Changes Needed To Modernize Torque-and-Drag Software
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 104609, "Step Changes Needed To Modernize T&D Software," by C.J. Mason, SPE, BP Exploration, and D. C.-K. Chen, SPE, Halliburton Sperry Services, prepared for the 2007 SPE/IADC Drilling Conference, Amsterdam, 20-22 February.
Torque-and-drag modeling is invaluable in well planning to predict and prevent drilling problems. The full-length paper outlines some limitations of current torque-and-drag models and proposes some functional requirements for future models. Probably the most important requirement in future models is a more realistic stiff-string model to account for the effect of tubular-stiffness, hole-clearance, and tortuosity effects correctly.

Introduction
The basic mathematical model used in most torque-and-drag software has not changed significantly since its original inception. By contrast, software user interfaces have improved dramatically as computer hardware, processor power, and software functionality have evolved. On the basis of more than 20 years of industry access to torque-and-drag drillstring-simulation software, the time is right to reflect on the state of current models and identify future requirements.

Torque-and-Drag Software
Torque-and-drag software commonly is used during well planning to ensure that the proposed well path can be drilled and completed with available equipment. It also is used for rig sizing and drillstring optimization to ensure that well designs are feasible. At a more advanced level, it is used to determine drilling limits for field-development options.
For challenging or complex wells, torque-and-drag software often is used in real-time mode. Torque-and-drag trend lines are predicted in advance and monitored during operations for adherence. Virtually all torque-and-drag software uses steady-state models.

Soft-String Model
The soft-string model ignores any tubular-stiffness effects; this means that the pipe is treated as a heavy cable, chain, or rope lying along the wellbore. This means that axial tension and torque forces are supported by the string and contact forces are supported by the wellbore. This model assumes that the loads on the string result solely from the combined effects of gravity and frictional drag, a result of contact between the string and the wellbore.
Virtually all torque-and-drag soft-string models assume that the drillstring is made up of short elements joined by connections that transmit tension, compression, and torsion but not bending moment. The calculations start at the bottom of the string and proceed upward to the surface. Each short element contributes increments of torque, axial drag, and weight. Forces and torque values are summed to produce the cumulative loads on the drillstring. Initial conditions usually are specified at the bit, and for drilling operations, the inputs would be weight-on-bit (WOB) and bit-torque values.
The most significant assumption is that the string is in continuous contact with the wellbore over its entire length. This also means that radial clearance effects are ignored and that the bending moment is not considered in the model.