Maintaining optimum circulation rates is important in aerated mud drilling operations. However, reliable predictions of the optimum rates require accurate modeling of the frictional pressure loss at bottom-hole conditions. This paper presents a mechanistic model for underbalanced drilling with aerated muds. Extensive experiments in a unique field-scale high pressure and high temperature flow loop were performed to verify the predictions of the model. This flow loop has a 150 mm × 89 mm (6" × 3.5") horizontal annular geometry and is 22-m long. In the experiments, cuttings were introduced at a rate of 7.5 kg/min, representing a penetration rate of 15m/hr in the annular test section. The liquid phase flow rates were in the range of 0.30 - 0.57 m3/min, representing superficial liquid velocities in the range of 0.47 - 0.90 m/s. Gas liquid ratio (gas volume fraction under in-situ condition) was varied from 0.0 to 0.38. Test pressures and temperatures ranged from 1.28 to 3.45 MPa, and 27°C to 80°C, respectively. Gas-liquid ratios were chosen to simulate practical gas-liquid ratios under downhole conditions. For all the test runs, pressure drop and cuttings bed height over the entire annular section were measured. Flow patterns were identified by visual observations through a view port. The hydraulic model determines the flow pattern and predicts frictional pressure losses in a horizontal concentric annulus. The influences of gas liquid ratio (GLR) and other flow parameters on the frictional pressure loss are analyzed using this model. Comparisons between the model predictions and experimental measurements show a satisfactory agreement. The present model is useful for the design of underbalanced drilling applications.


Aerated mud drilling has been recognized as having many advantages over conventional mud drilling, such as higher penetration rate, less formation damage, reduced lost circulation, and lower drilling cost. A good prediction of hydraulics of drilling with aerated mud requires knowledge of the flow pattern and determination of the properties of each phase under borehole conditions.

Although several hydraulic studies have been performed to predict frictional pressure loss in conventional drilling, very limited numbers of studies have been conducted in the area of aerated mud applications. To our knowledge, no studies are reported in the SPE literature concerning the hydraulics of aerated fluids at elevated pressures and elevated temperatures.

Extensive aerated mud flow experiments were conducted by Sunthankar1 in a large-scale flow loop that has a 30-m long annular (200 mm × 114 mm) test section. Tests were carried out in a horizontal position with and without drillpipe rotation. Water and an aqueous polymer solution were used as the liquid phase. The effect of drillpipe rotation on the pressure drop for air-water flow was insignificant. However, in the case of air-polymer solution, the pressure drop decreased with drillpipe rotation. A higher pressure drop was observed in the case of air-polymer solution flow as compared to air-water flow. The phenomenon of turbulent drag-reduction, which occurs in a polymer solution flow, may influence air-polymer solution as well. At present, the effect of drag reducers on gas liquid flow is unknown.

Barnea and Doron2 examined the effect of gas injection on the flow of solid-liquid mixtures of coarse particles in horizontal pipes. They suggested that the effect of gas injection is due to the formation of gas pockets that reduce the pressure drop at low slurry flow rates (flow rates that are less than the critical flow rate required to clean the pipe completely).

The motions of sand particles in horizontal pipe flow with aerated fluids were experimentally investigated by Holte3. Experiments were conducted using different water-air-sand mixtures in horizontal pipe. An experimental facility that has a 30-m long test section with 100 mm inside diameter was used for the investigation. Gas and liquid limits for the formation of sand beds were determined.

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