Production tank protection eliminates traditional corrosion mechanisms

Standard construction practices for storage tanks typically result in regular site maintenance and replacement that compromise environmental protection and regulatory compliance. After millions of dollars are spent to drill and complete a well, many production sites are constructed for only a few percentage points of the total well cost. This minimum investment in permanent production fixtures means most sites will be replaced many times over the life of the reservoir.

The maintenance demands this creates are significant. Secondary containments constructed of dirt and gravel must be continuously maintained to ensure they meet regulatory standards for volume and allow safe access to the site. Even metal wall containments with high-density polyethylene lining require significant work every few years to keep the containment fit for purpose.

Rust and pitting due to corrosion of the production tank is a common result of traditional construction methods. (Images courtesy of Falcon Technologies)

Historical oilfield practices for supporting metal tanks also create an environment for accelerated corrosion. Production tanks commonly sit on steel rings filled with gravel that retain moisture or produced liquids, which is in constant contact with the tank. This fluid, whether it is water or petroleum, is held in the interconnected pore spaces between the packed gravel granules that form the raised bed.

Oil and gas fields commonly produce varying levels of corrosive liquids. The combination of moisture and these produced liquids can result in an environment that quickly degrades tanks and may require replacement within one year of installation. In addition, when leaks and spills accumulate around the base of the tanks, these contaminate the gravel and earthen beds with pollutants that pose a potential environmental threat.

Corrosion process

An electrochemical cell, which leads to corrosion, is formed when the area where the metal that is exposed to fluid with higher oxygen concentration becomes the “cathode” and the lower concentration area becomes the “anode.”

Corrosion is commonly defined as the deterioration of a metallic body through metal loss via electrochemical action. An electrochemical reaction is an action by which energy and metal body are transferred from one body into another medium. The metal transfer results in deformations of the metallic body, such as rusting, pitting, cracking, or embrittlement.

An electrochemical cell, commonly known as a battery, is a system where two metallic bodies of different electrode potentials are connected to form a conductive circuit. The electro-potential difference creates the driving force or potential for electric current flow. This circuit can be either solid or liquid in the presence of an electrolyte. Corrosion will not take place in the absence of any element of this electrochemical cell.

Tank corrosion mechanisms

Storage tank corrosion is the result of electrochemical and microbiological mechanisms. Electrochemical corrosion is explained by the concentration cell mechanism, which describes the corrosion of a metal surface when exposed to a fluid of varying electrolyte concentrations or of varying aeration.

Differential aeration is the more important of these two pathways. In this condition, the area where the metal is exposed to fluid with higher oxygen concentration becomes the “cathode” and the lower concentration area becomes “anode.” This generates a differential aeration current that results in corrosion.

Varied oxygen concentration in the gravel bed can lead to corrosion of the storage tank.

Moisture and varied oxygen content create conditions for electrochemical corrosion.

The gravel bed promotes an environment that fosters bacterial propagation.

The tank base provides a moisture barrier that prevents corrosion attacks.

Traditionally, field storage tanks are set on top of raised gravel beds within the secondary containment. Although these gravel beds have been widely used to expedite operations, this solution actually increases the risk of corrosion.

The interconnected pore spaces between the gravel granules form a network of channels that retain and transport fluid via capillary action, resulting in a tank bottom constantly exposed to fluid. Due to the placement of the tanks on top of the gravel beds, air flow through the gravel bed can only occur circumferentially.

As a result, water entrained within the gravel bed pores has a varying concentration of oxygen with respect to the radial extent of the gravel bed. This differential oxygen concentration across the bottom of the storage tank forms an electrochemical corrosion cell.

Microbiological corrosion is the second mechanism that is problematic for production tanks. In nature, many species of bacterium possess the ability to corrode metal in both aerobic and anaerobic environments. To propagate, most bacteria commonly require a set of conditions to survive and grow, including a carbon-based food source, a pH range from slightly acidic to slightly basic, a temperature range of near-freezing to 49°C (120°F), an energy source (either oxygen for aerobic or nitrogen for anaerobic systems), and moisture. As with an electrochemical cell, the absence of any of these propagation elements prevents the bacteria from developing.

Safeguarding against corrosion

Strictly speaking, eliminating any one of the critical elements required by electrochemical or microbiological corrosion mechanisms prevents tank corrosion. The most practical and cost-effective method is the prevention of moisture coming into contact with the tank bottom.

This is being achieved by replacing the traditional gravel bed with a tank base surface that effectively prevents the moisture contacting the tank. The patent-pending base has been installed in more than 5,500 tank locations since 2008.

The elastomer-encapsulated tank base eliminates fluid retention and transmissivity mechanisms. A modified polymer lining provides nonpermeable, inert protection from degradation and eliminates the retention of pollutants that can generate environmental waste. As a result, tank life is greatly extended and environmental issues are mitigated.

The tank base is a 6-in. platform that raises the metal tank surface out of the moisture and ensures reliable support through the life of the well. The base exceeds required compression strengths across multiple sizes and configurations and maintains its properties through changes in weather and ultra-violet exposure. Straight lines between tank base components provide a drainage path for any moisture that remains after setting the tank.

Reducing the costs and environmental risks associated with production tank corrosion requires a break with traditional construction designs. These gravel bands present long-term maintenance issues by creating conditions that promote rather than prevent corrosion. A unique tank base design, however, breaks the chain of corrosion mechanisms to eliminate problem sources and extend tank life.