Focus question

How does the viscosity of drilling mud help remove cuttings from a drill hole?

Essential materials

• Wooden dowel about 30 cm (about 12 inches) long and about 1.3 cm (about 1 / 2 inch) diameter
• Clear plastic tubing about 15 cm (about 6 inches) long and about 1.3 cm (about 1 / 2 inch) inside diameter
• Sand
• Clear plastic graduated cylinder about 2.5 cm (about 1 inch) inside diameter
• Water
• Clear shampoo
• Cellophane tape
• Sandpaper
• Paper towels
• Safety goggles

Main ideas and background information

• Viscosity is the property of a fluid that causes it to resist flowing. Thick liquids such as oil and shampoo are more viscous than water.
• A viscous liquid resists the motion of an object moving in it. The more viscous a liquid is, the more it will resist the movement of an object. Thus, an object can move more freely in a liquid of low viscosity, such as water, but moves more slowly in a liquid of higher viscosity, such as oil or shampoo.

• A highly viscous liquid might resist the motion of an object so much that it appears to be suspended or motionless.
• When a viscous drilling mud is pumped into a well hole, it can bring cuttings up to the surface as it curculates. If the drilling mud is not sufficiently viscous, the cuttings will drop back toward the bottom of the hole.
• A related factor (not covered in this activity) is the speed of the drilling mud. The greater the upward speed of the mud, the better it can carry cuttings to the surface.
• Note that the viscosity is not related to the density of the mud, although density is an important mud property. Engineers regulate the density of the mud to balance the pressure in the well hole so as to avoid both fluid leakage (caused by the pressure of overly dense mud) and well blowout (caused by insufficient pressure from mud that is not sufficiently dense).

Procedural tips

• Students should first add the sand to the bottom of the graduated cylinder before adding water or shampoo.
• When inserting the dowel into the tubing, make sure that the dowel can move smoothly and still make contact with the full inside of the tubing.
• For best results, push the dowel into the tubing until the end of the dowel is even with the end of the tubing.
• Insert the tubing and dowel into the cylinder until the ends are just below the surface of the liquid. Keep the end of the dowel at this level while pushing the tubing the rest of the way down. The goal of pushing the tubing down to the bottom of the cylinder is to fill the tubing below the end of the dowel with liquid.
• When pushing down on the dowel, the emphasis is on steadiness and not speed. Although they may be tempted, students should not push forcefully on the dowel. This movement is not intended to produce a high speed “squirt” of fluid, but rather to mimic the stroke of a pump pushing the drilling fluid steadily down into a well hole.
• Encourage students to observe closely the movement of the sand while the dowel is being pushed down. This is the critical moment in the well hole, when the drilling fluid disturbs the cuttings and lifts them up.
• When cleaning up after the using the shampoo, we suggest that the students empty as much shampoo as possible into paper towels, and then throw the towels into the trash bin. This would minimize the amount of foam and bubbles in the sink when using running water to finish the cleanup.

Discussion

1. Describe and explain what happened when the water flow started and then stopped.  (The outflow of water from the bottom of the tube disturbed the sand particles and started to move them upward. However, when the water flow stopped, the sand dropped back to the bottom. The water was not viscous enough to suspend the sand particles.) 

2. Describe and explain what happened when the shampoo flow started and then stopped. (The outflow of shampoo from the bottom of the tube disturbed the sand particles and started to move them upward. When the shampoo flow stopped, the sand particles stayed where they were. The shampoo was viscous enough to suspend the sand particles.) 
   

3. What do you think would happen to the sand particles if the flow of water was continuous? (The flowing water would tend to keep the sand on the bottom stirred up. Perhaps some of the sand particles would be carried upward for a short distance. However, the sand particles would tend to keep sinking downward because of the low viscosity of the water. If the water flowed upward at a speed greater than the falling speed of the sand, the sand particles could be carried upward.)

4. What do you think would happen to the sand particles if the flow of shampoo were continuous? (The flowing shampoo, being more viscous than the water, would suspend the sand particles and carry them steadily upward.)

Assessment

Are students able to describe how the viscosity of drilling mud helps remove cuttings from a drill hole? (A viscous drilling mud that is pumped into the bottom of a drill hole can suspend the cuttings and carry them upward as it flows back to the surface. A mud that is not as viscous might not be able to do this because the cuttings can fall more easily through it.)

Extensions and further investigations

• Have students experiment with liquids of other viscosities.

• Place the tubing under a water faucet to set up a continuous flow. Explore different water flow speeds to determine if all of the sand can be removed from the bottom of the cylinder. 

Career connections

• Petroleum engineer

• Hydraulic engineer

• Drilling fluid engineer

• Oil drilling specialist

Glossary/vocabulary

viscosity
viscous

Resource links

Drilling mud—from Wikipedia, the free encyclopedia
More extensive description of drilling mud

Drilling Fluid: Mud Cycle At A Wellsite
A SEED web page with additional detail about drilling mud

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