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Preventing Corrosion in New Mexico Oilfields

Corrosion control methods include: In addition, monitoring corrosion and scale formation is an important part of the prevention process, allowing producers to better understand the environment and to be able to predict problems. Field experience indicates that a systematic, integrated process of monitoring and failure analysis will, over time, reduce failure rates, and effectively lower production costs.



Cathodic Protection

In general, cathodic protection is an approach where the metal surface to be protected is made into the cathode of a corrosion cell. Since corrosion and material loss occurs at the anode, this approach protects the metal.

There are two types of cathodic protection, the sacrificial anode and the impressed-current method. The sacrificial anode method is the simpler method, and utilizes galvanic corrosion. Sacrificial anodes are pieces of metal usually electrically connected by a wire or steel strap to the structure to be protected. The metals used must be less noble than steel (the common oil-field material), such as magnesium, zinc, or aluminum. The sacrificial anodes are preferentially corroded, protecting the (cathodic) steel from corrosion. Magnesium and zinc are usually used in soils, and zinc can also be used in brine environments. Sacrificial anodes are most often used when current requirements are relatively low, electric power is not readily available, and when system life is short, which calls for a low capital investment.

Impressed-current method uses an external energy source to produce an electric current that is sent to the impressed-current anodes, which can be composed of graphite, high-silicon cast iron, lead-silver alloy, platinum, or even scrap steel rails. Impressed-current cathodic protection is used when current requirements are high, electrolyte conductivity is high, fluctuations in current requirements will occur, and when electrical power is readily available.

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Coatings


Protective coatings can be used to protect tubing, downhole equipment, wellhead components, Christmas trees, and various flowlines and pressure vessels. Coatings work by reducing the cathodic area available for corrosion reaction. Since no coat can be 100% holiday-free (without pinholes or defects), coatings are often used in conjunction with cathodic protection or chemical inhibition.

Quality control parameters for coating include surface finish/preparation, application techniques, coating thickness, holiday detection, joint condition, and inspection. Coated equipment must be carefully handled after coating to prevent defects in the coat.

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Corrosion Inhibitors


There is a wide variety of corrosion inhibitor formulations available that can be selected to handle most environments in oil and gas production systems except those where oxygen is present. The application technique must match the systems mechanical and process considerations in order to assure that the inhibitor will reach the metal surface where needed. Most corrosion inhibitors, which are typically organic amine-based compounds, function by establishing a film that protects metal from corrosive fluids.

The choice of a specific inhibition program is a combination of technical and economic considerations. The programs should be modified or adjusted periodically to optimize the program for cost effectiveness. Additionally, programs should be monitored and periodically reviewed because systems are continually changing.

Types of inhibitors

There are many types of corrosion inhibitors for various applications. Generally they can be grouped into two broad categories, organic and inorganic. Inorganic inhibitors are most often used in cooling tower water, heating/cooling mechanisms, dehydration glycol, and sweetening amine solutions. Organic film formers are used in oil, gas, and water wells, water and gas systems, and flowlines.

Inorganic corrosion inhibitors are usually metal salts, which act to passivate the metal surface. They have limited use because they require constant concentrations, are often pH sensitive, and usually don't work if there are chlorides present.

Organic corrosion inhibitors are the most common corrosion inhibitors in use in oilfield systems. The majority of these are "organic film forming inhibitors". These are organic chemicals with a polar ("water loving") head and a long hydrocarbon ("oil loving") tail. When applied, these compounds align with the polar head towards the metal and the tail towards the outside, effectively establishing an oil-wet film on the metal surface. This inhibitor film breaks the corrosion cell by separating the metal surface and the electrolyte containing water.

Inhibitor selection

The first step in selecting an inhibitor is to review the system, its physical layout, mechanical considerations, and fluids being handled, locating any special or unique factors.

The second step is to select the application method(s) that assure the inhibitor gets to where it will be effective, i.e., the metal surface. This step is very important - large numbers of "inhibitor failures" were actually application failures.

Common application methods for wells with packered annuli include formation squeeze, tubing displacement, partial tubing displacement and yo-yo, and treating strings. Less common application methods include weighted liquids, dump bailers, wash bailers, inhibitor sticks, chemical injector valves, and gas lift gas addition.

Application methods for wells with open annuli include annular batch - operator applied or by treater truck, and continuous, with a chemical injector pump.

The third step is to review the properties required for the application technique. Inhibitor properties that should be taken into consideration during selection include solubility and dispersability, both in the carrier and in produced fluids (water, oil, gas); emulsification properties, viscosity, freeze point, thermal stability, corrosiveness, foaming properties, partitioning (between oil and water), compatibility with other chemicals, mobility of individual components, compatibility with downstream process, and environmental concerns.

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Petroleum Recovery Research Center, Socorro, NM-87801