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Natural Gas: Classroom Activities

Reservoir and Recovery Volume


Goal

To better understand the relationship between porosity and permeability and an oil and gas reservoir.

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Background Information

Understanding the porosity, or amount of pore space, of petroleum and gas reservoirs is important for oil and gas extraction. Rocks with a greater porosity are able to store a greater volume of hydrocarbons. Exploration geologists perform many tests to assess the porosity of rocks in new wells. They use very powerful microscopes to examine rock chips, especially the grains and the pores between them. Another method involves lowering devices into the drill hole to take measurements. This process is called well logging. Readings of electrical resistance provide an indication of the fluid content within the pores of the host rock. The ease with which fluid flows out of rocks depends on the interconnection between pores. This attribute of porous media is called permeability.

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Activity Overview

Students design an experiment to see if gravel or sand can hold more oil within the pores between grains. They determine the relationship between grain size and porosity. By allowing the oil to flow out of the sediments they also begin to develop an understanding about permeability.

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Materials and Equipment

two clear plastic 500 mL bottles with the base cut away, fine cheesecloth, electrical tape, sand, gravel, 50 mL graduated cylinder, 200 mL of vegetable oil, water

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Activity

Part A: Reservoir Volume

  1. Ask students which material can hold more natural gas: gravel or sand? Have them explain their thinking.
  2. Have students develop a hypothesis related to the question.
  3. Ask students to use the materials available to design an experiment to test their hypothesis.
  4. Check the students' designs before approving their experiments.
  5. Have students draw their designs and record their methods.
  6. Have students record their observations.
    Students' answers will vary according to the type of sand and gravel they use. If the sediments are well supported they will have greater porosities. Typical porosities vary from 25-50 percent. Well sorted gravel has a higher porosity than well sorted sand. If either type of sediment is not well sorted then they will have lower porosities because smaller grains fill the voids between larger grains.
  7. Explain to students that porosity is the ratio of the pore space to the total volume of the solid material and pore space. It is usually expressed as a percentage by multiplying the ratio by 100.
  8. Have students complete the following:
    1. What is the porosity of the gravel?
      Answers will vary depending on the sediments used in the investigation.
    2. What is the porosity of the sand?
      Answers will vary depending on the sediments used in the investigation.
    3. Imagine a geologist discovers an oil field in a sandy reservoir rock. The total volume of the reservoir (oil plus rock) is 80 million barrels. If the porosity is 0.25, how many barrels of oil are in the reservoir?
      20 million barrels.
    4. How would this be different if the reservoir were gravel?
      For uniform gravel the porosity is greater and therefore the yield could also be greater.

Part B: Recovery Volume

  1. Create a set-up of containers holding sand, gravel, and vegetable oil that looks similar to the following:
    figure of bottles and containers
    Source: EarthComm
  2. Ask students to describe what would happen if the caps were removed from the containers holding the gravel and oil and sand and oil.
  3. Have them consider the volume of oil that would flow out from each reservoir.
  4. Ensure that a cheesecloth is secured to each bottle using electrical tape. Unscrew the caps from the bottom of the reservoirs of gravel and oil and sand and oil. Allow the oil to flow into collection devices for 5 minutes.
  5. Have students complete the following:
    1. What was the volume of recovered oil?
      Student's results will vary depending on the type of sand and gravel used. In particular, the degree of sorting will greatly affect permeability. If the gravel is well sorted it should yield a great volume of oil than the sand. Gravels typically have a greater flow capacity (hydraulic conductivity) or permeability than sands if smaller particles do not block cavities between grains.
    2. Calculate the percentage of oil recovered for each reservoir.
      Answers will vary.
    3. How do you explain your results?
      The greatest yield will result from a higher volume of interconnected pores in relation to their size.
    4. If the model represented two reservoirs, each with 250 million barrels of oil, how many barrels would be removed from each reservoir?
      Answers will vary.
  6. Have the class discuss ways to recover a greater percentage of oil.
    Answers may include: to dissolve the oil with a soapy fluid, to heat the reservoir and reduce the viscosity of the oil, or to reduce the contacts between grains by artificially fracturing the sediments using vibrations.

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