Abstract
One of the problems concerning the prevention of corrosion of oil and gas prevention of corrosion of oil and gas well casings is that of determining the current requirements for cathodic protection. protection. This paper discusses the causes of corrosion on well casings, suggests well completion practices that would minimize corrosion problems, and explains and discusses two methods of determining current requirements for cathodic protection. Graphs and drawings are included to describe and substantiate both methods.
The casing potential profile tool is the most used tool to locate and evaluate external casing corrosion. It can also be used to determine the current requirements for cathodic protection.
The E-Log-I survey (sometimes referred to as the log-current-potential method) cannot locate or evaluate external casing corrosion, but is a relatively inexpensive method of determining current requirements for cathodic protection. It's accuracy has been verified in tests with the casing profile tool and it's dependability has been substantiated through proven results with the successful application of cathodic protection.
A. CAUSES OF EXTERNAL CASING CORROSION ON OIL AND GAS WELLS
There are five (5) major items which are normally thought to be the cause of well casing corrosion. These are:
Dis-similar metals
Dis-similar soil
Differential aeration
Bacteria
"Stray current" interference
Dis-similar metals cause a form of corrosion brought about by the use of different metals electrically coupled together (such as steel pipe and brass gate valves); the use of metals unintentionally caused to be dissimilar by the manufacturing process in either the steel or the pipe; the use of "old" and "new" pipe in the same casing which can include pipe in the same casing which can include oxide scales, etc., and marks and scratches on the pipe left by tools.
