New Developments in Aerated Mud Hydraulics for Horizontal Well Drilling

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 due to 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 mudflows 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 X 4.5, 90' 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 75-360 gpm and air in the range 10-1000 scfm. Measurements of pressure drop and average liquid holdup over the entire annular section were carried out. To our knowledge, no such data has 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. To account for these changes, a pipe flow model for prediction of flow pattern boundaries was modified for flow in annuli. 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, modified for two-phase flow in annuli, considerably under-predict pressure drop occurring during the two-phase flow in annuli.