Coated Well Casings Reduce Cathodic Protection Power and Costs
Authors: Cassinis Steve A. , Catte Darrell R. 
Published: December 2007

Cathodic protection (CP) technology is well-known throughout the oil industry to be an effective method of protecting well casings from external corrosion. In remote areas, however, providing electric power to run a cathodic protection system can be expensive, if not impractical in some cases. For onshore wells, AC power for the cathodic protection system is typically provided via electric power lines. As the distance from the well to the nearest electric power line increases, solar photovoltaic power becomes the most economic choice in Saudi Arabia.

To reduce the cost of the solar photovoltaic system, testing with Fusion Bond Epoxy (FBE)-coated casings was conducted. Testing revealed that coating the casings reduced the current design requirements for the well by 92%. A lower current requirement means fewer solar modules and fewer anodes. The total solar photovoltaic system cost was reduced by 56%, making solar photovoltaic systems economically competitive with electric power systems where power lines are more than 1 km distant.

Introduction

Saudi Aramco spends millions of dollars annually, installing new cathodic protection systems. This effort has virtually eliminated external casing leaks for onshore wells.

Recognizing the importance of cathodic protection, a study was initiated to better understand its application/usage, evaluate system performance and determine whether costs could be reduced.

Though the study is still ongoing, this paper documents the successful work associated with installing coated well casings with cathodic protection systems for remotely located onshore wells.

Basic Principle of Cathodic Protection

The cathodic protection process is fundamentally an electrochemical method of redirecting corrosion from the surface of a critical structure such as a well casing or a pipeline to the surface of a structure that has been installed and designed to corrode. The structure installed to corrode has the basic property of a slow corrosion rate and is most typically made from a cast iron alloy.

The corrosion process is an electrochemical process that can be referred to in a unit form as a corrosion cell. It requires four components:anodic surfacecathodic surfaceelectrolyte common to both the anodic and cathodic surfaceselectrical continuity between the anodic and cathodic surfaces

When a corrosion cell exists on a steel surface, the rate of metal loss follows Faraday's Law, which states:

weight of metal reacting = kIt

where I is the DC current in amperes, t is time in seconds and k is a constant specific to the metal involved.

Faraday's Law reveals an important fundamental concept to cathodic protection: The amount of metal that corrodes over a given time is a function of the DC current flowing in the corrosion cell. In general terms, an engineered cathodic protection system introduces an overwhelming anodic surface into an existing corrosion cell and effectively reverses the flow of current that was initially present.

The amount of electrical power required to reverse the flow of corrosion current is a function of the amount of corrosion current present, which in turn is a function of the area of the corroding surface.

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