Porosity and Permeability in Petroleum Reservoirs

This activity created in partnership with AGI.

The word petroleum means “rock oil” or “oil from the Earth.” Petroleum, also called crude oil, is a smelly, yellow-to-black liquid that is sometimes runny but can also be quite thick. When refined into specific fuels, it is used to power vehicles and heat homes. It also provides the chemical basis for plastics and other common products of modern life.

Image courtesy of

U.S. National Park Service.

Petroleum forms from tiny animals and plants that lived in oceans and shallow seas hundreds of millions of years ago. As these plants and animals died, their remains sank and piled up on the sea floor. Over millions of years, the remains were covered by layers of mud and sand. Heat and pressure from these layers changed the remains into petroleum and turned the layers into rock. Later, the petroleum traveled sideways and upward from where it was formed to other rock formations or even to the Earth’s surface.

American Geosciences Institute, 2012.Underground areas that contain petroleum are called reservoirs. Only a certain kind of place is likely to be a petroleum reservoir. An essential characteristic of a petroleum-bearing region is a thick bed of sedimentary rocks. In addition, only certain types of sedimentary rocks can hold and transmit petroleum. Our animation shows you how petroleum forms and migrates into reservoirs.

Video ©American Geosciences Institute, 2012.
The video Oil Formation over Time shows how oil develops in rock layers.

Our Experiment

Petroleum reservoirs are associated with certain types of underground rock formations, which are not found in all places around the world. In addition, not all reservoirs hold the same volume of petroleum. An added complication is that petroleum flows out of some reservoirs much more easily than others.

In this activity, you will consider some of the factors that determine whether or not a sedimentary rock might be a good reservoir for petroleum. First, you will run a porosity experiment to determine whether sand or gravel can hold more cooking oil. This will help you to understand the types of rocks that best hold petroleum. Then, you will test the permeability of sand and gravel by allowing the oil to drip out of each container while tracking the time it takes. This will give you an idea about how much petroleum is actually recovered from a reservoir.

Tools and materials

Two 500-mL (approximately 16-oz) clear plastic bottles
Coffee filter
Electrical (vinyl plastic) tape
Two clear containers that can hold the plastic bottles when they are inverted. Empty jars, or plastic or glass cups, can be used.
200 mL (7 oz) of sand
200 mL of gravel
50-mL (2-oz) or larger graduated cylinder
200 mL of cooking oil
Scissors

What to do

Cut off the bottoms of the two 500-mL clear plastic bottles with your scissors.
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Safety note: Handle the scissors with care. Always cut away from you.

Fit a piece of coffee filter over the open top of each bottle. Secure the filter tightly with the vinyl tape.

Screw the caps onto both bottles.

Hold one of the bottles upside down so that the open end is facing up. Place the 200 mL of gravel into the bottle. Then sit the inverted bottle into a cup that can hold the bottle.

Repeat this process with the second bottle and the 200 mL of sand.

Put 50 mL of cooking oil into the graduated cylinder and then pour that oil into the bottle with gravel, until the sediment is completely saturated. The level of the liquid should not exceed the sediment level. Be sure to note the exact volume of cooking oil added.

Repeat this process with the bottle containing the sand. Be sure to note the exact volume of cooking oil added. Note that getting the oil to flow down through the sand is a little tricky and requires patience. You can tell when the sand is fully saturated because it will turn a shade darker. One way to keep the oil flowing through the sand is to unscrew the cap of the bottle first. This allows air to flow through the grains as oil moves downward. Once the oil nears the bottom of the inverted bottle, replace the cap. This process may take about 15 minutes.

Calculate the porosity (percentage of open space) of your gravel and sand samples. Porosity is the ratio of pore space to total volume of solid material and pore space, usually expressed as a percentage by multiplying the ratio by 100. For example, suppose you poured 25 mL of sediment into a container and found that 10 mL of water completely filled the spaces between the grains. The porosity of the sediment in this example would be:

10 mL = 0.4 x 100 = 40% porosity
25 mL
Record your data in a table like the one below:
Type of material Volume of material Volume of oil (pore space) Porosity percentage
Gravel
Sand
Imagine that a geologist discovers an oil field in a gravel reservoir. The total volume of the reservoir is equal to a volume of 100 million barrels. How many barrels of oil are in the reservoir?
If the reservoir described above were sand, how many barrels of oil would it contain?
In the second part of this activity we will look at the rate at which oil passes through the pore spaces of rock, or the permeability. Porosity and permeability are directly related to each other. For a rock to have a high permeability it must not only have large pore size, but also the pores must be connected to each other for the oil to flow easily. We will take a look at this concept using the gravel and sand that is mixed with oil from above.
Suppose you were to remove the cap at the bottom of the plastic bottles. What volume of oil do you think would drip out of the containers? Explain your answer.

Now test your hypothesis from the question above by unscrewing the cap from the bottom of the containers of gravel and sand. Allow the oil to seep through the containers and drain into the holding cups for five minutes.
Record your answers to the following:
1. What is the volume of oil recovered?
2. Calculate the percentage of oil recovered.
3. How do you explain your results?
4. Using the information from the first part of the activity and assuming the total volume of the reservoir was equal to 100 million barrels, use the porosity percentage and the oil recovery percentage to determine how much oil could actually be recovered from the gravel reservoir and the sand reservoir.

Course:

Science

Result/Solution(s)

We set up each bottle according to the instructions, and then using a 50-mL (2-oz) graduated cylinder, we poured oil into each bottle until the sediment was saturated. The gravel bottle was easy to fill. The sand bottle took significantly longer, but we were able to follow the downward progress of the oil because the wet sand got notably darker. To speed up the downward movement of the oil we unscrewed the bottle cap on the bottom. This allowed air to flow through the grains and helped the oil’s movement. We screwed the cap back on as the oil neared the bottom of the sand sample.

It took 92 mL (3.1 oz) of oil to saturate the gravel container. It took 76 mL (2.6 oz) of oil to saturate the sand container. According to our calculations, the gravel sample had a porosity of 46% while the sand sample had a porosity of 38%.

Type of material	Volume of material	Volume of oil (pore space)	Porosity percentage
Gravel	200 mL	92 mL	46%
Sand	200 mL	76 mL	38%

Note: Your results may vary from ours. Factors such as variations in the sizes of gravel and sand that you use can cause results to vary slightly

If the total volume of a gravel-based petroleum reservoir were equal to 100 million barrels, we calculated that it would contain 46 million barrels of oil (100, 000,000 x 46% = 46,000,000). If the reservoir were sand, we calculated that it would contain 38 million barrels of petroleum (100,000,000 x 38% = 38,000,000).

Next we measured the permeability of the sand and the gravel. We unscrewed the cap at the bottom of each bottle to allow the oil to seep through the coffee filter at the bottom of the bottles and into the holding jars. Initially, the oil flowed quickly out of the gravel. But, after five minutes, it was dripping at a very slow rate. The volume of oil recovered from the gravel was 74 mL (2.5 oz), which is about 80.4% of the amount initially poured into the sample. If our gravel model represented a discovery of 46 million barrels of oil, about 37 million barrels of oil would actually be removed from the reservoir.

The oil very slowly seeped out of the sand. After five minutes the oil in the sand had not flowed out at all. All we could see was a small drop forming at the bottom of the coffee filter. This represents a 0% oil recovery rate. We waited longer and found that after ten minutes about 1 mL (0.03 oz) of oil had escaped from the sand. At this very slow rate, we would be recovering only about 380,000 barrels of petroleum from the sand reservoir.

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