Nitrified Water as UBD Medium - Perfect Recipe for a Corrosion Nightmare:: Issues and a Solution

Abstract

Kuwait Oil Company embarked on an ambitious project to look for gaseous Hydrocarbons in a hitherto un--proven formation in the North Kuwait Field. As the target formation was extremely depleted and usually was drilled through under--total--loss conditions, these wells were designed to be Under--Balanced--Drilling (UBD) wells.The design conditions for an Under--balance or drawdown of 10% below formation pressure of the target zone called for Nitrified water as the fluid medium. Design simulations showed a normal Nitrogen pumping rate of around 1100 scfm for maintaining the UB conditions. Further testing of the target formation called for a huge drawdown of a maximum of 30%. This led to extreme conditions of pumping Nitrogen at the rate of 2000 scfm. The normal operating conditions with membrane NPU resulted in 5% Oxygen being pumped in with Nitrogen. This resulted in a highly corrosive environment under Bottom--hole conditions, especially when we started producing highly saline formation water. The above conditions led to very severe corrosion of our Drill string and UBD Coring equipment, creating a potential hazard of losing our whole string.Conventional corrosion inhibition completely failed to arrest the corrosion of the string. This led to intensive study and debates within the team to find a solution to the problem and quickly too. A multi--pronged solution comprising of controlling pH, usage of Phospate Ester Inhibitor formulation, filming amine and monitoring through usage of Corrosion coupon rings in the drill string led to a dramatic improvement of the corrosion damage.Corrosion hazards in UBD wells are a reality and a heady--mix of ideal conditions for corrosions could mean disaster. Conventional Corrosion programs usually fall short of dealing with the severe corrosion existing down-hole. Special operations such as Underbalanced Coring could fail dramatically before achieving objectives. A multi--pronged approach needs to be designed and the severity of the problem understood before embarking on such ambitious projects. The authors are going to present a strategy for putting the best foot forward when dealing with such issues. Being forewarned will always mean being forearmed.