Science Buddies: "Ask an Expert"

I am trying to come up with a science project that simulates hydraulic fracturing. I don't know where to begin to make a scale model showing some of the side effects of hydraulic fracturing such as how the fluids can leak into the aquifer,etc. There is a lot of information on hydraulic fracturing, but none on how to set up an experiment. Please help.

Hello:
How can I use the Science Buddies experiment on Darcy's Law in a hydraulic fracturing experiment? How would I show the permeability of shale or how fast contaminants that leach through shale can reach the aquifer?

Thanks for your help.

Hi djcmudder,

You've picked an interesting topic. I would suggest that instead of building a scale model of a fracking system, you focus one particular aspect of fracking, such as how fractures can enable fluids to flow through rocks more easily. The experiment described in this Project Idea is a good starting point:

https://www.sciencebuddies.org/science- ... ml#summary

The background information on Darcy's law is important, so let me know if it doesn't make sense to you. Fracking changes K, the hydraulic conductivity of the rock by making it easier for fluids to flow through the rocks. The different materials (gravel, etc.) you use in the soda bottle all have different K's.

You'll also want to understand the words porosity and permeability, too. Porosity relates to how much empty space there is between pieces of rock. A rock with high porosity has lots of empty spaces in it. The empty spaces are called pores. Permeability is how well the empty spaces, or pores, are connected. A rock with high permeability has well-connected pore spaces, which makes it easier for fluids to flow through them.

This webpage discusses these concepts in more detail:

http://www.tulane.edu/~sanelson/geol111/groundwater.htm

Take a look at the section on "movement of groundwater".

Now, I mentioned that the Project Idea was a good starting point. In other words, it's a good foundation to build on. If you're interested in how fracking can lead to contaminated aquifers, I would suggest doing the following. Follow the directions in the Project Idea. Then, with a helper still pouring in water, add a few drops of food coloring to the top of the bottle and time how long it takes the food coloring to start coming out of water from each of the four holes. Write those times in a data table. Do that for each of the different materials you test (e.g., sand, gravel, etc.), and see which materials have the shortest transportation times. The food coloring is like contamination entering an aquifer, and this experiment will help you see how porosity and permeability (hydraulic conductivity, K) affect how fast contaminants can enter an aquifer.

Let me know if I can help with anything else!

Hello Terick:

Thank you so much for your reply, it was most helpful! We are going to go with your suggestions on the effects of hydraulic fracturing on water aquifers. We might need some help with Darcy's Law and the K coefficient. If so, I will be contacting you.

Thank you for your help!

Hi Terik:

We are going to start the experiment next week, so I would like to ask a few questions:
1. Could you show me a Darcy's law equation example and how to calculate each of the parts. I'm not quite sure how to do it.
2. When we fill the bottles up with the different types of materials, should we leave a space at the top for water? If so, how much space? How much water should we put in each time?
3. What material should we use to emulate shale?

Thank you so much for your help!

djcmudder

Hi djcmudder,

I'll answer these questions in a post to the original topic thread. We ask that you keep questions about the same topic on a single thread--it helps us help you better, which is what we are all about.

Hi djcmudder,

I'll answer your second and third questions first, since the first question will have the longest answer.

2) The important thing is that you fill your bottles to the same level. So, for simplicity I would suggest filling them to the bottom of where the cap screws onto the bottle. That gives you a little bit of room to adjust how fast you are pouring the water, in case you are pouring faster than the water is going through the bottle.

Instead of pouring in a certain amount of water, you want to measure the water that comes out of the outlet for a certain amount of time (e.g., 30 seconds). For materials with high K (high porosity and permeability), you will have to pour water pretty quickly to keep up with how fast it comes out the bottom. For impermeable materials, like clay, you won't have to pour very much water at all.

3) Shale is basically clay that has been turned into a rock. (We say the clay has been "lithified".) So, if you have clay-rich soil in your backyard, you could use that. Or, you could use mud from a stream. Be sure to ask permission from whoever owns the stream, if you go that route.

1) Let me try to break down Darcy's law for you, and point out how you will measure the different terms. We can make several simplifying assumptions with this particular experimental setup. After making those assumptions, for this project Darcy's law reduces to Q = KA. If you want to see how I got that equation, take a look at the text in between the dashes. Otherwise, you can just use this simplified version.

Now, you will actually measure Q and A. So, we can rearrange this equation into a more useful form: K = Q/A.

A is the cross sectional area of the soda bottle. To calculate that, I would trace a circle around the bottom of the soda bottle and then measure the diameter of that circle. The area, A, is (diameter/2)*Pi*(diameter/2). You could also wrap a piece of string around the middle of the soda bottle, measure how the length of the string. That would give you the circumference of the soda bottle. Divide the circumference by pi and then divide that number by 2 to find the radius of the soda bottle. The cross sectional area, A, is radius*radius*pi.

Q is the flow rate. Remember that you will measure how much water comes out of the outlet hole in a specific amount of time. For example, you could measure the volume of water that comes out over a 30 second period. Let's say, for example, that you measured 30 milliliters in 30 seconds. The flow rate, Q, would be 30 milliliters/30 seconds = 1 milliliter per second. A handy conversion factor is that 1 milliliter = 1 cubic centimeter. So, if you measure the volume of water in milliliters, time in seconds, and cross sectional area in centimeters squared, you will get K in units of cm/s.

If the equation is too complicated, you can also simply measure the flow rate, Q, and compare the flow rates for different materials.

Here's an example. I'm just making up numbers here. Let's say that I traced a circle around the bottom of my soda bottle. I measured the diameter of the circle. It was 10 cm. Now I calculate A:

A = (diameter/2)*pi*(diameter/2) = (10 cm /2)*pi*(10 cm /2) = (5 cm)*pi*(5 cm) = 78.5 square centimeters.

And, if I measure 60 milliliters of water over 30 seconds, then...

Q = 60 milliliters/30 seconds = 2 milliliters per second.

Now I can calculate K:

K = Q/A = (2 milliliters per second)/78 square centimeters = (2 cubic centimeters per second)/78 square centimeters = 0.02 cm/s.

So, K is 0.02 cm/s. This is roughly the value of K you would get if the bottle were filled with a mix of sand and gravel.

One last comment - if you can't measure the volume of water in milliliters, you can type "convert # of cups to milliliters" into Google, and Google will make the unit conversion for you.

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Here's Darcy's law, as written in the background of the Project Idea:

Q = KA (h1 − h2) / L

In this project, you will measure Q, the flow rate, A, the cross sectional area, h1 and h2 (the inlet head and outlet head, respectively), and L, the path length of flow. You will calculate K. So, a more useful form of this equation for this project is:

K = Q*L/[A*(h1-h2)]

Let me also clarify what h1 and h2 are. "Head" is related to pressure. What we are interested in is the difference in pressure between the where the water enters the bottles and where it leaves the bottles. For the purposes of this project, the difference in head is equal to the difference in height. So, we can rewrite Darcy's law yet again, to make it a bit friendlier:

K = Q*L/[A*D]

Where D is the distance between the outlet hole and the top of the sediment. We can make an approximation, which is that in this case the distance between the outlet hole and top of the sediment (D) is about the same as the path the water has to flow to get from where you pour in into the bottle to the outlet hole (L). Since D is about equal to L, we can simplify this equation even more:

K ~ Q/A

Where ~ means "approximately equal to". "Approximately equal to" is good enough for the purposes of this project.

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Hi djcmudder,

I realized I forgot to answer your question from your other post: How would I show the permeability of shale or how fast contaminants that leach through shale can reach the aquifer?

Shale is quite impermeable. The K values for shale (or clays) are often around 0.0000001 cm/s. So, you will find that water passes very slowly through the clay. As I've been thinking about your interest in fracking, I wonder if it would be better to just use 2 types of material: coarse sand or small gravel (be sure the sand is coarse), and clay (an analog for shale). You could tests with 4 bottles: sand only, clay only, a layer of clay sandwiched between layers of sand (make sure the clay covers the whole area, with no gaps around the edges of the bottle), and a layer of clay, with a gap (or "crack") in the middle (make sure there really is a clear gap), sandwiched between two layers of sand. You probably only need to do experiments with the outlet hole at a single level, not several levels like they have in the Project Idea. Just make sure the outlet holes are all at the same level.

The bottle with a continuous layer of clay sandwiched between 2 layers of sand is like shale layers before fracking. They are barriers between aquifers. The layer of clay with a gap or crack in it is like the shale layers after fracking, which puts lots of fractures in the rocks. Those fractures can create links between aquifers that were previously separate The other two bottles are controls. Don't be surprised if you can't get any water to come out of the all clay bottle; the permeability of clay is really, really low. To show how contaminants can leach through shale, pour water through the bottles, and add a drop of food coloring to the water at the top of the bottle. Time how long it takes for that color to appear at the outlet hole. Differences in how long it takes the dye to appear at the outlet hole tell you about how fast contaminants might move through unbroken versus fractured shale layers. It won't be a direct model of fracking, but this experiment will help you see the key processes at work.

Hi djcmudder,

I have nothing substantive to add, but I wanted to promote Terik's idea in the last post. This system is elegant, simple, and will allow you to probe some important processes involved in fracking. It's a great idea.

Cheers,
Colin

Hi Terik:

I need your advice on my hydraulic fracturing project. You suggested using clay to simulate shale. What can I use for clay. Are you speaking about play doh type of clay, or another type of solid clay, or a pourable clay.(I cannot get it from my backyard). If a solid clay, I guess I could cut the bottle down the side and tape it back together? I need your assistance with this issue.

I appreciate your help,
Thank you

If you use plastic soda bottles can you cut the tops off so that they are basically open cylinders into which you can stuff the clay? As for clay type, how about clay meant for firing pots from an art supply store?

Cheers,
Colin

Hi Colin:

Thanks so much, your reply was most helpful!

Hello Terik,

I im currently talking about hydraulic fracturing for a regional science fair project so i was wondering if you can help me with it and walk me through it so i can finish my project your help is needed

Thank you