Finite service life evaluation method of production casing for sour-gas wells

The material selection and design for oil and gas wells containing hydrogen sulfide and carbon dioxide is very difficult due to the uncertainties such as the changeable production capacity, partial pressure of corrosive medium (like CO₂ and H₂S). For oil and gas wells, there is a high production rate in the initial stage while the production rate is very low in the rest of stage. Meanwhile, some wells may have water outflow in the very early period but they even become flooded in the following process. According to relevant standards, Nickel-based alloys must be used in these conditions. However, using Nickel-based alloys could greatly increase construction cost. In this paper, we propose an evaluation method for those types of wells based on a finite life design theory, which utilizes a high pressure high temperature (HPHT) flow corrosion test. Furthermore, phase state and transformation kinetics are introduced into the evaluation. The simulation covers the formation of water corrosion in three periods, which include the condensate water gas production period, the water-carrying gas production period and the water accumulation period. Corroded casing strength was derived according to API Specification 5CT, and the residual strength including residual tensile strength, residual internal pressure, and the residual collapsing strength of the casing. Finally, residual strength data was used for the finite life theory based on the resistance safety method, and compared with the Offshore Drilling Manual, the finite service life can be predicted.