# An Illustration and A Twisted Finger

Let’s illustrate in an easier way what torque and drag is:

Hold your index finger tightly in the fist of your other hand.

Now twist your finger.

Do you feel how your finger does not want to twist?

Yes!

You know why?

Because it's not built to be twisted.

The pain you feel is because of the torque you are putting on the joints.  - That's torque.

Do you feel how your finger resists the pull because you have a good grip on it with your fist? - That's drag.

Torque and drag can have a dramatic increase in horizontal and extended-reach wells and can become the limiting factor in determining the horizontal length or extended-reach of a well. For this reason, precise calculations of torque and drag are necessary for drilling operations. Torque and drag are the results of friction caused by a moving pipe inside the wellbore: torque occurs when rotating the pipe along the wellbore and drag occurs when moving the pipe.

When drilling horizontal or extended-reach wells, excessive torque and drag may become troublesome both in the drilling operations and later in the completion operations. Estimating torque and drag is very important, but the calculation of drag in the build section of a well is complicated by the effect of the axial force (tensile or compressive) on the lateral contact force which produces the sliding drag and in turn causes changes to the axial force itself. The axial force has a great effect on the torque and drag calculations in the build section. When the axial force (tension or compression) becomes large enough to let the pipe contact only one side of the wellbore, the torque and drag in the build section will increase proportionally with the increase in the axial force.

The most common way to calculate approximate torque and drag values in the build section involves monotonous numerical calculations: dividing the build section into many small pieces, assuming the axial force remains constant in those small pieces, calculating the friction factor for each of the pieces, and then summing these values to get the total drag over the entire build section. This process is both time-consuming and difficult for field engineers.

The analysis of torque and drag is made easier by today’s technology. There is a comprehensive torque and drag software in the market that removes many of the risks during the drilling process. This software was developed by PVI and it’s called TADPRO (Torque and Drag).

This software comes with features that help users to:

• Calculate hookload and surface torque
• Identify potential buckling
• Perform sensitivity analysis
• Determine side force
• Analyze forces downhole
• User-friendliness and graphical outputs

Illustrations have always been a great learning method and today we have learned two things:

1. Fingers were not made to be twisted.
2. Likewise a pipe is not built to be twisted, but the torque and drag inevitably occurs during horizontal drilling, but with the help of TADPRO, torque and drag can be calculated and predicted, therefore the risks are reduced.

# Connecting Dots with Lines Using Drilling Software

Dots are isolated incidents, individual cases, snap shots, discrete numbers, etc. Looking at dots, we obtain limited information, but more often, we miss the big picture if our vision is restricted to the dots only.

One of the uncertainties in torque and drag analysis for drilling is the friction factor, because it is dependent on many things such as mud type (oil-based or water-based), pipe moving in casing (steel) or in open hole (rock), cutting concentration, etc. If we can observe hookloads or surface torque for certain operation at various depths, we can calibrate friction factor (back calculation). The following picture is a screen from TADPRO (torque and drag model). You can see from the dots (field measurements) and the lines (model prediction) that the friction factor of 0.32 is a good estimation for slack off operation.

Individual case study with one set of input data only tells one story. If we can compile the output from multiple runs of computer model, we can see the trend. This process is automated in many of our drilling software models. The following pictures show the impacts of open hole excess on top of cement (TOC) and hydrostatic pressure difference.

We call this powerful feature of automatic run on multiple cases “Sensitivity Study”.

Steve Jobs once said this in his famous commencement address to Stanford University: “You can’t connect the dots looking forward; you can only connect them looking backwards. So you have to trust that the dots will somehow connect in your future.”

Connecting dots in our lives gives us confidence to follow our hearts even when it leads us off the well worn path.  Connecting dots in drilling engineering reveals the trends of operation date and window of safety to operate.

Let our drilling software assist you connect more dots.

# Drilling Software: Your Hot Model

Everyone including drilling engineer seems to be busier than before – There are always more wells to be drilled, more projects to be managed, more well planning to be done and reports to be run. Let us face it: you and I are not supermen or ironmen. We are just normal John and Joe, facing the increasing demanding.

Any help would be helpful. Imagine an assistant, who could be on-call 24/7! And she does not charge overtime or complain about tasks. One might think this kind of helper is a far-fetched idea. But what is in my mind is an assistant you can actually own and this assistant is most likely a super model: drilling engineering model.

All of us hate to do repeated tasks, and that is why we invented dish washer and vacuum cleaner. From abacus, to calculator, to computer, we calculate more and faster. It might be fun to find the greatest common divisor in 3rd grade, but hand calculation of a buoyant weight of pipe in mud is more of a tedious step than an exciting destination. We have important technical decisions to be made in office and on rig floor.

That is why drilling engineering software such as PVI’s TADPRO (torque and drag model) comes handy. It frees us from repeated manual calculation and errors associated. These drilling models do not discriminate vertical, deviated, horizontal or ERD wells. Provided with proper input data, these model assistants perform analysis, run reports, show animation within a few seconds. If you are not sure about certain variable such as friction factor in open hole sections, they like many attentive helpers, even give you sensitivity study to show you the best and worst scenarios.

Sometimes, these models are just like robots, receiving orders and performing tasks. Other times, they give you crucial advices and identify potential problems, such as helical buckling or excessive downhole pressure. They are like a senior consultant who is amiable and does not charge premium.

These models are strictly trained to be friendly. They do not care much if you give them a job after 5pm or in weekends. They are very tolerant if you accidently key in the wrong data. They do not give you an eye. Instead, they kindly remind you of the error and even suggest you the correct input in many cases.

Did I mention that these models are very pretty? Their faces (what we call interface) are carefully designed so that users will not get lost and interaction creates pleasant experience.

Drilling is tough, find good software - your hot model.

# Torque and Drag - Nuts and Bolts

No matter it is an operation of drilling or casing running; any pipe movement in the deviated wellbore produces torque and drag (T&D) along the pipe. T&D is our weapon to drill a well or run a casing to the bottom. However, excessive T&D will cause equipment and operation failure.

Basically, axial movements such as drilling ahead or tripping creates drag, while rotation produces torque. The magnitude of T&D is determined by the combination of these two movements. Rotation shifts the resistance from drag to torque. In other words, you can shift the drag to torque by rotating the pipe. That is why people tend to rotate the pipe little bit if pipe gets stuck.

Torque and Drag Calculation

Typical T&D analysis starts by dividing the pipe into small elements. Calculation begins from the element at the bottom of the pipe, where weight on bit (WOB) or torque on bit (TOB) is expected. For each element, force and torque are balanced and the T&D at the top of the element are calculated step by step and from bottom to top, calculation is performed for each pipe element, until it reaches the rig floor. We call the torque and drag at the top of pipe surface torque and hook load (with block weight), respectively.

Torque and Drag Common Terms

Some terms often used in torque and drag analysis are listed here with explanations:

• Friction Factor (F.F.) - the representation of the friction between the wellbore/casing and the work string. The friction factor is dependent on mud type, pipe and wellbore and cutting concentration. Higher cutting concentration leads to higher friction factor.
• Rotating Off Bottom (ROffB) – pipe rotates without any axial movement, such as rate of penetration or tripping. There is no WOB or TOB because bit is not engaged with formation.
• Rotating On Bottom (ROnB) – pipe rotates without any axial movement, such as rate of penetration or tripping. However, WOB and TOB are present because bit is engaged with formation.
• Drilling – pipe rotates with certain rate of penetration and with the presence of WOB and TOB.
• Slide Drilling - Drilling with no drill string rotation. (only axial movement, no rotation)
• Sinusoidal Buckling - Sinusoidal buckling occurs when compressive forces on the string become too high, resulting in a snake-like bending in the string. Note that in this mode, the pipe deforms, but still in a 2D plan.
• Helical Buckling - a more extreme form of buckling which occurs when compressive forces pass through sinusoidal buckling and exceed the helical buckling limit. Helical buckling causes contact between the pipe and the wellbore, exerting force on the wall of the hole. Both drill string fatigue and interference with weight transfer to the bit occur. Helical buckling should be avoided.
• Helical Lockup - Helical lockup occurs when compressive forces on a string in helical buckling prevent axial movement. Forces at surface are not transmitted to the bit.
• Tension Limit- The tension limit of a material is based on its yield strength, which is measured in psi. When the minimum yield strength is exceeded, pipe will plastically deform. Plastic deformation occurs when pipe that has stretched does not return to its original shape.
• Make Up Torque- The rotational force used to make up a connection in the string. Drill pipe failure may occur when the make-up torque of a connection is exceeded.
• Stress in the String - The various stress that TADPRO models are axial, bending, torsional, and shear stresses. These stresses are summed up in the Von Mises Stress. Various failures occur as a result of repeated stress to a string, including cracking, washouts, and twist offs, etc.
• Stress in the String - The various stress that TADPRO models are axial, bending, torsional, and shear stresses. These stresses are summed up in the Von Mises Stress. Various failures occur as a result of repeated stress to a string, including cracking, washouts, and twist offs, etc.
•  Casing Wear- Prolonged, repeated axial and rotational movement within casing will wear both at the string and the casing, potentially leading to string and casing failure.