The effect of previous sedimentation on the filtration and mudcake properties of Newtonian and non-Newtonian fluids

Abstract The drilling fluids may be confined in the annulus for months until the oil production initiates, forming a settle layer of weighting materials at the bottom of this region. When production starts, the temperature rises in the wellbore and a volumetric expansion of the confined fluid increases the annular pressure, a phenomenon known as Annular Pressure Buildup, APB. Pressure increasing is one of most important problems faced by the oil industry and if is not well controlled may cause problems such as failure and tubing collapse. A methodology to APB mitigation was perceived in oils wells when confined fluid was maintained in contact with rock formation. Therefore, the increase in pressure can lead to formation, or even the cement, fracture and a filtration process starts over the mudcake, reliving the pressure inside the annulus. Laboratory experiments were performed to investigate how the mudcake properties affect this process of pressure relief inside the annulus of oil wells. Filtrations preceded or not by seven days sedimentation were performed with Newtonian and non-Newtonian fluids in a HTHP (high temperature, high pressure) filtration cell. Newtonian fluids were composed by glycerine, deionized water and B-1 barite, while non-Newtonian fluids were composed by a solution of xanthan gum or carboxymethylcellulose and PCS-150 barite. Results indicated that for Newtonian fluids the volume of filtrate is lower when filtration is preceded by sedimentation, different from non-Newtonian fluids that filtration volume increased with previous sedimentation. It was also observed that when sedimentation was performed the mudcake thickness increased for all fluids, independently of the pressure applied and the fluid rheology. Parameters such as permeability and porosity were also obtained. Simultaneously, sedimentation tests in a graduate cylinder allowed was to observe that for Newtonian suspensions the particles had already settled down after 24 h, while for non-Newtonian fluids they remained in suspension even after seven days.