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Acidizing

Also known as
acid stimulation, matrix acidizing, acid job, acid treatment

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.

What is acidizing?

Acidizing is one of the oldest and most common ways to stimulate a well. Acid is pumped down the well to react with the rock, with mineral scale, or with other material restricting flow, dissolving it to open or create flow paths. It is used in producing wells, in injection and saltwater disposal wells, and as a step in other operations, such as the acid stage pumped ahead of hydraulic fracturing.

Types of acid treatment

  • Acid washing cleans the wellbore and perforations of scale and debris without pushing acid into the rock.
  • Matrix acidizing injects acid into the rock below the pressure that would fracture it. In carbonate rock, hydrochloric acid dissolves channels called wormholes that bypass damage. In sandstone, mixtures of hydrochloric and hydrofluoric acid dissolve clays and fines that plug pore throats.
  • Acid fracturing pumps acid above fracture pressure in carbonate rock so that it etches the fracture faces unevenly, leaving conductive channels when the fracture closes.

Additives

Acid systems contain additives to make them effective and safe:

  • Corrosion inhibitors to protect steel tubing and casing.
  • Iron control agents to prevent iron compounds from precipitating as the acid is spent.
  • Surfactants and mutual solvents to improve penetration, water-wet the rock, and prevent emulsions.
  • Anti-sludge agents to prevent acid sludge when acid contacts some crude oils.
  • Diverters to spread acid across zones instead of letting it enter only the most permeable one.

Some acid systems are formulated as microemulsions to improve contact with the rock or deposits.

Risks

Acid that is poorly matched to the rock or fluids can cause formation damage of its own: precipitates, sludge, emulsions, or loosened fines. Treatment design depends on mineralogy, temperature and the nature of the damage.

Acidizing at Locus

Locus’s AcidBoost is a biosurfactant microemulsion added to acid treatments, which Locus describes as improving acid contact with carbonate scale near the wellbore and controlling sludge without solvents.

Frequently asked questions

Matrix acidizing is pumped below fracture pressure, so acid flows into the rock’s pores to dissolve damage. Acid fracturing is pumped above fracture pressure to create fractures, whose faces the acid etches to keep them conductive.

Hydrochloric acid is the most common, especially for carbonates. Sandstones are often treated with mixtures of hydrochloric and hydrofluoric acid. Organic acids such as acetic and formic acid are used where slower reaction or lower corrosion is needed.

  • 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.

  • 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.

  • Acid sludge

    Acid sludge is a thick, insoluble material that can form when acid used in well treatments reacts with certain crude oils. Asphaltenes and other heavy components precipitate at the acid-oil contact, especially when dissolved iron is present, and the sludge can plug the formation.

  • 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.

  • Well stimulation

    Well stimulation is any treatment that restores or increases the flow of oil, gas or water between a well and the reservoir by improving the rock around the well. Hydraulic fracturing and acidizing are the two main methods.

  • Microemulsion

    A microemulsion is a clear, thermodynamically stable mixture of oil, water and surfactant, often with a co-surfactant, that forms spontaneously. Its oil and water domains are only nanometers to tens of nanometers across, much smaller than the droplets of an ordinary emulsion.

Sources

  1. acidizing; matrix acidizing; acid fracturing, SLB Energy Glossary
  2. Acidizing, SPE PetroWiki