Abstract
Stress corrosion cracking (SCC) of stainless steels and Ni-base alloys in chloride environments has been widely studied and the results to date suggest that SCC only occurs at potentials above a critical potential. Many researchers have demonstrated that this critical potential is the repassivation potential for localized corrosion of the material of interest. Corrosion resistant alloys (CRAs) have been widely used in the oil and gas production environments where low alloy steels typically have lower chance to withstand the corrosive conditions. It is highly likely that SCC of CRAs only occurs above the repassivation potential of the material in H2S containing environment. This paper summarizes the work conducted to determine the repassivation potential of 2507 super duplex supermartensitic stainless steel (UNS S32750). It was demonstrated that the model predicted repassivation potential data agree well with the experimental values. Crack growth rate (CGR) tests were performed and showed the correlation of SCC occurrence at potential above the repassivation potential.
Introduction
The oil and gas industry is moving into deeper water and deeper reservoirs for drilling and production as a result of dwindling production from easily accessible shallow reservoirs. The new discoveries since mid-80’s and early 90’s in the coast of Brazil, the Gulf of Mexico, West Africa, South China and the North Sea have the water depths greater than 1500 m (4,900 ft).1 Temperature and pressure in these deep reservoirs can dramatically increase with the depth of the well. Pressures and temperatures approaching 160 MPa (23,000 psi) and 300°C, respectively, have been forecasted for depths greater than 10,000 m (32,800 ft). 2-7
The environments in the high pressure and high temperature (HPHT) wells are usually composed of a mixture of H2S and CO2. While a moderately sour gas could have between 20 and 100 ppm of H2S, the CO2 concentration of such system could be as high as 1-3%.8 In addition to H2S, extreme environments usually have concentrations of CO2 (g) greater than 3%.8, 9 Elemental sulfur (S0) could be found if the concentration of H2S is greater than 5-10%.9-12 Finally, traces of Hg and organic acids (mostly acetic acid, HAc) are not uncommon in the gas phase of deep and ultra deep wells.10 The composition of the water will change with the life of the well. Typically, the condensed water in the gas wells contains a very low concentration of chlorides whereas the produced water extracted from the field will have very high concentration of chlorides. The condensed water is generally not buffered (i.e., possibly low pH) while the produced water is often a buffered environment. In the NACE survey,13 a wide spectrum of chloride concentrations in produced waters is reported, ranging from 2,000 to 200,000 ppm.
