Technical Glossary

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Acid Stimulation

  1. n. [Well Completions, Well Workover and Intervention]

The treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sandstone formations, the acid reacts with the soluble substances in the formation matrix to enlarge the pore spaces. In carbonate formations, the acid dissolves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of reservoir fluids. Matrix acidizing operations are ideally performed at high rate, but at treatment pressures below the fracture pressure of the formation. This enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation.


  1. n. [Drilling Fluids]

An additive that kills bacteria. Bactericides are commonly used in water muds containing natural starches and gums that are especially vulnerable to bacterial attack. Bactericide choices are limited and care must be taken to find those that are effective yet approved by governments and by company policy. Bactericides, also called biocides, can be used to control sulfate-reducing bacteriaslime-forming bacteriairon-oxidizing bacteria and bacteria that attacks polymers in fracture and secondary recoveryfluids. In polymers, the degradation of the fluid is controlled, thus avoiding the formation of a large biomass, which could plug the formation and reduce permeability.


  1. n. [Drilling Fluids]

A chemical that reduces the viscosity of a fluid by breaking long-chain molecules into shorter segments. Drilling fluids are commonly emulsified or contain long-chain molecules that have sufficient viscosity to carry cuttings to surface. After the drilling fluid has done its job, a breaker may be added to reduce the viscosity of the fluid by breaking down the long chain molecules into shorter molecules. A surfactant may be added to an emulsion to reduce its viscosity.

Cement Dispersant

  1. n. [Well Completions]

A chemical additive that reduces the cement slurry viscosity to improve fluid flow characteristics. Adequately dispersed cement slurries exhibit improved fluid-loss control, can displace drilling fluid more efficiently and be successfully mixed and pumped at higher densities.

Clay Control

  1. n. [Well Workover and Intervention]

A chemical additive used in stimulation treatments to prevent the migration or swelling of clay particles in reaction to water-base fluid. Without adequate protection, some water-base fluids can affect the electrical charge of naturally occurring clay platelets in the formation. Modifying the charge causes the platelets to swell or migrate in the flowing fluid and, once these are dispersed, it is likely that some clay plugging of the formation matrix will occur. Clay stabilizers act to retain the clay platelets in position by controlling the charge and electrolytic characteristics of the treatment fluid.


  1. n. [Well Workover and Intervention]

A compound, typically a metallic salt, mixed with a base-gel fluid, such as a guar-gel system, to create a viscous gel used in some stimulation or pipeline cleaning treatments. The crosslinker reacts with the multiple-strand polymer to couple the molecules, creating a fluid of high, but closely controlled, viscosity. Treatments using crosslinkers should take account of the conditions needed to break the gel structure to ensure satisfactory cleanup and disposal.

Enhanced Oil Recovery

  1. n. [Enhanced Oil Recovery]

An oil recovery enhancement method using sophisticated techniques that alter the original properties of oil. Once ranked as a third stage of oil recovery that was carried out after secondary recovery, the techniques employed during enhanced oil recovery can actually be initiated at any time during the productive life of an oil reservoir. Its purpose is not only to restore formationpressure, but also to improve oil displacement or fluid flow in the reservoir. The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion). The optimal application of each type depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity. Enhanced oil recovery is also known as improved oil recovery or tertiary recovery and it is abbreviated as EOR.

Equivalent Circulating Density - ECD

  1. n. [Drilling Fluids]

The effective density exerted by a circulating fluid against the formation that takes into account the pressure drop in the annulus above the point being considered. The ECD is calculated as: d + P/(0.052*D), where d is the mud weight (ppg), P is the pressure drop in the annulus between depth D and surface (psi), and D is the true vertical depth (feet). The ECD is an important parameter in avoiding kicks and losses, particularly in wells that have a narrow window between the fracture gradient and porepressure gradient.

Gas Migration

  1. n. [Drilling Fluids]

A generic term referring to all possible routes for annular gas entry and propagation through and around the cement sheath. Gas migration is also known as annular gas flow.

Hydraulic Fracturing

  1. n. [Shale Gas, Well Completions, Well Workover and Intervention]

stimulation treatment routinely performed on oil and gas wells in low-permeability reservoirs. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formationProppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area.

Lost Circulation

  1. n. [Drilling]

Solid material intentionally introduced into a mud system to reduce and eventually prevent the flow of drilling fluid into a weak, fractured or vugular formation. This material is generally fibrous or plate-like in nature, as suppliers attempt to design slurries that will efficiently bridge over and seal loss zones. In addition, popular lost circulation materials are low-cost waste products from the food processing or chemical manufacturing industries. Examples of lost circulation material include ground peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials.

Lost Circulation

  1. n. [Drilling Fluids] –A type of lost circulation material that is long, slender and flexible and occurs in various sizes and lengths of fiber. Fiber LCM is added to mud and placed downhole to help retard mud loss into fractures or highly permeable zones. Ideally, fiber LCM should be insoluble and inert to the mud system in which it is used. Examples are cedar bark, shredded cane stalks, mineral fiber and hair. Often, granular, flake and fiber LCM are mixed together into an LCM pill and pumped into the well next to the zone of fluid loss to seal the formation that is taking mud from the system.Synonyms: LCMAlternate Form: fiber LCM, fiber lost-circulation material


  1. n. [Geology]

The phase that occurs after successful exploration and development and during which hydrocarbons are drained from an oil or gas field.


  1. n. [Well Completions]

A chemical agent used to increase the thickening time of cement slurries to enable proper placement. The need for cement retardation increases with depth due to the greater time required to complete the cementing operation and the effect of increased temperature on the cement-setting process.

Scale Control

  1. n. [Well Completions]

A chemical treatment used to control or prevent scale deposition in the production conduit or completion system. Scale-inhibitorchemicals may be continuously injected through a downhole injection point in the completion, or periodic squeeze treatments may be undertaken to place the inhibitor in the reservoir matrix for subsequent commingling with produced fluids. Some scale-inhibitor systems integrate scale inhibitors and fracture treatments into one step, which guarantees that the entire well is treated with scale inhibitor. In this type of treatment, a high-efficiency scale inhibitor is pumped into the matrix surrounding the fracture face during leakoff. It adsorbs to the matrix during pumping until the fracture begins to produce water. As water passes through the inhibitor-adsorbed zone, it dissolves sufficient inhibitor to prevent scale deposition. The inhibitor is better placed than in a conventional scale-inhibitor squeeze, which reduces the retreatment cost and improves production.

Secondary Emulsifier / Low End Rheology Modifier

  1. n. [Drilling Fluids]

A type of organic acid derived from animal and vegetable fats and oils. Fatty acids are the raw materials used in the manufacture of many drilling-fluid additives, such as emulsifiers, oil-wetting agents and lubricants. Tall-oil fatty acids are distilled from conifer trees. Animal and vegetable fats and oils are triglycerides, which are hydrolyzed to give fatty acids (and glycerol). Fatty acids from animals are mostly saturated acids, having single bonds between carbon atoms. Tall oils and vegetable oils yield both saturated and unsaturated (double- and triple-bond) fatty acids.


  1. n. [Well Completions]

A viscous fluid used to aid removal of drilling fluids before a primary cementing operation. The spacer is prepared with specific fluid characteristics, such as viscosity and density, that are engineered to displace the drilling fluid while enabling placement of a complete cement sheath.


  1. n. [Enhanced Oil Recovery, Well Completions, Well Workover and Intervention, Shale Gas]

A treatment performed to restore or enhance the productivity of a well. Stimulation treatments fall into two main groups, hydraulic fracturing treatments and matrix treatments. Fracturing treatments are performed above the fracture pressure of the reservoirformation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area. Stimulation in shale gas reservoirs typically takes the form of hydraulic fracturing treatments.


Sweep Material

  1. n. [Well Completions]

A relatively small volume of viscous fluid, typically a carrier gel, or fiber package that is circulated to sweep, or remove, debris or residual fluids from the circulation system.