Experimental Study and Modeling of Cuttings Transport with Aerated Mud in Horizontal Wellbore at Simulated Downhole Conditions
Authors: Zhou L. , Ahmed R.M. , Miska S.Z. , Takach N.E. , Yu M. , Pickell M.B. 
Published: December 2007

Abstract
Experiments were carried out in a unique full-scale flow loop which includes a 73-ft (22.25 m) long annular section of 6-inch (152 mm) casing and 3.5-inch (89 mm) concentric drillpipe at elevated pressures and elevated temperatures (EPET) ranging from 185 to 500 psi (1.28 to 3.45 MPa), and 80 to 175°F (26.8 to 79.44°C) respectively. The gas-liquid ratio of the aerated fluids varied from 0.0 to 0.38. The in-situ cuttings concentration (i.e. volumetric concentration) was determined by using a special designed multiphase measurement system consisting of an air expansion tank, quick-closing valves, cuttings weighing system and two nuclear densitometers. The following test parameters were recorded during the experiments: liquid and gas flow rates, cuttings weight in the annulus, liquid holdup, mixture density and pressure losses.
The results clearly show that in addition to liquid flow rate and gas-liquid ratio (i.e. injection gas volume fraction calculated at test temperature and pressure), temperature essentially affects the cuttings transport efficiency and the associated frictional pressure drop. The volume of cuttings which accumulated in the annulus was very sensitive to the liquid flow rate. Elevated temperature was found to cause a significant increase in the cuttings concentration at given flow conditions. The injection of gas has a positive effect on the cuttings transport at high liquid flow rates (greater than 150 gal/min). However, it was found that at lower liquid flow rates (less than 150 gal/min), increasing gas-liquid ratio (GLR) results in a decrease in cuttings transport efficiency.
A mechanistic model for cuttings transport with aerated fluids under EPET conditions has been developed to predict frictional pressure loss and cuttings concentration in the annulus. The model is based on mass and momentum conservation equations and wall equations. Comparisons between the predictions of the model and experimental results show satisfactory agreement. In summary, this paper presents several important new aspects of cuttings transport that will be very useful for practical underbalanced drilling (UBD) design.

Introduction
The need for technologies to reduce cost and improve recovery from existing hydrocarbon reserves is well known. One of the most effective methods of cost reduction relies on improvements in drilling technologies. Particularly, development of UBD technology is beneficial for drilling partially depleted reservoirs and as well as and re-entry wells. During conventional (overbalanced) drilling, mud filtrate penetrates the near-wellbore formation because of high equivalent circulation density (ECD). This alters near wellbore pore-flow properties. As a result, well productivity decreases significantly. As a result, UBD is often used to minimize problems associated with formation damage, lost circulation and differential sticking. It has great potential to reduce drilling time and cost. High rates of penetration and longer bit life can be obtained using UBD. This technology is also important in offshore, deep water drilling to avoid fracturing of unconsolidated formations.
In field applications, many different techniques are available for achieving underbalanced conditions. These mostly involve circulating low density fluids such as aerated mud. Nonetheless, multiphase flow behavior of aerated muds is complex and it is difficult to predict cuttings transport efficiency of aerated muds. As a result, cuttings accumulate in the borehole when this technique is applied. In-situ concentration of cuttings in the wellbore is not equal to the concentration near the drill bit. Similarly, in-situ gas mass fraction is not the same as the injection gas mass fraction. This will significantly affect flow behavior of the fluid. Using low density fluids alone does not always guarantee underbalanced conditions. Excessive frictional pressure loss due to poor hole cleaning may result in overbalanced conditions even with low density fluids. Therefore, transport mechanisms of cuttings with aerated fluids should be well understood to control ECD and optimize hole cleaning and hydraulics.
Several solids transport models have been proposed in the literature to predict solids transport in pipe flow. However, very few of them are related to aerated muds. Compared to the pipe flow, little work has been done for flow through annuli. To our knowledge, no studies are in the literature concerning cuttings transport using aerated fluids at elevated pressures and elevated temperatures.