Corrosivity of water-base drilling fluids in deep, hot wells

Abstract

Corrosive behavior of mild steel type 1018 in a typical drilling fluid used in drilling deep and hot wells was studied. The studies were carried out under simulated flow conditions, temperature, and pressure; which drilling fluid and drill pipe are exposed to while drilling such wells using a dynamic flow loop. Corrosion monitoring and measurements were carried out by weight loss and electrochemical techniques. The electrochemical techniques are: Polarization resistance, Potentiodynamic plots and AC impedance test. They were used to characterize the corrosion behavior of the steel in drilling fluid. Scanning electron microscope was also used to study the general morphology changes of steel samples when exposed to drilling fluid under simulated downhole conditions. Results show that corrosion rate increases with increase in temperature and decrease in pH. Corrosion rates are low at mildly alkaline to mildly acidic pH range. The drilling fluid generally attacks the grain boundaries of the steel. Diffusion was found to be the rate limiting step for the corrosion reaction. Once diffusion resistance is overcome by increased flow rate or agitation, the corrosiveness of mud increased tremendously. Diffusion resistance led to a capricious and non-linear kinetic behavior. Experiments conducted at high agitation will give a better understanding of mud corrosion reaction kinetics.