PSI - Issue 80

G. Mubarak et al. / Procedia Structural Integrity 80 (2026) 157–168 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

162 6

The pH profiles in Figures 6 and 8 result from the combined effects of varying CO₂ solubility and partial pressure along the depth. Throughout the tubing, the dominant flow regime is churn (or plug in horizontal terms), maintaining continuous liquid contact with the internal surfaces. Notably, pH values are lower during the first 30 days of production compared to the last 30 days, due to initially higher system pressures leading to increased CO₂ partial pressure and greater acidification. The protective potential of corrosion product scaling , primarily iron carbonate (FeCO₃), is examined through the scaling temperature distribution shown in Figure 9, which is compared with the actual local temperature profile. In the deepest sections of the well, the local temperature exceeds the scaling temperature, promoting the formation and stability of a protective FeCO₃ layer . These positive scaling conditions are further supported by the high pH values at the same locations, which together render this region the least susceptible to corrosion. Conversely, in the mid-depth region where the pH is lowest, the scaling temperature exceeds the local temperature, hindering scale formation and marking this zone as the most vulnerable to corrosion. Upstream and downstream of this middle section, although consistent scale formation is absent, the relatively higher pH moderates the corrosion risk.

Table 3: Results of Rockwell B hardness testing

Location

Adjacent to fracture

Away from fracture

Location 1 Location 2 Location 3

89 88 86 88

87 88 87 87

Average

Approximate ultimate tensile strength, UTS, ksi (MPa)

86 (593) 75 (517)

84 (579) 75 (517)

Minimum UTS for API 5CT J55, ksi (MPa)

Pitting corrosion rates calculated at the lowest pH points using the iFILMS ® software are summarized in Table 4, all of which fall into the “severe” category as per NACE SP0775 [29]. These rates are higher during the initial production period due to elevated CO₂ partial pressure, lower pH, increased flow velocities and consequently increased wall shear stresses driven by large water production volumes. These factors collectively intensify mass and ion transfer, thereby accelerating localized corrosion. Overall, the results confirm that depth-dependent changes in pH, CO₂ behavior, temperature, and hydrodynamics create a critical mid -depth region with unstable scaling and acidic conditions that align with observed pitting damage and reported tubing failures in similar field operations.