EFFECTS OF HIGH-SPEED PIPE ROTATION ON PRESSURES IN NARROW ANNULI

Variations in annular geometry, eccentricity, and pipe rotational speed strongly affect pressure loss of a fluid flowing in the narrow annulus of a slimhole well. Due to these factors, accurately calculating and controlling pressures in slimhole wellbores are difficult. Accurate pressure calculations are crucial for safely controlling formation pressures and protecting wellbore integrity. Attempts to model non-Newtonian fluid flow in narrow annuli with high-speed pipe rotation have been hampered by the lack of quality data. The results of numerous annular flow experiments presented herein partially correct this deficit. These results supplement annular pressure data from a 2500-ft slimhole test well and standpipe pressure data from a slimhole exploration well. Sensitive pressure measurements were used to characterize fluid flow in concentric narrow annuli created by a 1.25-in. diameter (D-p) steel shaft inside clear acrylic tubes with 1.375-in. to 1.75-in. inside diameter (D-h) Similar tests were conducted in a fully eccentric annulus formed by the steel shaft inside an acrylic tube with D-h = 1.50 in. Maximum shaft rotational speed was 900 rpm and maximum fluid flow rate was 12 gpm. Test fluids included water, glycerin solutions, viscosified clear brines, and several slimhole drilling muds. Models selected from the public domain were used with varying success to calculate results from the hydraulics tests. Simple models typically used by the drilling industry calculated annular pressure loss for non-rotating cases with reasonable accuracy. However, the simple models seldom calculated absolute effects of pipe rotation even though calculated trends correctly match those in measured data. For turbulent flow, annular pressure loss increased with increasing pipe rotation. For laminar flow annular pressure loss decreased with increasing pipe rotation. In all cases, annular pressure loss increased with increasing mud theology and decreased with increasing eccentricity.