Lab and field corrosion testing was completed on materials commonly used downhole in Steam Assisted Gravity Drainage (SAGD) to evaluate general corrosion rates, and how they vary with well depth, and operating environment to determine a dominating mechanism.
Lab testing utilized autoclaves to allow for corrosion rate and characterization measurements under controlled conditions. Field coupon testing consisted of installing corrosion coupons at various elevations in the annulus of a producing SAGD well. Corrosion rate analysis consisted of weight loss and various electrochemical measurement techniques, visual inspections, and x-ray diffraction analysis to determine scale compositions. Materials tested included various carbon steels, alone and in combination, as well as steels with hard facing and galvanized finishes.
Field coupons showed corrosion rates decreasing from 0.0178mm/y at the bottom of the well to 0.0145mm/y at higher well elevations. This corresponded to a decrease in iron sulfide scale content from the well bottom upwards. Based on the scales composition, operating conditions, and fluids present, it is likely that the scales were formed through the well-known solid-state reaction between aqueous H2S and the metal. High average corrosion rates of 0.263mm/y were measured in the lab, compared to a low 0.0183mm/y in field studies.
SAGD is a high temperature oil recovery technique used to produce bitumen from relatively deep, usually greater than 200 m1, heavy oil sands deposits. SAGD is a relatively new and advanced form of oil recovery, where a pair of horizontal wells are drilled into the reservoir with a steam injection well drilled only a few meters above a second production well2. Steam is injected into the upper well at temperatures at or above 240°C to heat the bitumen and reduce its viscosity from approximately being close to that of a solid (>1,000,000 cp) down to ~20 cp so that it can drain and be produced economically using one of a variety of artificial lift methods. Figure 1 shows a typical SAGD drilling and production description diagram.