Inspection of the below grade well head equipment has shown corrosion damage to the buried landing base, casing spools and surface casing, especially in water injection and supply wells in onshore fields in Saudi Arabia. This paper describes the problem, examines possible reasons for the occurrence of corrosion and then discusses procedures and standards that have been developed to regularly inspect all wells, perform the required repair work in a safe and cost-effective manner and provide durable protection with field-applied corrosion resistant coating on all buried equipment.
Occurrence of corrosion damage in the buried well head equipment and surface casing immediately below the landing base has been a concern in the onshore fields in Saudi Arabia. Initial random inspections of the below grade wellhead equipment in the mid-eighties showed corrosion damage to the buried landing base, casing spools and surface casing. The damage was occurring in spite of an apparently successful cathodic protection program that has reduced the number of casing leaks due to external corrosion damage.
The possible causes of the corrosion damage are: leakage of water from surface piping and wellhead valves during various operations on water related wells, presence of highly saline and corrosive water close to surface in "subkha" terrain, and, impediments to effective cathodic protection at shallow depths.
In view of the safety and environmental hazards associated with possible shallow leaks from corroded casing or failure of wellhead equipment, a number of steps have been taken to control the damage. These include regular inspection and repairs at regular intervals, protection with field-applied corrosion resistant coatings and a requirement to coat all new wells immediately after the rig release.
Regular inspection and protection of the below grade wellhead equipment has successfully reduced potential hazards associated with casing failures at shallow depths. Repair and renovation costs have been effectively reduced by establishing guidelines for safe acceptable thickness limits and by adopting rigless repair procedures.
Typical landing base and surface casing equipment for onshore wells is depicted in Figure-1. The 13-3/8" casing is either welded or screwed on to the 13-3/8"x13-5/8" landing base. The 18-5/8" conductor pipe is cemented at a distance ranging from a few inches to 2–3 feet below the landing base.
A typical landing base inspection operation involves excavating the cellar to below the landing base to expose three to six feet of the surface casing or until hard cement is encountered below the landing base, which ever is earlier. The exposed section is sand blasted and then inspected for evidence of corrosion. The data from such inspections for the last six years (1991 through 1996) is presented in Table-1, while Figures 2 through 4 illustrate some cases of severe corrosion damage on the landing base and surface casing on oil as well as water wells.