This month’s cover features a variety of new bits optimized to drill some of the best-known US shale plays. Bit designs are tailored to specific well construction requirements such as drilling lateral or curve sections. Performance continues to improve, saving drilling time and setting new records.

Bit companies use finite element analyses and drilling simulators to model and assess designs. Varel International engineers used SPOT, a virtual drilling simulator, to design the new Diamond Edge series, featuring an asymmetric blade cutting structure. Hughes Christensen engineers modeled bit dynamics and used a downhole simulator to optimize force distribution in the new Quantec Force bits for stability and drilling efficiency.

Bit use

Polycrystalline diamond compact (PDC) bits have overtaken all other bit types in the US. According to Baker Hughes, PDC bits were used to drill more than 80% of US land footage in 2009. Roller-cone bits with tungsten carbide inserts are often used to drill particularly abrasive formations, such as the Wasatch sandstone in the Pinedale or extra-hard zones in the Permian Basin.

Reaching shale reservoirs can also involve drilling through abrasive, interbedded formations such as hard sandstones, cherts, carbonates, or pyrite-rich zones. Drillers contend with the abrasive Atoka sandstone and Bend conglomerate to reach the Barnett shale; Smith Bits markets its FHi25B bit for directional work in the Atoka.

The Hosston, Travis Peak, and Cotton Valley sequence of ratty hard sands, soft shales, and limestones challenges drillers in Texas and Louisiana. Ulterra’s MS1688CDEF bit drills these sections in the Haynesville play. The bit has a matrix body and eight blades with double rows of 16-mm cutters and extended (4-in.) gauge pads.

There’s a trend toward toughening up PDC bits to handle more abrasive formations as well as softer shale sections. This means assessing the configuration of cutters, blades, nozzles, and gauge pads to maximize stability and durability.

Cutters

Diamond cutters work most effectively when shearing the rock face. But abrasive formations can dull the cutting edge, which is why PDC bits are pulled and replaced with roller-cones through certain hard sections. Hughes Christensen adds a carbide interface to its newer PDC bits to protect the cutting edge.

Varying weight on bit (WOB) can overload and damage diamond cutters. Increasing WOB for deeper cutting can lead to stick-slip and stress build-up on the cutting edges. Reducing WOB for shallower cutting can induce “bit whirl” and vibrations that shock the cutting surface. Both stick-slip and intense vibration can break cutters.

One solution is Ulterra’s TorkBuster, coupled with a United Diamond bit. The UD513 bit is a market leader in the Haynesville shale, used by 12 operators, and has drilled the fastest overall lateral and achieved the highest footage/day. This five-blade bit has a flat cutting profile with single rows of cutters and extended gauge to help maintain angle while rotating. Long (6-in.) gauge pads are available for rotary steerable assemblies. Extra-long (10-in.) gauge pads are used with turbine-drive assemblies. Short and step-gauge designs are used to build curve sections (10-14°/100 ft).

Another bit designed to minimize stick-slip is NOV’s MSR616 SystemMatched directional PDC bit with SmoothTorque inserts. This bit can be used in different plays by altering the depth of cut, which directly affects torque response, critical to the directional capability and stability of the bit. Predictable and smooth torque response to applied WOB reduces torque variance, reduces torsional vibration, and improves directional control.

Layout

PDC blades are most often in a spiral or kerfing pattern. Varel’s VM519HU bit has five blades and seven nozzles, specifically designed to drill lateral sections in the Barnett. Smith Bits’ MDSi613 bit is designed to drill out and long footage intervals in the Barnett and Woodford, and the MDi711 bit is designed for curve and horizontal drilling in the Haynesville.

The gauge area contributes stability, varies in length, may include hardened inserts, or be entirely absent. Encore Bit’s SideWinder bits have no gauge section as the cutting structure wraps entirely around the flank. These bits were designed to aggressively drill the curve in horizontal wells. Encore uses the S-6166 bit in the Haynesville, the S-7136 and SW-6133D in the Woodford, and the SW-3136D in the Eagle Ford shales.

Halliburton’s Security DBS designed the FX73 bit for drilling the curve and lateral sections in the Haynesville. It features a flatter profile for directional control, a shorter gauge section for directional responsiveness, and increased blade count for better torque response. Security DBS runs the FXD55 bit in the Barnett.

Well-designed bits work efficiently, drill faster, and last longer.