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Formation damage

Also known as
skin, skin damage, near-wellbore 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.

What is formation damage?

Every barrel of oil a well produces, or every barrel of water it injects, passes through the rock immediately around the wellbore. Formation damage is anything that lowers the permeability of that rock. Because flow converges on the well, even a thin damaged zone in the near-wellbore region can sharply cut production or injectivity.

Engineers express formation damage as skin: a dimensionless number describing the extra pressure drop near the well. Positive skin indicates damage; negative skin indicates stimulation, as after hydraulic fracturing.

Common causes

  • Drilling and completion fluids: solids and filtrate that invade the rock.
  • Fines migration and clay swelling: particles loosened or swollen by changes in water chemistry or flow rate that plug pore throats.
  • Water blocks: water held in the rock by capillary forces.
  • Emulsion blocks: viscous emulsions formed in the rock.
  • Inorganic deposits: mineral scale and iron sulfide.
  • Organic deposits: paraffin and asphaltenes.
  • Wettability changes: surfactants or oil components that make rock oil-wet, reducing flow of oil.
  • Bacteria: biofilms and byproducts that plug pores.

Preventing and removing damage

Prevention starts with fluid design: choosing drilling, completion and injection fluids compatible with the rock and reservoir fluids, and keeping injected water clean. Remediation depends on the cause:

  • Acidizing to dissolve scale and damage in carbonate and sandstone rock.
  • Solvents, dispersants and surfactants for organic deposits, emulsions and water blocks.
  • Fracturing to create a path that bypasses the damaged zone.

Frequently asked questions

Skin is a number that describes the additional pressure drop near a well compared with an undamaged well. Positive skin means damage is restricting flow; negative skin means the well is stimulated.

Signs include production or injectivity below what the reservoir should deliver, a rising skin value in well tests, and declines following workovers, treatments or changes in injected water.

  • Near-wellbore region

    The near-wellbore region is the zone of reservoir rock immediately surrounding a well, typically extending from inches to several feet. All produced or injected fluid passes through it, so its condition has an outsized effect on well performance.

  • Water block

    A water block is a reduction in the flow of oil or gas caused by water held in the rock near a well or fracture face. Capillary forces trap the water in small pores, lowering the rock's permeability to hydrocarbons.

  • Acidizing

    Acidizing is a well treatment in which acid is pumped into a well to dissolve rock, scale or other damage near the wellbore and restore or improve the flow of oil, gas or water. The main forms are acid washing, matrix acidizing and acid 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.

  • Mineral scale

    Mineral scale is a hard, inorganic deposit that forms when dissolved minerals in water, such as calcium carbonate or barium sulfate, crystallize on surfaces. It builds up in wells, pipes, heat exchangers and equipment and restricts flow and heat transfer.

  • Asphaltene

    Asphaltenes are the heaviest, most polar fraction of crude oil, defined by solubility: they dissolve in aromatic solvents such as toluene but not in light alkanes such as heptane. When they come out of solution they form sticky deposits that can plug rock, wells and equipment.

Sources

  1. formation damage; skin, SLB Energy Glossary
  2. Formation damage, SPE PetroWiki