New Developments in Aerated Mud Hydraulics for Horizontal Well Drilling

Abstract

Summary Drilling using aerated muds offers a major advantage in controlling mud effective density, enabling both underbalanced and balanced drilling. However, planning of aerated mud drilling operations is difficult because of complex hydrodynamics of aerated mud flow, resulting in a lack of confidence in predicting and controlling operational variables. It is, therefore, necessary to better understand the hydraulics of aerated mud flows in order to be able to accurately calculate bottomhole pressure and optimal flow rates for drilling. Pressure losses are a strong function of gas-liquid flow patterns. The existing methods of predicting flow patterns are based on an extrapolation of results from pipe flow to flow in annuli. Therefore, to verify the applicability of this existing practice, the present study focuses on hydraulics of aerated fluids through a large-scale annulus. Extensive experiments were performed in a unique field-scale low-pressure flow loop (8×4.5 in., 90 feet long) in horizontal position with and without drillpipe rotation. The liquids used were water and aqueous polymer solution (CMC+XCD+water) at flow rates in the range of 75 to 360 gpm and air in the range of 10 to 1000 scfm. Measurements of pressure drop and average liquid holdup over the entire annular section were carried out. To our knowledge, no such data have been published before. It was observed that intermittent flow was present for most of the cases, which is contradictory to some existing field and simulation practices that assume homogeneous gas-liquid flow. Also, the flow pattern boundaries are shifted as compared to pipe flow, which necessitates the modification of flow transition criteria. Higher frictional pressure drop was observed in case of flow with drillpipe rotation. Also, a higher frictional pressure drop was found in case of air-non-Newtonian fluid flow as compared to air-water flow. The results of this study show that two-phase pipe flow models, when modified and applied to two-phase flow in annuli, considerably underpredict pressure drop occurring during the two-phase flow in annuli.