Porosity is the amount of open space in a rock or sediment. Permeability is the extent to which pores are interconnected. A rock can have high porosity but low permeability Permeability in fluid mechanics and the earth sciences (commonly symbolized as κ, or k) is a measure of the ability of a porous material (often, a rock or an unconsolidated material) to allow fluids to pass through it.Permeability
Aquifer
An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt) from which groundwater can be extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydroge…
What is an example of a rock with high porosity but low permeability?
The one key exception is that aquitards can have high porosity and hold lots of water however, due to the their low permeability they are unable to transmit it from pore to pore and therefore water cannot flow within an aquitard very well. A good example of an aquitard is a layer of clay.
Why do some soils have high porosity but low permeability?
Some surface soils in the area have a high clay content (very small particles), so they have high porosity but low permeability. Adding sand helps increase the average soil particle size, increasing the permeability.
Do rocks with a high porosity have a high permeability?
The greater the permeability, the easier it is to extract oil from the rock. Rocks such as sandstone have a very high porosity and permeability and make a productive oil or natural gas well. Looking at the permeability of rocks is one way that geologists can determine where a good location for an oil well is.
Is there a relationship between porosity and permeability?
For groundwater to be able to get into a rock with good porosity it must also have good permeability. For a rock to be permeable and for water to move through it, the pore spaces between the grains in the rock must be connected. Permeability is therefore a measure of the ability of water to move through a rock.
Can a material have a high porosity but a low permeability?
A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability.
Why do pumice and shale have high porosity but low permeability?
For example, clay and rocks like pumice may have high porosity, but because the pores do not connect with each other, the permeability of these rocks is usually low.
Which rock has the highest porosity and permeability?
Gravel and sand are both porous and permeable, making them good aquifer materials. Gravel has the highest permeability.
What does high porosity rock mean?
Porous rock contains empty space in which fluids, such as compressed air, can be stored. Porosity is defined as the percentage of a rock that is empty and can be used for storage. A porosity of >10% is needed for CAES (sandstone, shale, and limestone are examples of such rocks).
Can a material be porous and impermeable?
Rocks such as pumice and shale can have high porosity, yet can be nearly impermeable due to the poorly interconnected voids.
Are porosity and permeability inversely related?
So it key to understand that Porosity and Permeability are inversely related, if a soil has a higher percentage of Porosity then it would be lower in Permeability.
What is the permeability and porosity of rocks?
Porosity and permeability are both properties of rocks and soil. The main difference between porosity and permeability is that porosity is a measurement of space between rocks whereas permeability is a measurement of how easy it is for fluids to flow between rocks.
What makes a rock porous?
It results from the spaces between grains of sediment, or from bubbles of gas that get trapped in lava as it cools. Secondary porosity is created after a rock has already formed. It can be caused by things like the dissolution of minerals, or the formation of fractures (cracks) in the rock.
Why do clay soils have such low permeability despite being very porous?
Many clay soils have porosity greater than that of sand as the clay soils have more surface area onto which water can adhere and move, but on the opposite side, the pores are poorly connected because the clay has fine particles that fill in much of the pore space blocking or limiting movement of water through them.
Why do clay soils have low permeability?
Clay textured soils have small pore spaces that cause water to drain slowly through the soil. Clay soils are known to have low permeability, which results in low infiltration rates and poor drainage.
What does high soil porosity mean?
Porous soils have a low holding capacity for water and become saturated quickly. Large pore spaces allow water to drain through the soil quickly, and porous soil often holds fewer nutrients than other soils. Particles of clay and organic matter help hold nutrients in the soil.
What are the factors affecting porosity of soil?
Porosity varies depending on particle size and aggregation. It is greater in clayey and organic soils than in sandy soils. A large number of small particles in a volume of soil produces a large number of soil pores. Fewer large particles can occupy the same volume of soil so there are fewer pores and less porosity.
What is the porosity of a material?
Porosity is an intrinsic property of every material. It refers to the amount of empty space within a given material. In a soil or rock the porosity (empty space) exists between the grains of minerals. In a material like gravel the grains are large and there is lots of empty space between them since they don’t fit together very well. However, in a material like a gravel, sand and clay mixture the porosity is much less as the smaller grains fill the spaces. The amount of water a material can hold is directly related to the porosity since water will try and fill the empty spaces in a material. We measure porosity by the percentage of empty space that exists within a particular porous media.
What is the property of permeability?
Permeability is another intrinsic property of all materials and is closely related to porosity. Permeability refers to how connected pore spaces are to one another. If the material has high permeability than pore spaces are connected to one another allowing water to flow from one to another, however, if there is low permeability then the pore spaces are isolated and water is trapped within them. For example, in a gravel all of the pores well connected one another allowing water to flow through it, however, in a clay most of the pore spaces are blocked, meaning water cannot flow through it easily.
What is the opposite of an aquifer?
The other type is a confined aquifer that has an aquitard above and below it. An aquitard is basically the opposite of an aquifer with one key exception. Aquitards have very low permeability and do not transfer water well at all. In fact, in the ground they often act as a barrier to water flow and separate two aquifers. The one key exception is that aquitards can have high porosity and hold lots of water however, due to the their low permeability they are unable to transmit it from pore to pore and therefore water cannot flow within an aquitard very well. A good example of an aquitard is a layer of clay. Clay often has high porosity but almost no permeability meaning it is essentially a barrier which water cannot flow through and the water within it is trapped. However, there is still limited water flow within aquitards due to other processes that I won’t get into now.
How do aquifers get recharged?
Aquifers are replenished when surface water infiltrates through the ground and refills the pore spaces in the aquifer. This process is called recharge. It is especially important to ensure that recharge is clean and uncontaminated or the entire aquifer could become polluted. There are two main types of aquifer.
What is it called when water is pumped out of a rock?
When a water-bearing rock readily transmits water to wells and springs, it is called an aquifer . Wells can be drilled into the aquifers and water can be pumped out. Precipitation eventually adds water ( recharge) into the porous rock of the aquifer.
What is confined aquifer?
Sometimes the porous rock layers become tilted in the earth. There might be a confining layer of less porous rock both above and below the porous layer. This is an example of a confined aquifer. In this case, the rocks surrounding the aquifer confines the pressure in the porous rock and its water. If a well is drilled into this “pressurized” aquifer, the internal pressure might (depending on the ability of the rock to transport water) be enough to push the water up the well and up to the surface without the aid of a pump, sometimes completely out of the well. This type of well is called artesian. The pressure of water from an artesian well can be quite dramatic.
Why do rocks shrink in size?
On the average, however, the porosity and permeability of rocks decrease as their depth below land surface increases; the pores and cracks in rocks at great depths are closed or greatly reduced in size because of the weight of overlying rocks.
