Laboratory Evaluation of Corrosion Inhibitors Used in the Oil and Gas Industry
|Publication Date:||1 October 2014|
A corrosion inhibitor is a substance that, when added in small concentrations to a corrosive environment, reduces the corrosion rate of a metal in that environment. Commercial inhibitor systems consist of corrosion inhibitor raw materials formulated with surfactants and solvents. Inhibitors are classified in terms of corrosion control mechanism, environment to which they are added, metal to be protected, or inhibitor chemistry. Some of the more commonly encountered classifications include anodic, cathodic, passivating, oxidizing, film-forming, organic, inorganic, environmentally benign, high-temperature and high-pressure stable, vapor phase, and volatile. Corrosion inhibitors are categorized in accordance with their solubility, i.e., water soluble or oil soluble.
An effective corrosion mitigation program consists of selecting the proper corrosion inhibitor for a given system and applying it using the most appropriate technique. The treatment technique is selected based on the corrosiveness of the field conditions, amount and composition of produced fluids, production technique, system history, and equipment availability. Depending on the system design, metallurgy, corrosiveness, etc., inhibitors are applied down the well, in pipelines, surface equipment, or in the formation.
Corrosion inhibitors are applied using either continuous injection or periodic batch application techniques based on predicted and/or assessed corrosiveness of the system and the adopted corrosion mitigation program. Continuous injection is used in corrosive systems where chemicals are added to the system at a selected rate determined by the chemical program in order to maintain a predetermined concentration of inhibitor in the production fluids. During continuous treatment, a low concentration of corrosion inhibitor is maintained in the system using continuous injection. In general, batch application is used for periodic treatment to address a specific problem in a less corrosive system. In some cases, inhibitors are batch treated because of lack of continuous injection equipment, cost, field remoteness, etc. Periodicity of batch application is determined by the production system and its parameters. During the batch treatment, a high concentration of chemical is allowed to contact the metal surface to be protected for a short period of time.
Corrosion inhibitors are subjected to thorough laboratory evaluation before being used for a field application. Laboratory evaluation of corrosion inhibitors includes assessment of some physical and chemical properties, as well as performance under simulated field conditions. Certain chemical and physical characteristics of inhibitors are considered, along with their ability to provide corrosion protection. Field-ready inhibitor formulations are chemically and thermally stable at expected temperature conditions (hot and/or cold) of the field. Inhibitor formulations are soluble or dispersible in field fluids and consist of components to minimize formation of emulsions or foam. Viscosity of chemicals is such that chemicals have easy flowability and pumpability characteristics under given field conditions. This report provides methodologies used for laboratory evaluation of oil and gas industry corrosion inhibitors' chemical and physical properties. Definitions for terms relevant to this report are found in Appendix A.