Drilling Fluids Rheological and Volumetric Characterization Under Downhole Conditions

Abstract Accurate estimation of equivalent circulating density (ECD) and circulating bottom hole pressure (CBHP) are essential components of successful managed pressure drilling operations. Normally, calculated frictional pressure losses do not match actual frictional losses recorded during drilling operations. Discrepancies between calculated and measured pressure losses are more severe in operations where synthetic based drilling fluids are used because synthetic based drilling fluid rheologies vary with changes in downhole conditions. In order to deal with differences between actual and theoretical pressure losses, rheological and volumetric characterization of the fluid under downhole conditions was performed. N-paraffin-based drilling fluid (NPDF) was used in this study. Effects of pressure and temperature under HPHT conditions were determined using a mercury-free PVT cell. The volumetric behavior of the NPDF was compared with the behavior of water-based muds and less toxic mineral oil-based muds. An empirical relation for density as a function of pressure and temperature is introduced. In addition, rheological characterization of the NPDF was conducted using an HPHT rotational viscometer. The rheological model that defines the shear stress – shear rate relation of the drilling fluid used in this study at all pressure and temperature conditions is the Yield Power Law model. The effects of downhole conditions on the rheological parameters consistency index (K), flow behavior index (n) and yield stress (τo) are analyzed.