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Porosity and Permeability


Let's talk a little more about those tiny pore spaces in rocks. Did you know that many rocks have tiny holes, or pores, in them? They are called "porous" rocks. A rock also has to be "permeable" to hold carbon dioxide. A permeable rock is a rock with pore spaces that are connected together so that liquids or gases can flow between the pores. You can even push air through these permeable rocks with a bicycle pump!

pump

These kids are using their hands, their eyes and their brain along with the bicycle pump to measure the permeability of 3 different rocks.

They assigned a measurement of 10 to the rock that they were able to push lots of air through to make lots of bubbles. Do you think this rock was highly permeable or impermeable?

The second rock got a score of 4 or 5. They could still push air in, but it was much harder to do and fewer bubbles came out of the rock.

The third rock scored 0! It was a rock with NO connected pores - what a geologist would call "tight". Was this rock permeable or impermeable?

Do you think a scientist could collect permeability data using a bicycle pump? Why or why not? Scientists must be able to talk to each other about their measurements and make the same measurements at different labs around the world. It seems like there is a variable in this experiment that would be very difficult for them to control for. Do you think that all scientists have the same amount of muscle in their arms? How could they describe to each other how much force it took for them to push down the handle of the bicycle pump? Well, they couldn't. Scientists have to use a very expensive piece of equipment to measure permeability of rocks. It is called a permeameter.

miniperm

Using this machine, scientists can measure how fast nitrogen flows through a piece of rock and come out with very reproducible data. Reproducibility is very important in science. Scientists in different parts of the world must be able to come up with the same answers when working with the same materials and they also must be able to communicate with one another about their results.

 

The posters below have more information about porosity and permeability.

  • Poster 1 (PDF) A poster about porosity and permeability.
  • Poster 2 (PDF) A poster about measuring permeability and with photographs of porous rocks taken through a microscope.
  • Poster 3 (PDF) This poster

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