A significant challenge for oil and gas operating companies worldwide is to maximize drill bit run intervals within interbedded, hard-to-drill rock sections. In these more challenging applications, polycrystalline diamond compact (PDC) and roller cone insert designs are pushed to their limits and often fail due to PDC thermal fatigue, severe abrasion, bearing failures, or impact damage. This translates into additional trips for replacing bit types or cleaning the hole from junk left in the hole, representing significant added costs.

An additional cost that operators incur is represented by the utilization of multiple bits to finish these intervals when a single diamond-impregnated bit can complete the interval using a mud motor or a turbine. Standard mud motors have a maximum circulating life of 250 to 300 hours depending on temperatures, fluid type in the string, and a variety of other downhole environmental variables. When a challenging zone is encountered that results in the need for a high-speed drive technology, high-speed mud motors or turbines are often used with impregnated-diamond bits to complete these intervals. Tool selection criteria will consider longevity, torque capability, and temperature tolerance, amongst other variables. From the directional drilling aspect, traditionally only mud motors were used, but currently both motors and turbines are used to steer the well toward the objective.

Advanced design

To meet the industry’s needs, National Oilwell Varco developed the DuraDiamond Evolution diamond-impregnated line of drill bits. The drill bits were designed as a result of knowledge gained through experience combined with the latest materials engineering that have pushed the evolution of conventional designs, incorporating innovative design concepts coupled with the advances in manufacturing technology to respond to the hardest rock drilling market.

The drill bits combine advanced cutting structure design and materials engineering to deliver improved durability and ROP, enhancing the conventional diamond bit range of applications. The bits are proficient in drilling hard, abrasive, and interbedded formations faster. The bits were designed for multiple drive types and have three profiles for application-specific requirements. The design also includes antisticking features for bit sticking problems. The gauge retention has been dramatically enhanced as well.

Bit durability

The design enhances durability of the bit, creating a long-lasting cutting structure that can stay in the hole longer and achieve longer intervals. New design features are included to mitigate specific problems like localized wear in specific lithologies via change in grit mixes, profiles for drive type/directional trajectory and formation abrasion potential, selection in blade layouts and sleeve materials (matrix or steel), and API pin and short-gauge configurations for directional applications on motor assemblies. With these features, the product becomes more versatile for drilling not only hard and interbedded rock layers but also interbedded sequences of varying compressive strengths. The bits consist of two series: Sub Series 5 and Sub Series 7. The Sub Series 5 bit design includes abrasion- and impact-resistant PDC cutters placed in optimal positions in the profile of the bit. The sunken thermally stable polycrystalline diamonds were engineered to the cone and nose areas, providing increased wear protection. This series has sharp and straight bit blades for drilling through softer formations that require shearing action. It has an interruptions option for additional shearing and cleaning in softer formations. It also has optimized hydraulics for efficient cooling and cleaning.

The Sub Series 7 bit has an increased blade count added to the center of the bit to improve durability. The optimizing of the blades’ layout increases the contact surface area, which improves the ability to remove more rock. The round blade geometry provides more cutting surface, and the shoulder of the bit has dedicated ports and improved hydraulics, enabling it to get sufficient fluid, proper cooling, and cleaning. Selection criteria for either sub series will depend on the specific application.

Due to the high rotational speeds, wear in abrasive lithologies tends to be accelerated; therefore, the use of maximum gauge pad protection and PDC protection of the turbine sleeve is recommended. The use of steel turbine sleeves in conjunction with a turbine through a very abrasive formation is not often recommended due to the lower abrasion resistance of the steel turbine sleeves.

With their thicker and stronger blades and wider water courses, these bits have overcome the issue of having occasional blade breakage and have reduced the tendency of balling when drilling softer formations.

The bit features include a heavy-set gauge that protects the bit and prevents undergage dull condition. Other features of the bit overcome mechanical bit sticking, which has represented a significant problem in some areas.

Setting records

The bits allow for outstanding ROPs and have set numerous records in the field.

Recently in Oman, an 8½-in. DD4540 drilled an interval of 1,276 m (4,187 ft) at 2.9 m/hr (9.6 ft/hr) on turbine assembly, generating savings of more than US $400,000 for the operator. The bit performed exceptionally well, delivering the directional objective and achieving above-average ROPs compared to direct offset diamond impregnated drill bit runs.

In a challenging turbine application in Texas, a 6½-in. DD5760 drilled an interval of 934 m (3,062 ft) at 2.7 m/hr (8.8 ft/hr), achieving significantly more footage than direct offset runs in a single run. The assembly built inclination from 73° to 89° through a sequence of hard sandstone, shale, and siltstone sections.

In a deep well in Algeria with a depth out of 5,425 m (1,780 ft), two 6-in. DD5560S bits drilled from shoe to total depth, where an average of three to four bits was required on direct offset wells. The bits drilled 222% farther and reduced cost per meter by 35% compared to offsets.

A 6-in. DD5760 achieved a country record in Colombia in terms of interval drilled by one single bit through the reservoir section, completing the entire 2,336 m (764 ft) through very hard and abrasive sequences. The bit outperformed offset runs by 97% in intervals drilled and replaced three impregnated offset bit runs. The bit showed an overall good dull condition.

The diamond-impregnated drill bits provide a solution to the industry problem of needing a bit that maximizes drill bit run intervals within interbedded, hard-to-drill rock sections.