The rheology and corrosivity of water-base drilling fluid under simulated downhole conditions

The rheological property and corrosion behavior relationship of mild steel type 1018 in a typical drilling fluid used in deep drilling and hot wells was studied. The tests were conducted under conditions that simulate flow, temperature, and pressure encountered during drilling operations. Physical properties that were considered are: shear stress-shear rate relationship, effective and plastic viscosities, yield strength and gel strength. The properties were determined under high temperature and pressure by using a flow loop, the Baroid roller oven and the FANN-70 viscometer. The corrosion measurements were carried out by weight loss and electrochemical techniques. The effective and plastic viscosities of the drilling fluid decrease with increase in temperature and increase in time of exposure to downhole conditions. The corrosion rate of 1018 mild steel increase with decrease in pH of the fluid. The corrosion rates are lower at the mildly alkaline pH and higher in the mildly acidic pH range. The drilling fluid generally attacks the grain boundaries of the steel samples. Diffusion was found to be the rate limiting step for the corrosion reactions.