Experiment

Background

I prepared this experiment for my daughter's elementary school class when I was invited to animate a full-day activity on the "World of Oil." The experiment was intended to put physics in motion in the middle of maps, videos, and geologic descriptions. The activity was so successful that I had to repeat it several times!

Drilling mud is pumped down through the center of the drill pipe and returns to the surface in the space between the drill pipe and the wall of the borehole.

One of the major risks in drilling an oil well is a blowout, which occurs when gas pressure inside the well suddenly forces out the oil. This demonstration, and the related animation, illustrate how a blowout can happen and how it can be prevented.

The experiment also highlights factors that must be considered when drilling an oil well: drilling mud, blowout prevention, hydrostatic pressure, pressure balance, gas compressibility, and density.

Tools and Materials

• Transparent vinyl tube with a 6 mm (0.2 in) inside diameter
• Transparent vinyl tube with a 6 mm (0.2 in) outside diameter
• Party balloons
• Wooden board, 1 m (3.3 ft) x 20 cm (8 in)
• Electric wire staples
• Ribbon tape measure
• Film canister or small plastic container
• Foam packing
• 6 mm (0.2 in) brass cross connection
• Needle valve
• Bicycle pump
• Duct tape
• Water
• Salt
• Kitchen scale
• Small funnel

What to Do

The model is built out of commonly available materials easily found at a home improvement  or hardware store. The diagram shows how to put it together. The transparent vinyl tube with the 6 mm inside diameter simulates an oil well. The party balloon is the oil reservoir. I used the larger transparent tubing (6 mm outside diameter) from an aquarium air pump for the manometer, or pressure gauge, although it results in a small capillary error. Larger tubes can be used, but not too large, or the set-up will not remain practical.

I fixed the tubes to the board using round electric wire staples. Ribbon tape measures from a furniture shop worked very well as graduated scales to measure the water level on the well and the gauge. The well and gauge must be carefully aligned to the same horizontal reference point. I used a film canister or small plastic container as the water tank for the manometer. I put a layer of thick foam packing in the lid, and forced the well and gauge tubing through the lid and foam packing, to make the entries airtight. I then attached the well tubing and the reservoir balloon to the 6 mm cross connection and needle valve.

Some creativity will be needed to make the connections air-tight with more or less makeshift fittings. Duct tape or elastic bands can be used to help with the attachments. If you are doing this project in a classroom, rehearsal is recommended to familiarize yourself with the small errors of such a simple setup.

Experiment 1

1. Close the needle valve. Fill the tube simulating the well with fresh water up to the top edge. Also fill the gauge tank with fresh water.
2. Use the bicycle pump to bring the water level in the gauge to just below the water level in the well tube. If the balloon inflates too much, install two balloons, one inside the other.
3. Stop pumping, and make sure that it does not leak. Gently open the needle valve. The water level on the well will decrease slightly to the same height as in the gauge. At this moment we are at equilibrium. The hydrostatic pressure of the water column in the well matches exactly with the reservoir pressure, keeping the fluids (in this case air) in place.

   Mark the exact level on the gauge measuring tape for future reference. If the level drops far below the edge of the well tube, close the needle valve, replenish the well and put a slightly higher pressure before opening the needle valve again.

4. Let's now simulate an underbalance. Gently press the balloon, thus increasing slightly the reservoir pressure, and observe the first air bubble entering the tube. Note how its size increases as it travels up the tube, as the hydrostatic pressure decreases.

5. The increase in the volume of the air bubble displaces water, which spills out of the well tube. As the amount of water in the well decreases, so does the hydrostatic pressure, and more air enters the tube, displacing more water and very quickly all the water will be spilled out, showering everyone around!

This scenario might well happen while drilling an oil well, resulting in a blowout with catastrophic consequences. In fact, primitive drilling consisted of hammering a peg in the ground until the reservoir was hit and the fluid blew out. Apart from being dangerous, wasteful, and polluting, this technique, or rather lack of it, could only be used for shallow wells. In modern rotary drilling, specially-formulated mud is used in the well to keep the reservoir fluids in place; the mud has other functions as well. The drilling mud is constantly circulated in the well through the drill pipe and out of the well through the annular space between the drilling pipe and the well bore. The returning drilling fluid is continuously monitored to detect the presence of gas.

If gas is detected, the driller immediately lowers the drill pipe as low as possible and clamps it with the blowout preventor pipe rams, stopping fluid loss from the well. Then, mud of higher density is injected through the drill pipe and annulus fluid is choked out to remove the gas and the lighter mud while maintaining the pressure. The heavier mud travels down the drill pipe and up through the annulus. As the mud column gets heavier, less and less gas is detected at surface. Once the heavier mud is in place and the reservoir fluids contained, the pipe rams are opened and drilling resumes. We can also model this in our simulator.

Experiment 2

1. Place a container with enough capacity for the liquid required to fill the well tube on the kitchen scale and set it to zero. Fill with fresh water and weigh.
2. Add salt until the weight is increased by 5% or 10%. Stir to dissolve the salt completely
3. Close the needle valve and fill the well tube with the salt water.
4. Operate the pump until the level in the gauge tube is 5% or 10% higher than in Experiment 1, depending on the amount of salt added in step 2.
5. Gently open the needle valve and observe how a shorter column of heavier, or more dense, fluid in the well balances a taller column of lighter fluid in the gauge tube.
6. If air bubbles enter the well it means that the pressure is too great—you've pumped too much and the level of the gauge is too high. Conversely, if fluid from the well tube is lost, it means that the reservoir is overcompensated—the pressure in the well is too great. (Note that a little loss is unavoidable because of the relatively low accuracy of our measurements.
7. We are now again at equilibrium and a slight pressing on the balloon will produce the same result as in Experiment 1.

Further Thoughts

Click on the image to try this virtual blowout simulator.