Porosity facts for kids

Kids Encyclopedia Facts

Porosity, also called void fraction, is a way to measure how much "empty" space is inside a material. Think of it as the tiny holes or gaps within something, like a sponge or a rock. It tells you what fraction of the total volume is made up of these empty spaces. This fraction is usually between 0 (no empty space) and 1 (all empty space), or it can be shown as a percentage from 0% to 100%.

Sometimes, when we measure porosity, we only count the empty spaces that are connected to the outside surface. These are called "accessible voids."

Scientists and engineers use the idea of porosity in many different areas. This includes making medicines, working with ceramics, studying metals, creating new materials, and understanding how water moves through the ground.

Porosity in Earth and Building Materials
Measuring Porosity
Images for kids
See also

Porosity in Earth and Building Materials

In fields like geology, soil science, and building, porosity describes the amount of empty space in materials like rocks or soil. These empty spaces can be filled with things like air or water.

Porosity is calculated using a simple idea: it's the volume of the empty spaces divided by the total volume of the material. For example, a solid granite rock has very little porosity, usually less than 0.5% empty space. But materials like peat (decayed plant matter) or clay can have a lot of empty space, sometimes more than 50%.

Knowing the porosity of a rock or soil layer is very important. It helps us figure out how much water or hydrocarbons (like oil or natural gas) it might hold. The porosity of sedimentary rocks changes based on many things. These include how fast and deep the material is buried, what kind of fluids are already in it, and the type of layers above it.

We can also estimate porosity if we know the material's bulk density (how heavy it is for its size), the density of the fluid filling its pores, and the density of the solid particles themselves. If the empty space is filled with air, the calculation becomes even simpler.

Porosity and Water Flow

Porosity can be linked to how easily water can flow through a material, which is called hydraulic conductivity. For example, if you have two sandy areas, the one with more empty spaces (higher porosity) will usually let water flow through it faster.

However, this connection isn't always straightforward. Clays, for instance, have very high porosity, meaning they can hold a lot of water. But their tiny pore spaces make it very hard for water to move quickly through them. This is why clays have low hydraulic conductivity, even with high porosity.

How Sorting Affects Porosity

This picture shows how well-sorted materials (left) have more empty space than poorly sorted materials (right). Black is solid, blue is empty space.

When all the grains in a material are about the same size, we call it "well sorted." Well-sorted materials tend to have more empty space than "poorly sorted" materials. In poorly sorted materials, smaller particles can fill in the gaps between larger ones.

The picture shows this clearly. When smaller grains fill the spaces, they drastically reduce the empty space where water could flow. This lowers both the porosity and how easily water can move through the material.

Porosity of Rocks

Solid rocks like sandstone or granite can have different types of porosity. We can think of it as "dual" porosity. This means there might be connected empty spaces and unconnected ones. Connected porosity is easier to measure because gas or liquid can flow into it. Unconnected pores are like tiny, sealed bubbles that fluids can't reach.

The amount of empty space in a rock depends on its type, how the pores are spread out, and how the rock formed over time. Interestingly, the size of the grains in a rock doesn't control porosity. Instead, it's about how the grains are packed together.

Rocks usually become less porous as they get older and are buried deeper underground. However, there can be exceptions to this rule, depending on the rock's history.

Porosity of Soil

The porosity of soil at the surface often decreases as the size of the soil particles gets bigger. This is because tiny soil particles can clump together, forming "aggregates." These clumps make the soil more resistant to being squashed, creating more empty space.

For example, sandy soil usually has a bulk density between 1.5 and 1.7 grams per cubic centimeter. This means its porosity is between 36% and 43%. Clay soil, on the other hand, has a lower bulk density (1.1 to 1.3 g/cm³), giving it a higher porosity (51% to 58%). This might seem odd because clay soils are often called "heavy." This term usually refers to how much effort it takes to dig or plow through them when they are wet.

Deeper soil layers usually have less porosity than surface soil. This is because the weight of the soil above compacts them.

Soil porosity is quite complex. Scientists use different models to understand it better, especially how environmental factors affect the shape and size of the empty spaces.

Types of Geologic Porosities

Primary porosity: This is the original empty space system in a rock or loose sediment. It's the porosity that formed when the material first came together.
Secondary porosity: This is a new or additional empty space system that forms later in a rock. It often increases the rock's overall porosity. This can happen when minerals dissolve away or when cracks (fractures) form in the rock. It can replace the primary porosity or exist alongside it.
Fracture porosity: This type of porosity is found in cracks or faults within rocks. It can create new empty spaces in rocks that wouldn't normally hold fluids, like some igneous rocks.
Vuggy porosity: This is a type of secondary porosity where large holes or "vugs" form in rocks, especially in carbonate rocks. These holes are created when parts of the rock, like old shells or fossils, dissolve away.
Effective porosity: Also called "open porosity," this is the part of the total empty space where fluids can actually flow. It includes pores that are connected and those that are "dead-end" (meaning fluid can enter but not easily flow through). It does not include closed pores that are completely sealed off. This is very important for understanding how water and oil move underground.
Ineffective porosity: Also called "closed porosity," this is the part of the total empty space where fluids or gases are present but cannot flow effectively. These are the sealed-off pores.
Dual porosity: This idea suggests that there are two different, but connected, empty space systems working together. For example, in fractured rocks, the solid rock and the cracks might be thought of as two separate systems that interact.
Macroporosity: In solid materials (not including soil), macropores are empty spaces larger than 50 nanometers in diameter. Fluids move through these larger pores by simple diffusion.
Mesoporosity: In solids, mesopores are empty spaces between 2 nanometers and 50 nanometers in diameter. Fluid movement in these pores is a bit slower.
Microporosity: In solids, micropores are empty spaces smaller than 2 nanometers in diameter. Movement in these tiny pores is very slow and happens through a process called activated diffusion.

Measuring Porosity

Scientists and engineers use several methods to measure porosity:

Direct methods: This involves finding the total volume of the porous sample and then finding the volume of the solid material without any pores. The difference between these two volumes is the volume of the pores.
Optical methods: You can look at a sample under a microscope and measure the area of the solid parts versus the area of the empty spaces. For materials with a random structure, the area of pores seen on a surface is similar to the total volume of pores.
Computed tomography (CT) scanning: This method uses special X-ray scans to create a 3D image of the material. This image shows both the outside and inside, including all the empty spaces. Computer software then helps analyze these voids.
Imbibition methods: This involves putting the porous sample into a fluid, often under vacuum, so the fluid fills the pores.

* Water saturation method: You soak the sample in water, then measure how much water is absorbed. The volume of absorbed water tells you the pore volume. * Water evaporation method: You weigh a sample when it's full of water and then again after it's completely dry. The difference in weight, divided by the density of water, gives you the pore volume.

Mercury intrusion porosimetry: This method pushes mercury into the pores under pressure. By measuring how much mercury enters at different pressures, you can figure out the size and volume of the pores. (Newer methods use other liquids because mercury can be toxic).
Gas expansion method: A sample of known volume is placed in a container. Gas is then allowed to expand from this container into another empty container. By measuring the pressure changes, you can calculate the volume of the pores. This method works best for pores that are connected.
Thermoporosimetry and cryoporometry: These methods involve filling the pores with a liquid and then freezing it. Small crystals of a liquid in tiny pores melt at a lower temperature than larger amounts of the same liquid. By measuring the melting temperature, scientists can learn about the size of the pores.

Images for kids

This image shows a thin slice of rock under a microscope. The purple areas are empty spaces (pores), while other colors are solid rock grains. This sample is from a type of sand dune rock in the Bahamas.