Abstract
A downhole corrosion and scale monitoring (DCSM) tool was developed and applied in sour oil wells to monitor the corrosion and scale formation under real downhole flow conditions. The cylindrical test coupon was made of T-95 carbon steel, which was identical to the metallurgy of the downhole completion tubing. The corrosion and scale deposited on the surface of the cylindrical test coupon effectively simulated the corrosion and scale deposited on the surface of downhole completion tubing.
The DCSM was installed at a desired depth in the downhole and retrieved after three months. The retrieved coupons were characterized thoroughly to assess the corrosion and scaling mechanisms using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) analyses, X-ray diffraction (XRD) and surface profilometer. The results showed that a thin layer (~300 micron in thickness) of deposit, mainly composed of pyrrhotite, was formed on the surface of the T-95 coupon. However, an overall weight loss (1.3194 g) due to the combined effects of the iron sulphide deposition (weight gain) and abrasion/corrosion of the outer layer from the surface (weight loss) was observed. After removing the scale deposition, both general corrosion and under-deposit pitting corrosion were detected by the surface profile scan on the T-95 coupon, where the under-deposit pitting corrosion was considered to be the major concern.
