Bicenter Bit: Eccentric Underreaming Design for Well Re-Entry and CWD

Key Takeaways

• Bicenter bit geometry: pilot OD, wing OD, and gauge step-out calculation: The two critical dimensions of a bicenter bit are: (1) the pilot diameter — the OD of the central cutting structure that passes through any existing restrictions; and (2) the gauge diameter (or wing diameter) — the OD of the borehole drilled by the combination of pilot and wing cutter after the bit exits the restriction and rotates freely. The gauge step-out (the amount by which the wing cutter enlarges the borehole beyond the pilot) equals (gauge diameter - pilot diameter) / 2, which is the radial eccentricity of the wing cutter axis relative to the string axis. For a typical WCSB 9-5/8 inch casing deepening application: pilot OD = 8.5 inches (passes through 8.625 inch drift), gauge OD = 10.625 inches, step-out = (10.625 - 8.5) / 2 = 1.0625 inches. The wing cutter sweeps this 1.0625-inch step-out at each point of its rotation, creating the 10.625-inch gauge borehole. The bit must be passed through the casing in a specific orientation — typically with the wing cutter folded back (in folding or retractable designs) or oriented parallel to the casing axis (in fixed wing designs) — before emerging below the shoe and rotating to gauge. Fixed-wing bicenter bits (most common PDC designs) rely on the casing drift clearance between the wing cutter OD and the casing drift to be just sufficient for the bit to pass with light lateral force; the bit is rotated slowly (10-20 RPM) while running in hole through the casing, and the wing cutter OD must be smaller than the casing drift (with 1/16" to 1/8" clearance) to ensure passage without getting hung up in the casing joints.
• PDC bicenter bit design: cutter layout on pilot and wing: A PDC bicenter bit places cutters on both the pilot portion and the wing cutter portion of the bit body to create a combined cutting action that efficiently removes formation material across the full gauge diameter. On the pilot section, PDC cutters are arranged in a standard conical profile (flat, apex, taper, shoulder, gauge zones) at cutter densities of 2-4 cutters per blade, sized 13-16 mm cutter diameter for medium formations. The wing cutter section adds a laterally offset blade with 4-8 cutters at 16-19 mm diameter, sized for higher loading because the wing cutters operate at larger effective cutting radius (higher peripheral velocity at the same RPM as the pilot) and must remove the step-out formation outside the pilot diameter. The wing cutter geometry must balance two competing requirements: aggressive cutting action (to maintain acceptable ROP from the wing) and geometric stability (to maintain the gauge diameter accurately without the wing cutter drifting inward and creating an undersized borehole or outward and creating an oversized borehole that would not seat a subsequent liner string). Most WCSB bicenter bit designs place a single row of gauge-trimmer elements (polished gauge OD PDC cylinders) at the extreme gauge point of the wing cutter to maintain a consistent diameter even as the cutting PDC elements wear during drilling. Wing cutter blade count: 1-2 blades on most bicenter designs (fewer than the 4-6 blades on a standard PDC bit) to maintain the slim profile necessary for passing through casing while providing sufficient lateral stability in the drilled borehole.
• WCSB well deepening applications: bypassing formation restrictions: In the WCSB, bicenter bits are used in three main operational scenarios. (1) Well deepening through existing casing: a producing well with a known hydrocarbon target below an existing casing shoe is deepened by drilling through the casing shoe with a bicenter bit that passes the casing shoe drift and then drills to the new target depth at a gauge large enough to run a production liner. For example, a Viking well with 5-1/2 inch production casing set at 950 m depth is deepened to a Mannville target at 1,420 m by running a bicenter bit through the 4.778-inch drift of the 5-1/2 inch casing (wall thickness 14.14 lb/ft, J55 grade), which requires a bicenter pilot OD of 4.75 inches and drills a 6.0-inch gauge borehole at the wing — sufficient to run a 4-1/2 inch production liner. (2) Sidetrack through casing exit windows: when a casing exit window has been milled by a casing window mill, the through-window OD (typically 2-4 inches smaller than the main borehole) limits conventional bit sizes for the sidetrack well. A bicenter bit that passes the window can begin building the sidetrack wellbore at full gauge below the window exit point. (3) Casing while drilling (CWD): in CWD operations where casing is drilled and cemented simultaneously without removing the bottom hole assembly, a bicenter bit integrated with the casing shoe assembly passes through the casing during drilling and enlarges the borehole just below the shoe to the required liner or open-hole size.
• Bicenter bit limitations: torque asymmetry and wellbore quality: The eccentric geometry of the bicenter bit creates technical challenges that limit its application relative to conventional PDC bits. (1) Torque asymmetry: because the wing cutter is offset from the string axis, the lateral cutting forces from the wing are not balanced by an equal force on the opposite side of the bit, creating a net lateral force (side force) on the bit that tends to push the bit against the borehole wall on the pilot side. This side force causes the bit to walk or deviate laterally during drilling, making it very difficult to maintain a straight or precisely planned wellbore trajectory. Bicenter bits are therefore generally unsuitable for directional well sections where azimuth and inclination control are critical; they are used primarily in vertical deepening applications or in horizontal re-entry sections where the well path is already established and minor lateral drift is acceptable. (2) Borehole quality: the sequentially swept cutting action of the wing (which cuts a different arc than the pilot per revolution) can create minor geometric imperfections in the borehole cross-section — a slightly non-circular hole that may cause tight spots for subsequent casing or liner running operations. Most WCSB bicenter bit applications size the wing gauge to be 0.5-1.0 inch larger than the liner OD to be run, ensuring adequate clearance even if the borehole is slightly non-circular. (3) ROP limitation: bicenter bits typically drill at 70-85% of the ROP of an equivalent diameter conventional PDC bit because the eccentric geometry creates intermittent cutting action (the wing contacts the formation for part of each revolution only) and higher vibration levels than symmetrical bits. The ROP compromise is accepted for the geometric access advantage the bit provides.
• Bicenter bit selection criteria for WCSB re-entry programs: Selecting the right bicenter bit for a WCSB well deepening or re-entry program requires specifying: (1) Pilot OD (must pass casing drift with minimum 1/16 inch clearance on radius, i.e., pilot OD ≤ casing drift - 1/8 inch); (2) Gauge OD (large enough for subsequent liner/casing of the target completion size plus cementing clearance of minimum 0.75 inch annulus on radius, i.e., gauge OD ≥ liner OD + 1.5 inches minimum); (3) Cutting structure compatibility with the target formation UCS (PDC for <138 MPa, hybrid PDC-roller cone for 138-206 MPa); (4) Hydraulics nozzle sizing for the target pump rate through the casing and open hole (bicenter bits have complex nozzle paths to route fluid through the pilot and wing sections); (5) Bit weight (WOB) and RPM range from the motor or rotary table (bicenter bits typically require reduced WOB, 30-50% of equivalent diameter conventional PDC, to prevent the wing from loading too aggressively and generating excessive side force). WCSB bicenter bit suppliers (National Oilwell Varco, Ulterra, Downhole Technology Solutions) maintain a range of standard sizes for the most common WCSB casing OD/liner OD deepening combinations, with custom sizes available for non-standard re-entry geometries at 6-10 weeks lead time.