Experimental study on drag reduction of aqueous foam on heavy oil flow boundary layer in an upward vertical pipe

• Four main flow patterns were observed in upward vertical oil–foam flow.
• A prediction model for pressure gradient of oil–foam flow was established.
• The drag reduction efficiency exceeds 50% at a lower critical foam–oil flow ratio.
• Ranges of foam injection for a stable foam annulus were recommended.

Flow patterns and pressure gradients of viscous oil flowing through a 25 mm ID vertical tempered borosilicate glass test pipe under the action of aqueous foam were investigated using 201 methyl silicone oil and AFS–2 aqueous foam. Measurements were made for flow rates of the oil at 8.33, 11.67, and 15.00 l/min, and the corresponding flow rates of the foam were 0.36–8.41 l/min, 0.58–11.86 l/min, and 0.32–15.44 l/min respectively. A model was established to predict pressure gradients of the oil–foam flow. The results indicate that flow patterns of the oil–foam upflow in vertical pipe mainly include thread–like flow, uniform and wavy annulus–like flow, non–uniform and wavy annulus–like flow and thick, non–uniform and wavy annulus–like flow. Injection of foam can lead to a stable foam annulus, which can isolate and lubricate the oil and inner pipe wall, and then reduce the flow resistances of heavy oil. All the drag reduction efficiencies exceed 50% when the flow ratios of foam to oil reach the critical values of 0.18, 0.20 and 0.24 respectively. Meanwhile the recommended ranges of foam injection under the experimental condition are 0.18–0.65, 0.20–0.60 and 0.55–0.87 separately. When a stable foam layer formed, the predicted pressure gradients are in good agreement with the measured ones, and the relative errors are basically kept within ±20%.