# 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.

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.

There is a good saying in oil drilling area: “Oil’s been found where it’s been found before.” This sentence has been proven to be so true again by the recent booming of oil drilling industry around USA.

A new drilling surge is happening around USA this year. Some people called it a “miracle”. Indeed it is a miracle due to the highly developing drilling technology. Horizontal drilling and fracking are two contributors to this miracle. Now in Houston, we saw that the energy companies, oilfield contractors and even landowners are rushing again into the profitable drilling industry.

Horizontal drilling is not a new thing. This drilling technique has been a hot topic for engineers and researchers for a while, but not been widely applied until 2003. Horizontal drilling is defined in Lynn Helm’s paper as "Horizontal Drilling": "Horizontal drilling is the process of drilling a well from the surface to a subsurface location just above the target oil or gas reservoir called the “kickoff point”, then deviating the wellbore from the vertical plane around a curve to intersect the reservoir at the “entry point” with a near-horizontal inclination and remaining within the reservoir until the desired bottom hole location is reached." Horizontal drilling has been going on for years in other states besides Texas around the country, including Colorado, Wyoming, New Mexico and Alabama.

However, extended-reach directional wells are becoming more prevalent today, which means that tubulars are exposed to greater amounts of torque and drag (T&D). If this torque and drag is not evaluated properly, it’ll result in stuck pipe, pipe failures and costly fishing jobs, not even mention the effects of environment contamination.

TADPRO, a comprehensive torque and drag software developed by PVI helps remove many risks of drilling program, completion design and specific tool operation. Limits in the length of a horizontal based on specific friction factors can be determined. The ability to get needed weight to a liner-top packer can also be evaluated. With the ability to analyze forces downhole, rig equipment specifications for torque and hookload can be predicted.

With unparalleled user-friendliness and graphical outputs in the industry, TADPRO provides both versatility and accuracy in its calculations, while also integrating advanced features that make it extremely easy to use and interpret results.

Although horizontal drilling didn't draw as much attention as "fracking", it is a marvel that truly benefits the drillers, just as drilling software does.

# Drilling Software: See the Invisible

We just had a T-Shirt design contest. Graphic designers were asked to put drilling software in perspective of the design.

Nowadays, drilling engineering software is an indispensable tool for drilling engineers. Directional and extended reach drilling and HTHP wells challenge us with more and more technical difficulties. Software is one of the solutions to these problems. However, because of its abstract nature, it is not easy to illustrate drilling software in a T-shirt design. At least, it is difficult to have a good design without using abused images of drilling rigs and computers.

One of the submissions is a pair of glasses as shown below.

As soon as we saw it, we liked it. It has a subtle message: our drilling software allows engineers to see the downhole condition, which is invisible to normal eyes.

Oil and gas well is created by drilling a hole of 5 to 50 inches in diameter into the earth with a drilling rig that rotates a drill string with a bit attached. During drilling process, engineers and drillers heavily rely on the limited information on the rig floor to determine the downhole condition, because they cannot see the subsurface.

Typically, only one tenth of iceberg is above water. Majority of iceberg is below water, making it difficult to determine its shape and size. Similar situation exists on the rig floor. Drilling engineers only have handful observations such as hookload, surface torque, pump pressure, ROP, RPM, etc. They can neither see what happens to drill string or formation nor accurately measure the buckling of the pipe. It is like a situation that we walk cross a muddy river: we cannot see the river bed, but our feet do their best sensing the water, mud and rock and send a message to our brains. Drilling operation is a dialog between drillers and formation in the dark. Drilling software turns on light to let engineers see the invisible.

# Friction: Drilling Engineers’ Friend and Foe

Friction, the resistance force between two rubbing surfaces, the very drag consumes our energy while we walk, run and drive, also prevents us from falling, colliding and accident.

Friction is everywhere. We can see it painting on roads when car makes sudden stop. We can hear it screaming when wheels stop rotating while car keeps the momentum. We can smell its anxiety when Native Indians rotate drive in wood making fire. These are some appearances of friendly sides of friction. And sometimes, we need to amplify its power by the means like the following.

For downside, we have seen plenty in our daily lines. Our shoe wears out (so our feet do not). We dress smart phone up by putting screen or cover so that scratch (a form of friction) only damages the dress, not the body.

This is friction: we hate, love and cannot get rid of it. And we had better use it toward our advantage.

In drilling and completion practices, we encounter friction whenever we move tubular inside wellbore. When moving pipe downward such as in drilling and casing running, friction slows us down. If we move pipe upward such as in trip-out, friction is pulling pipe down. Yes, friction always acts in the opposite direction of the moving object.

In torque and drag analysis, one graph is worth noting: the hookload for trip-in and trip-out operations, as shown here.

The green line is the hookload when pipe is stationary inside wellbore at various depths.

The blue line shows the hookload when pipe is moving downward. The red line is the hookload when pipe is moving upward.

Note that during trip-in, the frictional drag is against the gravitational force, so the hookload is smaller than the hookload during trip-out, when the frictional drag acts with the gravitational force.

In other words, during trip-in, friction helps hook to hold the pipe weight, while during trip-out, the friction is on the same side of gravitation to make hookload higher.

For coiled-tubing (CT) operation, since CT has relatively thin wall (could be as small as 0.125 inch), the tensile limit of CT may not withstand the pulling of drag and weight during trip-out, as shown in the following graph.

We can run the CT into a deviated well. However, we may not be able to pull the same CT out of the hole!

Friend or foe, friction shows up in various fashions. We just need to know his characteristics and dance with him.