Corrosion under the influence of CO2 and carbonates depositions
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Corrosion under the influence of CO2 and carbonates depositions

The intensity of the corrosion process of oil and gas equipment made from steel is characterized by the rate of transition of iron (Fe) from the surface of the metal into formation water as follows:

Fe → Fe2+ + 2ē      (1).

The presence of carbon dioxide (CO2) in the produced fluids actively stimulates this process, because, by dissolving in formation water, CO2 forms a carbonic acid that dissociates in water according to the reaction:

H2O + CO2 ↔ H2CO3 ↔ H+ + НCO3–    (2).

The resulting HCO3- anion reacts with Fe2+ cations in the formation water and forms iron bicarbonate (FeHCO3). The binding of Fe2+ cations with the bicarbonate anion HCO3- stimulates the corrosion of iron in the presence of CO2 due to the continuous transition to the formation water of the next "portions" of iron cations Fe2+, which are corrosion products. It is also well known that in the presence of CO2 in the produced fluids, and the presence of Ca2+ and Mg2+ cations in the formation water, bicarbonates Ca (HCO3)2 and Mg (HCO3)2 are formed, which are subsequently converted to carbonates:

(Ca, Mg)(HCO3)2 ↔ (Ca, Mg)CO3↓ + CO2↑ + H2O      (3).

The CaCO3 and MgCO3 carbonates formed by the reaction (3) precipitate in the form of a deposit on the walls and clog the oil and gas field equipment, thereby significantly reducing the productivity of the wells. The main causes for the formation of carbonate deposits of CaCO3, MgCO3, and FeHCO3 in oil and gas production equipment are the following factors (or their combinations):

•       decreasing the total pressure in the system;

•       changes in temperature (for example, the probability of precipitation of calcium and iron carbonates increases with increasing temperature);

•       changes in the chemical composition of water, which can be observed as when mixing water of different composition, and as a result of corrosion, when water is enriched with iron ions.

Deposits of carbonates that are formed on the metal surfaces of oilfield equipment adversely affect the productivity of wells and pipelines and lead to an aggravation of the corrosive situation in the system. So, if there is more than 1% CO2 in the formation water, the precipitates of CaCO3 and MgCO3 are very loose and do not interfere with the access to the metal surface of corrosive agents such as O2 and CO2, which leads to active stimulation of carbon dioxide corrosion of metal oil and gas production equipment. The friability and porosity of the layer of the above deposits on the surface of the metal, in the presence of CO2, stimulates the intensive development of local types of corrosion: ulcer, pitting, intercrystallite, knife, corrosion cracking of the metal, etc. As is known, it is local corrosion in all its types listed above that is much more dangerous than general corrosion, because despite the relatively small total metal losses, it is local corrosion that dramatically worsens the mechanical strength of metal equipment, and large amounts of ulcers, shells, fistula, etc. are formed on the metal surface. Frequent violations of technological processes of oil and gas production is a consequence of local corrosion of metals in the presence of CO2. Due to local corrosion, the main metal equipment, auxiliary equipment, and pipelines are broken down, which leads to large losses of produced and pumped products, that is: to significant economic and environmental costs of oil and gas producing enterprises.

The most effective way to reduce the corrosive effect of CO2 on metal oilfield equipment and to form precipitates of salts that are insoluble in formation water is the use of metal corrosion inhibitors and inhibitors of various deposits, including mineral deposits. It is very important to correctly select a reagent package that must effectively inhibit corrosion of equipment and remove deposits formed on its surfaces and be compatible with all other reagents applied in the field.

One of the most effective inhibitors of corrosion of metals and various deposits are multifunctional corrosion inhibitors of the DEFENDER series, developed and manufactured by Himipex Oil. The well treatment with our reagents is always economically beneficial for the customer. The degrees of protection against corrosion of oil and gas equipment by multi-functional corrosion inhibitors of the DEFENDER series are not less than 95%, and periods between cleaning from deposits are increased by 5-10 times.

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