Torque and Drag Calculator: Soft String Hookload and Torque

In a vertical well, the weight you see at surface is close to the actual weight of the string. In a directional or horizontal well, friction along the wellbore wall changes that picture completely, sometimes adding tens of thousands of pounds to the hookload or, in bad cases, preventing the string from moving at all. The soft string torque and drag model is the standard first pass tool for estimating how much friction will affect hookload and rotating torque before you ever pick up the pipe. Use the calculator above to run your own section by section estimate.

Where Drag Comes From

In an inclined or horizontal section, part of the pipe's weight presses sideways against the low side of the hole rather than pulling straight down the wellbore. That sideways, or normal, force generates friction whenever the pipe moves axially or rotates. Pulling pipe out of the hole, friction adds to the weight you have to overcome. Running pipe in, friction actually reduces the effective weight helping it fall, sometimes to the point where it has to be pushed or rotated in. The soft string model assumes the pipe lies against the low side of a smooth wellbore and ignores its own bending stiffness, which keeps the math simple while still capturing the dominant effect.

The Section by Section Calculation

For each straight, constant inclination section, the axial and normal force components come from basic trigonometry on the segment weight:

Axial component = segment weight x cos(inclination), normal component = segment weight x sin(inclination)

Tension is then built up from the bottom of the string toward surface, adding friction when pulling out and subtracting it when running in:

Tension change per section = axial component +/- (friction factor x normal component)

Rotating torque comes from the same normal force acting at the pipe radius, summed over every section regardless of tripping direction, since torque only depends on rotation, not axial movement.

Worked Example

For a well with a 2000 foot horizontal lateral, a 1500 foot build section averaging 45 degrees, and a 3000 foot vertical section, using 19.5 lb/ft, 5 inch drill pipe and a friction factor of 0.25, pulling out of hole gives a surface hookload of about 94,100 lb and a total rotating torque of about 3,109 ft-lb. Compare that to the pipe's actual buoyed weight of roughly 126,750 lb for the same string in a fully vertical well, and you can see how friction and geometry both add to and subtract from the number you would naively expect.

Why This Matters

• Predicted hookload and torque are compared against actual readings while drilling to catch hole cleaning problems or differential sticking early
• Excessive drag can mean the string cannot reach total depth, or cannot rotate freely, forcing a design change
• The soft string model tends to overestimate drag slightly compared to a stiff string model, making it a reasonable conservative first check
• Real planning also accounts for buoyancy and dogleg severity, both simplified out of this basic version

Related Calculator and Reference

For the buoyed string weight feeding into this drag calculation, see the Buoyancy Factor Calculator. The soft string approach used here follows the model originally described in Johancsik, C.A., Friesen, D.B., and Dawson, R. (1984), summarized in the Wikipedia entry on torque and drag modeling.