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Nanopore

Also known as
nanoscale pores, shale nanopores

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.

What is a nanopore?

A nanopore is a very small pore, with a width measured in nanometers. IUPAC classifies pores by size as micropores, narrower than 2 nanometers; mesopores, from 2 to 50 nanometers; and macropores, wider than 50 nanometers. In the oil and gas industry, nanopore is used more loosely for the nanometer-scale pores that make up much of the pore space in shale and other tight rocks, including the pores within the organic matter of source rocks.

Why nanopores matter

  • Very low permeability. Fluids move through nanometer-scale pore throats only slowly, which is why shale permeability is measured in nanodarcies and wells must be hydraulically fractured to produce.
  • High capillary pressure. Capillary pressure rises as pores shrink. In nanopores it can be very large, trapping oil and holding water, including fracturing fluid, in the rock; see water block.
  • Changed fluid behavior. In the smallest pores, confinement can alter how oil and gas behave, a subject of ongoing research.

Nanopores and treatment chemistry

For a treatment chemical to act on oil held in the rock matrix, it has to reach the pores where that oil sits. The size of a chemical’s molecules and aggregates relative to pore throats is therefore one of the factors that affect how far a treatment penetrates tight rock. Treatments for unconventional reservoirs often aim to lower interfacial tension and adjust wettability so that fluids can move between the nanopore matrix and the fractures.

Nanopores and Locus biosurfactants

Locus describes its oilfield biosurfactants as forming micelles of about 3 to 5 nanometers and cites that size as a reason they can reach micro- and nanopore networks. That is a Locus product claim; the size of any surfactant’s micelles depends on its structure and conditions.

Frequently asked questions

Nanopores are measured in nanometers. For comparison, a human hair is roughly 70,000 to 100,000 nanometers wide. Many shale pores are tens to hundreds of nanometers across.

Because fluids flow through them very slowly and capillary forces in them are very strong, so oil moves only from rock close to a fracture.

  • Micelle

    A micelle is a nanometer-scale cluster of surfactant molecules that forms spontaneously in water once the surfactant concentration passes the critical micelle concentration. The molecules' water-repelling tails gather in the core and their water-attracting heads face outward.

  • 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.

  • 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.

  • Permeability

    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.

  • Capillary pressure

    Capillary pressure is the pressure difference across the curved boundary between two immiscible fluids, such as oil and water, in a narrow pore. It increases as pores get smaller and as interfacial tension rises, and it is a main force trapping oil in reservoir rock.

Sources

  1. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (pore size classification), IUPAC Technical Report, 2015
  2. Shale pore systems, Peer-reviewed petrophysics literature