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Permeability

Also known as
rock permeability, millidarcy, mD

Permeability is the ability of a porous rock to transmit fluids through its connected pore spaces. It is measured in darcies or millidarcies, and together with porosity it is one of the main properties that determine how easily oil, gas or water can be produced.

What is permeability?

Permeability describes how easily fluid can flow through a rock. It depends on how large the connected pores and the pore throats between them are, and how well connected they are. A sandstone with large, well-connected pores can have high permeability; a shale with minute pores has extremely low permeability.

Permeability is defined through Darcy’s law, which relates flow rate to the pressure gradient, the fluid’s viscosity and the rock’s permeability. Its unit is the darcy; in reservoir work, the millidarcy (mD), one-thousandth of a darcy, is more common.

Typical values

  • Conventional reservoirs commonly range from a few millidarcies to hundreds of millidarcies or more.
  • Tight reservoirs are commonly defined as having permeability below about 0.1 millidarcy.
  • Shale permeability is often measured in nanodarcies, millionths of a darcy, which is why shale wells need horizontal drilling and hydraulic fracturing to produce economically.

Absolute, effective and relative permeability

  • Absolute permeability is measured with a single fluid filling the pores.
  • Effective permeability is the permeability to one fluid when others are also present.
  • Relative permeability is effective permeability divided by absolute permeability. It describes how oil, gas and water interfere with one another’s flow, and it depends strongly on saturation and wettability.

Porosity vs. permeability

Porosity is how much of a rock’s volume is pore space, which governs how much fluid it can hold. Permeability is how easily fluid can move through that space. A rock can be highly porous but poorly permeable if its pores are small or poorly connected, as in many shales.

What reduces permeability

Near a well, permeability can be reduced by formation damage: plugging by fines, scale or organic deposits, and water or emulsion blocks.

Frequently asked questions

A millidarcy is one-thousandth of a darcy, the standard unit of permeability. Reservoir rock permeability is most often reported in millidarcies.

Yes. If the pores are very small or poorly connected, a rock can hold a lot of fluid but let it flow only slowly. Shale is a common example.

  • Porosity

    Porosity is the fraction of a rock's volume that is empty space, which can hold fluids such as oil, gas and water. Effective porosity counts only the interconnected pores through which fluids can move. Porosity determines how much fluid a reservoir can store, not how easily it flows.

  • Pore throat

    A pore throat is the narrow passage that connects larger pore spaces in a rock. Pore throat size largely controls a rock's permeability and the capillary pressure needed for a fluid to enter it, making it a key factor in how oil, gas and water move through a reservoir.

  • Formation damage

    Formation damage is any reduction in the natural ability of reservoir rock to let fluids flow into or out of a well, usually in the rock near the wellbore. It is caused by drilling, completion, production or injection activities and reduces well productivity or injectivity.

  • Nanopore

    A nanopore is a pore with a diameter measured in nanometers, billionths of a meter. Shale and other tight rocks contain much of their pore space as nanopores, which gives them extremely low permeability and very high capillary pressures.

  • Unconventional reservoir

    An unconventional reservoir is an oil or gas accumulation, such as shale, tight sandstone or coalbed methane, whose permeability is too low for wells to produce economically without special techniques, typically horizontal drilling and multistage hydraulic fracturing.

Sources

  1. permeability; darcy, SLB Energy Glossary
  2. Permeability, SPE PetroWiki