drilling · torque drag
Soft-String Drag & Torque (Straight Inclined Section)
N = W sinθ; HookloadPOOH = W cosθ + μN; HookloadRIH = W cosθ − μN; Torque = μNr
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Inputs
lbf
°
—
in
Description
Estimates sliding (axial drag) and rotating (torque) friction for a single straight, constant-inclination wellbore section using the soft-string model — the simplified, closed-form basis of the full multi-segment Johancsik et al. (1984) torque-and-drag method used in directional drilling. Computes hookload while pulling out of hole (POOH), running in hole (RIH), and rotating torque.
Variables
| Symbol | Unit | Description |
|---|---|---|
| HLpooh | lbf | Hookload — Pulling Out |
| HLrih | lbf | Hookload — Running In |
| T | ft·lbf | Rotating Torque |
| W | lbf | Total buoyed weight of the straight, constant-inclination section being analyzed (from casing_buoyed_weight × section length, or a drillstring weight table). |
| θ | ° | Constant wellbore inclination through the section being analyzed. |
| μ | — | Sliding friction factor between the string/casing and the wellbore wall — typically 0.15-0.25 in water-based mud, 0.10-0.20 in oil-based mud, higher in unlubricated or high-angle open hole. |
| r | in | Outer radius of the pipe or tool joint contacting the wellbore wall (88.9 mm = 3.5 in radius, a common 7 in tool-joint OD). |
Assumptions
- Single straight section at constant inclination — no build, turn, or tortuosity within the section analyzed
- Soft-string model: pipe is assumed perfectly flexible, with no bending stiffness resistance (reasonable for most drillpipe in moderate-curvature wells)
- A single, constant friction factor represents the entire contact length — real wellbores show friction factor variation with mud type, cuttings loading, and hole condition
Limitations
- This is a single-segment approximation — a full wellbore torque-and-drag analysis requires summing forces/torques over many short segments along the entire trajectory (build sections, turns, and all straight sections), each with its own local inclination and azimuth change
- The soft-string model underestimates torque and drag in high-curvature or high-tortuosity wells where pipe bending stiffness (stiff-string effects) becomes significant
- Does not include buoyancy changes from a fluid column with varying density, or the effect of cuttings bed accumulation on the low side of the hole in horizontal sections
Use Cases
- → Quick drag/torque screening: Get a fast estimate of friction effects in a single curve or lateral section before running full multi-segment torque-and-drag software.
- → Friction factor back-calculation: Compare measured surface hookload/torque against this model to back-calculate an effective friction factor for the current wellbore condition.
- → Lateral reach feasibility check: Estimate whether a planned lateral length is achievable by slide drilling (RIH hookload must stay positive) before committing to the well design.
Related Calculations
Region Notes
Permian Basin
Long laterals (2-3 mi) in the Wolfcamp/Bone Spring routinely see friction factors of 0.20-0.30 in water-based mud, with torque and drag becoming the limiting factor on achievable lateral length well before torsional/tensile pipe limits are reached.
Bakken
Extended-reach laterals commonly use oil-based or synthetic-based mud specifically to hold friction factors near 0.15-0.20, extending achievable lateral length versus water-based systems.
Eagle Ford
High-friction intervals (e.g., through unstable shale sections prone to cuttings bed formation) can require friction-reducing additives or roller-equipped tubulars to keep RIH hookload positive in long laterals.
Global
Friction factors back-calculated from actual hookload/torque data on offset wells are far more reliable for planning than generic literature values — always calibrate against local field data where available.
References
Primary source
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