Roller Stem

Key Takeaways

• The friction reduction mechanism of the roller stem is based on the fundamental difference between static/sliding friction and rolling friction in the contact between two surfaces: in a horizontal or near-horizontal wellbore, the drill string lies against the low side of the borehole wall under its own weight (the normal force at the contact point equals the component of the string weight perpendicular to the wellbore axis), and any axial movement of the string (sliding in the wellbore while the pipe does not rotate, as occurs during sliding mode directional drilling with a mud motor) generates a friction force equal to the normal force multiplied by the sliding friction coefficient (typically 0.2-0.4 for metal-on-rock contact in drilling fluid); roller stems replace this metal-on-rock contact with rolling elements (carbide-faced rollers, bearing-supported wheels, or roller centralizer blades) whose rolling friction coefficient is 10-30 times lower than sliding friction, dramatically reducing the axial drag force on the string; the drag reduction allows more of the hook load (the weight applied at the surface) to be transmitted as bit weight to the bottom of the horizontal section, enabling effective drilling without excessive surface weight application that would cause sinusoidal or helical buckling of the drill string in compression.
• Torque and drag modeling is the analytical foundation for deciding when and where to place roller stems in a drill string, and the Johansick-Dawson-Williamson (JDW) soft-string model and its successors are the standard tools for this analysis: the drag force on a horizontal section of drill string is the integral of the product of the normal contact force per unit length (governed by string weight, buoyancy, and wellbore curvature) and the friction coefficient over the length of the section; substituting the rolling friction coefficient of the roller stem elements for the sliding friction coefficient of conventional drill pipe over the horizontal section reduces the computed drag significantly, and the model predicts whether the resulting hook load and bit weight can be achieved within the surface equipment constraints (drawworks capacity and top drive torque limit); in highly extended-reach wells (departure-to-depth ratios greater than 2.0), torque and drag modeling with roller stem elements frequently shows that conventional drill string assemblies cannot deliver adequate bit weight without buckling, while roller stem assemblies can, justifying the higher cost of the specialized component; the drag model also identifies the critical locations in the wellbore where friction is highest (typically at the build section entry and at points of wellbore curvature in the lateral) and guides the placement of roller stem sections to achieve the maximum drag reduction per unit of additional cost.
• Drill string buckling in horizontal sections is the failure mode that roller stems are specifically designed to prevent: when the compressive load in the drill string (the difference between the hook load and the weight of the string below the neutral point) exceeds the critical buckling load for sinusoidal buckling in the wellbore (a function of string stiffness, wellbore diameter, and inclination), the string adopts a sinusoidal shape against the wellbore wall instead of remaining straight; this increases the normal contact force, which increases drag, which requires more weight-on-bit to achieve the target drilling rate, which increases the compression and promotes helical buckling (the more severe buckling mode in which the string adopts a corkscrew shape); helically buckled drill string exerts large bending stresses at the tool joints that can cause fatigue cracks, severely increases drag (because the contact pattern wraps around the wellbore), and may damage the wellbore wall and formation; roller stems, by reducing the friction and therefore the required surface hook load for a given bit weight, lower the compressive load in the horizontal section below the sinusoidal buckling threshold, preventing the buckling-drag escalation cycle that limits drilling reach in conventional assemblies; the improvement in reach is commonly 20-40% over conventional assemblies in wells where drag was the binding constraint.
• Roller stem design considerations include the roller element material (carbide-faced rollers for abrasive formations, rubber or polyurethane-coated rollers for softer formations where wellbore damage is a concern), the roller spacing along the stem (closer spacing reduces the sag of the stem between contact points and the consequent increase in contact force at the rollers), the roller clearance relative to the wellbore diameter (too small a clearance causes the rollers to contact the wellbore wall even in straight sections where the string is not deflected, increasing drag unnecessarily; too large a clearance allows the string to deflect excessively between roller contact points, degrading the friction reduction), and the bearing design (sealed bearings lubricated with grease for extended service intervals, or open bearings continuously lubricated by the drilling fluid for applications where grease life at high temperature is a concern); the maximum roller load rating must exceed the maximum normal force expected at each roller contact point under the most severe wellbore curvature and string weight conditions encountered in the planned well, with an appropriate safety factor; roller stems are typically inspected and the roller elements replaced or refurbished at each bit run to maintain consistent friction reduction performance throughout the drilling of the horizontal section.
• Applications of roller stems extend beyond conventional oil and gas drilling to geothermal well drilling and underground gravity surveys (where extremely long horizontal sections are drilled in formations with very low natural permeability), and to coiled tubing drilling in horizontal wells (where the coiled tubing string cannot be rotated to reduce drag and must therefore rely on surface-deployed friction-reducing tools at the coiled tubing connector to the BHA); in coiled tubing drilling, a variant of the roller stem concept called a "friction reducer" or "tractor" uses rollers to reduce the tubing-to-casing friction in the horizontal section, allowing the coiled tubing to be pushed deeper into the lateral than would be possible by surface push force alone; the reach of coiled tubing in horizontal laterals without friction reduction is typically limited to 3,000-4,000 feet by the column of tubing pushing on the wellbore wall, while friction reduction tools extend this reach to 6,000-8,000 feet or more in favorable conditions; this reach extension directly determines the length of lateral that can be drilled and stimulated per coiled tubing intervention, which is a key economic driver for coiled tubing drilling programs in unconventional resource plays.