Cathodic Protection Requirements For Well Casings

Abstract

Abstract Failures of well casings with and without cathodic protection in the South Swan Hills and Nipisi fields were investigated. Surveys which were conducted to determine the adequacy of the existing or the need for cathodic protection included casing potential profile (CPP) surveys, E log I tests, and remote off-wellhead potential surveys. The extent of the casing damage was confirmed with casing inspection logs, internal casing calipers and visual inspection of recovered casing. The depths protected by past and present Current levels, as well as the depths that could be protected with additional current, are discussed. Resultsfrom the CPP surveys, E Jog I tests and remote off-wellhead potential surveys are compared. A deep protection pilot project initiated in the South Swan Hills field is described. Summary Well casing leaks in the South Swan Hills and Nipisi fields, caused by external corrosion, have been repaired by cement squeezes and casing replacement. Cathodic protectionof the well casings was initiated to prevent furtherexternal corrosion. This type of protection was selected as the best economic alternative to prevent further failures and the resultant high repair costs. In the South Swan Hills field, five of the nine external failures were attributed to the use of a low-pH gyp starch mud when the wells were being drilled. The other four failures were due to a field-wide corrosive water zone about 1,000 feet above the 8,000-foot producing formation. The CPP surveys and E log I tests were used to determine cathodic protection requirements. Casing potential profile (CPP) surveys showed that the thirty-one wells drilled with low-pH gyp starch mud could be cathodically protected to their average calculated cement top with a IS-ampere current. However, the required level of protection through the corrosive water zone could not be obtained using a 3D-ampere current. A test program has been initiated to determine whether the well casing can be polarized with time to provide protection through the corrosive water zone. Five external well casing failures have occurred in the Nipisi field. The depths of the leaks correlate to a waterbearing carbonate zone 1,000 feet above the average calculated cement top. Cathodic protection requirements were determined using CPP surveys. A IO-ampere current was found to be sufficient to protect the bare well casings through the corrosive water zone. Thirty-six wells which encountered this corrosive water zone were put on cathodic protection. Conclusions Detailed analyses of well completion data and geological studies to locate potential water-bearing formations arc necessary to determine what wells may be subject toexternal casing corrosion. Adequate cathodic protection of casing is best determined using the casing potential profile (CPP) survey in conjunction with a 0.5 ma/ft2 current density. E log I tests and remote off-wellhead potential methods are not always reliable. Downhole casing inspection logs and cprsurveys do not depict all the corroded intervals in thecasing. Cathodic protection to a depth of 8,000 feet may not be possible in the South Swan Hills field.