation strength and bottomhole assembly architecture, microcore fragments and even intact microcores can be recovered at the shakers.

Wide variety of formations cored

The MicroCore bit technology has been deployed in a wide variety of applications around the world, ranging from drilling conventional development wells to reducing the number of coring jobs required in exploration wells. Although originally developed for HP/HT environments, this technology was expanded to provide more conventional applications with benefits of improved drilling efficiency and quality formation samples.

Successful applications have been completed in many different environments, including highly confined reservoirs in the North Sea; compacted or interbedded formations in Saudi Arabia, North Africa, and Pakistan; sticky shales in Colombia, the North Sea, and the US; and the deepwater presalt hard carbonates in Brazil. Runs have been successfully completed on bent motors, rotary steerable systems, and turbines. While most of the exploration applications were vertical, field development applications were completed with an inclination of up to 65°. In each case these challenges were met with application-specific designs in a variety of bit sizes, types, and International Association of Drilling Contractors (IADC) codes. Micro-Core bit designs can be made in various cutting structure types including PDC, impregnated, hybrid, and natural diamond, in both matrix and steel bodies.

Case study

A major operator drilling offshore on an ultra-deepwater well wanted to obtain better formation cuttings while drilling hard formations in the 12?-in. section. This section of the well is typically drilled with an impregnated bit run on a turbine. Consequently, the cuttings typically produced are very fine and not useful for formation evaluation. Due to the high drilling costs, the operator was also concerned about the penetration rate and durability of the bit. Tercel Oilfield Products designed a 12?-in., eight-blade, 16-mm cutter, heavy-set PDC bit (IADC M422) for the application. The resulting bit drilled 96 m (317 ft) in 99 hours, which was very comparable to the offset bit performances. Not only did the bit produce a good run in both penetration rate and meters, it continuously produced large microcore fragments throughout the entire run.