Corrosion Mechanism and Material Selection of Casing and Tubing Steels under High-Temperature/High-Pressure CO2-H2S Synergistic Environment

To reveal the corrosion behavior and mechanisms of casing and tubing steels in CO2 and H2S synergistic environments under hightemperature and high-pressure conditions，dynamic corrosion simulation experiments were conducted on five commercial casing and tubing steels (N80-1 steel，L80-1 steel，L80-13Cr steel，P110SS steel，P110 steel) under extreme CO2 and H2S conditions (450 ℃，21.00 MPa)using a high-temperature and high-pressure autoclave.Combined with weight loss measurements and X-ray diffraction (XRD) characterization，the dominant corrosion mechanisms，phase transformation laws of corrosion products，and the influence of temperature alternation on the stability and cracking behavior of corrosion product films were explored.The results showed that: (1) In the high-temperature and high-pressure CO2-H2O system examined in this study，FeO was identified as the primary corrosion product phase of all five casing and tubing steels，the their corrosion resistance ranked in descending order as follows: P110 steel＞L80-13Cr steel＞P110SS steel＞L80-1 steel＞N80-1 steel.The corrosion rate was significantly increased by 77%－371%with increasing water vapor partial pressure，which was attributed to the enhanced ion activity in near-critical water and the reduced stability of corrosion product films.(2) Under the high-temperature and high-pressure CO2/H2S coexisting environment (CO2 partial pressure of 2.43 MPa；H2S partial pressure of 1.21 MPa)，corrosion was dominated by H2S，and Fe7S8 was formed as the main corrosion product.The corrosion resistance of the five casing and tubing steels to CO2-H2S-H2O was positively correlated with the Cr content in the steels，and the order from best to worst was L80-13Cr steel＞P110SS steel＞N80-1 steel＞L80-1 steel＞P110 steel.Notably，temperature cycling (from 450 ℃ to ambient temperature) was prone to triggering a “film rupture-hydrogen permeation-stress concentration” cascade effect，which markedly increased susceptibility to sulfide stress corrosion cracking and doubled the corrosion rate (e.g.，P110 steel: from 8.11 mm/a to 16.77 mm/a).(3)With a high Cr content (12.331%)，L80-13Cr steel effectively optimized the corrosion product film structure and suppressed hydrogen permeation，and exhibited superior corrosion resistance in both extreme environments；thus，it was identified as the preferred material for deep thermal recovery wellbore pipes.This study provided an important basis for the scientific selection and corrosion protection design of casing and tubing materials under extreme working conditions.