# Floating Egg

I did a simple science project with my 8-year-old daughter to show the action of buoyancy.

Step 1: Put a raw egg in a glass full of water. The egg sinks to the bottom.

Step 2: Keep adding salt and stir the water.  You will feel guilty of wasting too much salt, but eventually, the egg arises.

My daughter and I were both amazed by this simple demonstration of buoyancy of salt water. Recall that Archimedes’ principle states that the upward force on a submerged object (egg) is equal to the weight of the water that it displaces. Salt water weights more than fresh water, this difference of upward force (buoyancy) makes the egg arise.

In drilling mud situations, saltwater drilling fluids can be formulated with high-density brines, such as calcium chloride, calcium bromide, etc. The mud weight is dependent upon the specific gravity (SG) of salts added and concentration of these salts. The heavier the mud, the higher the bottom hole pressure is. Note that one of the main functions of drilling fluids is to provide hydrostatic pressure to prevent formation fluids from entering into the well. Heavy mud brings some advantages and some disadvantages to other aspects of drilling operations because of the buoyancy it creates.

Buoyancy makes drill string appear light. This reduces the side force, which pushes drill string against wellbore. Therefore, there is less torque and drag for a heavier drilling fluid system, but what helps also causes problems in this case: too much buoyancy revokes the effectiveness of transferring the pipe weight to the bit.

When cementing a casing, due to the large size of the casing and the big difference of fluid densities in the annulus and inside the pipe, the buoyancy could be so great that the casing could be pushed out of the hole if it is not chained down.

Let’s use Dr.DE (drilling engineering tool box software) to show a couple of examples.

Consider an 11.75” OD casing with weight of 47lb/ft. The casing shoe is at 12,345 ft in a vertical well. The annulus is full of cement slurry of 16ppg. We assume mud weight of 8.33 ppg. With the help of Dr.DE the calculation shows that the casing will be lifted up.

Casing Will Be Lifited Up

Now, change the mud weight to 9 ppg, due to this change, the hydraulic pressure inside the casing increases and the casing will not be lifted up by the buoyancy.

Casing will NOT be lifited up

# Merchant of Venice and Hook Load

I visited Venice after this year’s SPE/IADC Conference in Amsterdam in March. During that week in Venice, I stayed in a quiet world: no automobile, no motor cycle or train. You can simply add “water” in front of the names of our ordinary transportation to describe the local means of moving around: water-bus, water-taxi, and water-gondola.

While enjoying the Italian cuisine and limoncello, I suddenly thought of one of the famous play of Shakespeare’s: Merchant of Venice. The story with Venice as the venue goes like this.

A Venetian merchant Antonio has a friend Bassanio, who is desperately in need of money to court Portia, a wealthy heiress who lives in another city. Without enough money, Bassanio and Antonio finally secure the loan from Shylock, a Jewish moneylender, with Antonio as the loan’s guarantor. Shylock hates Antonio, but acts agreeably and offers to lend Bassanio three thousand ducats with no interest. Shylock adds, however, that should the loan go unpaid, shylock will be entitled to a pound of Antonio’s own flesh, to which Antonio agrees.

Later in story, news comes that Antonio has lost his ships, and that he has forfeited his bond to shylock. Shylock ignores the many pleas to spare Antonio’s life, and a trial is called to decide the matter. Portia, his friend’s fiancée, disguises as a young man of law and asks Shylock to show mercy, but he remains inflexible and insists the pound of flesh is rightfully his. Portia examines the contract and, finding it legally binding, declaring that Shylock is entitled to the merchant’s flesh. Shylock ecstatically praises her wisdom, but as he is on the verge of collecting his due, Portia reminds him that he must do so without causing Antonio to bleed, as the contract does not entitled him to any blood. Trapped by this logic, Shylock hastily agrees to take Bassanio’s money instead. The ending of the story has more drama.

We live in a world of measurements. Measurement is a cornerstone of engineering and science. For some reason, the story in my memory is that Portia reminds Shylock that he has to cut exactly one pound of Antonio’s flesh, not even one ounce more or less.

In drilling industry, we have mud weight (ppg) to measure the density of drilling fluid, pipe weight(lb/ft) to represent the thickness of pipe with given OD. The weight indicator - hook load is the total force acting (pulling down) on the hook on the rig.

Hook load is one of the few important readable operation parameters on rig floor. It’s basically the total force includes those of traveling assembly weight, buoyant pipe weight in a deviated well plus or minus the frictional drag caused by pipe movements inside borehole. Torque and drag (T&D) software such as TADPRO serves the purpose of predicting hook load, surface torque and other variables for drilling and tripping operations.

We can calculate hook load and surface torque precisely if we know all the details downhole. However, the uncertainties downhole, such as open hole sites, survey accuracy, make the prediction exactly match rig floor reading unlikely. This does not discount the importance of hookload prediction, because the significance of T&D calculation is its trend.

The following picture shows the hook load changes as we drill to TD. The hook load is increasing at beginning (due to longer pipe into the hole) and decrease later as we drill into build-up and horizontal sections (more drag).