Foam Stability - Does Well Inclination Matter?

Abstract To minimize fluid loss and the associated formation damage, underbalanced coiled tubing (CT) is one of the preferred methods to perform cleanout operations and re-establish communication with an open completion interval. Because of their high viscosity and structure, stable foams are suitable cleanout fluid when underbalanced CT operations are applied. However, unstable foams do not possess high viscosity and as a result, they are poor in cleanout operations, especially in inclined wellbores. This study is aimed to investigate the effects of wellbore inclination on the stability of foams. In this study, foam drainage experiments were carried out using a flow loop that has foam drainage measurement section and pipe viscometers. To verify proper foam generation and validate the accuracy of measurements, foam rheology was measured using pipe viscometers. Drainage experiments were performed with aqueous, polymer-based, and oil-based foams in concentric annulus and pipe under pressurized conditions. Tests were also conducted at an inclined orientation to examine the effect of wellbore inclination on the stability of foams. The foam bubble structure was examined and monitored in real-time using a microscopic camera to study bubble coarsening. The foam quality (i.e. gas volume fraction) was varied from 40 to 80%. The drainage rate was slightly higher in the pipe section than in the annulus. More importantly, the drainage rate of foam in an inclined configuration was significantly higher than that observed in a vertical orientation. The inclination exacerbated foam drainage and instability substantially. The mechanisms of foam drainage are different in inclined configuration. In inclined wellbores, drainage occurs not only axially but also laterally. As a result, the drained liquid quickly reaches a wellbore wall before reaching the bottom of the hole. Then, a layer of liquid forms on the low-side of the wellbore. The liquid layer flows downward due to gravity and reaches the bottom of the hole without facing major hydraulic resistance of the foam network. This phenomenon enhances the drainage process considerably. Although foam drainage experiments are reported in published literature, there is limited information on the effects of geometry and inclination on foam drainage and stability. The information provided in this article helps to account for the impact of inclination on foam stability to improve its CT cleanout performance in directional wells.