Unusual Corrosion of a Drill Pipe in Newly Developed Drilling Mud During Deep Drilling

In Japan, a newly developed drilling mud containing synthetic smectite is being used during deep drilling when the formation temperature exceeds 180°C. However, using this mud during long periods of drilling, localized corrosion at the outer surface of the drill pipe becomes a serious problem. In the present study, a corrosion mechanism was postulated by inspection of corroded pipes at a drilling site and by thermal degradation and corrosion testing in a laboratory. Corrosion was initiated by the combined effects of dissolved oxygen and carbon dioxide generated by the thermal degradation of the mud. Corrosion damage further developed by the establishment of a differential aeration cell caused by buildup of clay minerals at anodic sites. The general nature of drilling muds are that they dissolve carbon dioxide at high concentrations and keep carbon dioxide in them even at high temperatures, such as 70°C-influenced corrosion in the present study. The unique nature of the new mud containing synthetic smectite (i.e., its weak passivating effect) also contributed to this corrosion. A conventional inhibition method, using a water-soluble amine, was not adequate to mitigate this corrosion.