Why do we need to perform torque and drag analysis? Torque and drag removes superstition and reduces dependence on assumptions from well planning by taking into account all aspects of the plan from equipment specifications and formation properties. Changes made to specific components during planning is readily computed for the entire system. This allows for rapid trial and error testing during planning to find working solutions to reach desired well TDs.
Drillsoft HDX Torque and Drag module calculates axial tension, stretch, twist, hook load, and torque. It supports conventional and unconventional drilling operations which allows the modeling of all field operations including (RIH, POOH, Drilling and Rotating Off Bottom) for drill string, casing string, completion, and coiled tubing manufactured from steel and non-steel materials.
What is torque and what is drag? In our case, drag is an axial force, generated between the contact force between the drill string when it’s moving and the formation or the casing. Torque, is the rotational force between the drill string and the formation.
There are two main causes of torque and drag, side forces and friction. Side forces include the forces that exist between the wellbore wall and any element of the drill string. Side forces can also include the normal forces created by the drill string with the wellbore walls. Friction is caused due to the movement of the drill string. Other causes for torque and drag include hole cleaning, under-gauge holes, and wellbore stability.
Side forces and Components
There are a few main factors that can cause side forces. These factors include the weight of the drill string, tension due to dogleg severity, buckling, and stiffness of the drill string. The majority of side force is generated by the first two factors. However, while buckling and stiffness contribute less to generating side forces, they should not be ignored as they indicate problems with the equipment chosen.
The weight of the drill string acting on the wellbore is a major contributor to side forces. The weight of the drill string contributes to the magnitude of the normal force against the wellbore and as such, heavier drill strings induce greater side forces against wellbores. Therefore, it is important to minimize the weight of the drill string within safe limits.
Side forces created by tension typically only occur with doglegs. In build sections, the direction of pipe movement affects the direction of side forces. When moving down, the drill string will be lying on the low side, therefore the effect of tensile load is reduced and the resultant side force is in the direction of the weight component. When moving up, tripping out, the drill string will be in contact with the high side, therefore the effect of tensile load is increased and the resultant side force will be in the direction of the tensile component. In drop sections, regardless of whether the drill string is tripping out or running in hole, the drill string will be lying on the low side of the wellbore. Therefore, the resultant force will always act on the low side.
Buckling occurs when the drill string is in compression. Sinusoidal buckling occurs first, however, if the weight on bit is increased, helical buckling will begin. Buckling increases the contact surface area between the drill string and the wellbore, which increases side forces. Side forces from stiffness occurs when stiff drill strings pass through high dogleg zones. However, it is more likely for key-seating or stuck drill string to occur prior to experiencing side forces due to stiffness.
Friction and Components
When drilling, the rotation friction factor is primary source of concern as it affects the final torque output. It is the friction between the wellbore and the drill string when you rotate the string. The translation friction factor is an issue when tripping in and out of hole because rotation is not a factor during tripping. During back-reaming, both the rotation and translation friction factors are of concern because both rotation and axial movement occurs. The use of incorrect friction factors could lead to inaccurate designs.
Using Drillsoft Torque and Drag Analysis
Values that must be defined prior to performing torque and drag analysis include
• Mud Specs / Fluids
• Technical Limits
When all of the above data is specified, analysis can begin. In the Torque & Drag menu, click on the T&D Analysis button to acquire results.
The results of the torque and drag analysis will be shown in an analysis window. Here you will find graphs for various tension and torque scenarios during drilling. These different graphs can be viewed by selecting the corresponding option in the Graphic Options field. If no RPM was specified prior to performing torque and drag analysis, torque graphs will be empty. Torque and drag analysis must be performed for each desired RPM as results will only be valid for a single specified RPM at a time.
The lines representing weights for different scenarios can be toggled into and out of view with the checkbox found at the top right of the torque and drag analysis summary window. These weights include the pickup weight (PUW), slack off weight (SOW), and the rotating weight (ROT).
The Hookload While Tripping graph contains additional lines for possible open-hole friction factors. These are ±0.05 of what is specified in the Torque & Drag sidebar menu prior to performing analysis. It is possible to toggle the view of these
Figure 1: Hookload wile Tripping Graph
Figure 2: Drillstring Tension graph
Figure 3: Torque While Tripping graph
Figure 4: Drillstring Torque graph
Expected values for torque, weight, stretch, and windup, will be listed for different load cases. These cases include back-reaming, reaming, rotating, and sliding. This information can be found in the Load Case table in the torque and drag analysis summary window.
Figure 5: Load Case table