How Does Water Flow Through Rocks?

The following text is based on the books Physical Geology by Leet, Judson, Kauffman, published by Prentice-Hall and Physical Geology by Flint and Skinner, published by Wiley.

Layered sandstone. Picture from the image gallery at www.science.ubc.ca

Understanding how fluids move through rocks is important for understanding many things including the development of aquifers and oil and gas reservoirs. It is important for understanding and predicting groundwater movement and for developing plans for underground storage and containment of human produced pollutants. In addition, understanding fluid movement in rocks can help us understand how magma travels from within the Earth to the surface to erupt at volcanoes.

The limiting amount of fluid (such as water) that can be contained within a rock depends on the porosity of the rock. Porosity is defined to be the proportion (or percent) of the total volume of the rock that is pore space (open space). A very porous rock contains a large proportion of open space. Sedimentary rock such as sandstone are typically very porous, ranging between 20% or so in some sands and gravels to 50% in clays.

If the pore spaces within a rock become filled with deposits or cemented the porosity goes down. Igneous rocks such as lavas that flow from volcanoes and metamorphic rocks that have been under high temperatures and/or pressures typically have low porosity, except where cracks and joints have developed in them.

Water can flow through rocks, and rock permeability is the capacity of the rock for transmitting fluids. A rock of very low porosity is likely to have low permeability. However, high porosity rocks do not necessarily have high permeability values. Both size and interconnections between the open spaces influence permeability.

Although clay may have a higher porosity than sand, the particles that make up clay are very small flakes and the spaces between them are very small. Water passes more readily through the sand than the clay simply because of the molecular attraction of the rock surfaces. Molecular attraction is the force that makes a thin film of water stick to a rock surface despite gravitational forces – think of a wet film on a pebble that has been dipped in water. If the opening between two adjacent particles in a rock is small enough, the films of water that adhere to the two particles come into contact with each other. The force of molecular attraction extends across the open space. At ordinary pore pressures, the water is held firmly in place and the permeability is low. That is what happens in a sponge - before it is squeezed. As the diameters of the openings increase the permeability increases. Therefore gravel is more permeable than sand. Of course, no matter how large the spaces in a material are, there must be connections between them if water is to pass through. If the open spaces are not interconnected the rock is impermeable.

Compression squeezes the cylinder and makes it shorter and wider. Extension makes the cylinder longer and narrower. The strongest force is shown by the red arrows.

Both porosity and permeability can be increased when joints and cracks form in the rock. This can happen when stress is applied to the rock. A force applied to material that tends to change its dimensions is called stress. This is commonly unit stress, defined as the total force divided by the area over which it is applied. The effects of force applied to rocks has been studied in laboratories by measuring the strain produced by the application of the force. Strain is the relative change in shape or size of an object due to externally-applied forces. In most materials as stress increases, for a time strain increases and is proportional to the stress. If the stress is removed the strain goes back to zero. This is the range of elastic deformation. When the stress continues to increase it reaches the yield point, the strength of the rock is overcome, and deformation becomes plastic; that is if the stress is removed the strain does not go back to zero.

Examples of cylinders that have been compressed and extended. An undeformed cylinder is shown in the middle. The cylinders to the right are compressed - they are shorter and wider. The cylinders to the left are extended - they are longer and narrower. Notice that the cylinder on the far left and the one second from the right are cracked.

A special kind of permanent deformation is rupture or cracking. How a rock responds to stress depends on temperature and depth of burial. At depth, rocks will respond to stress by flowing before they rupture, whereas at the Earth’s surface they will break. In addition, rocks are generally weaker at high temperature.

Layered rocks that have been deformed by a compressive force acting parallel to the bedding planes. The rocks have been bent over or folded. Rupture probably did not occur.

What scientists know about stress and strain within the interior of the Earth is limited to what they can infer from rocks in the field that have been folded and faulted. They will never be able to see these processes in action though. Because of this scientists must recreate natural pressures and temperatures in the laboratory and measure the resulting deformation. They must study many types of rocks because different rocks have different yield strengths and deform differently under different conditions. The results of these laboratory experiments will further our understanding of many crustal processes including reservoir formation, ore deposition, and magma migration.