Exploring Porosity

The NEED Project

Activity Source: 

The NEED Project. Adapted with permission.

Earth scientists play a vital role in harvesting the energy resources on which we all rely. When preparing to drill for oil, for example, geoscientists must assess many aspects of a rock stratum (layer). For example, they must figure out the volume of the rock’s pores, or empty spaces, as compared with the rock’s total volume. This is called the rock’s porosity.

To help you understand porosity, think about different sizes of gravel. Which size gravel will have the greatest porosity? Why? In this activity, you will work in groups to explore the answers to these questions.

Type of Material

Volume (mL) of Water Poured

Volume (mL) of Material

Percentage of Pore Space in Material

Large gravel

 

 

 

Medium gravel

 

 

 

Small gravel

 

 

 

 

Materials

For each group:

  • 350 cm3 large gravel (coarse gravel)
  • 350 cm3 small gravel (aquarium type)
  • 350 cm3 medium gravel (pea gravel)
  • Water (can be dyed with food coloring)
  • 3 600 mL beakers or tall clear cups or jars
  • 100 mL graduated cylinder

Procedure

  1. Fill one beaker to the 350 mL mark with the large coarse gravel. Fill the second beaker with 350 mL of medium pea gravel. Lastly, fill the third beaker with 350 mL of small aquarium gravel. Make a prediction: Which size of gravel will have the greatest porosity? Give a reason for your prediction and record it.
  2. Now, fill the graduated cylinder with 100 mL of water.
  3. Slowly pour water into the first beaker until the water reaches the top of the rocks. Record exactly how much water you poured into the beaker. If you need more that 100 mL of water, fill the graduated cylinder again.
  4. Follow Step 3 again for the other two beakers filled with gravel.
  5. Calculate the porosity of the three materials using this formula:
    Porosity = (Volume of Water/Volume of Material) x 100
  6. Talk this over with your group: Which size of gravel has the greatest porosity? What do you think is porosity’s importance in the drilling process of an oil well?