Pilot size process visualization: Gravity fluid displacement method to stop annular gas migration

• We propose a new physical model of a casing annulus for better visualization.
• Immiscible heavy fluids are better than miscible heavy fluids for the purpose.
• Horizontal injection geometry generates impingement and enhances the process.
• We generated an empirical maximum allowable injection rate formula.
• The method has merit with suggested operational parameters.

Irreducible casing pressure, also known as Sustained Casing Pressure (SCP), cannot be bled off as it is caused by late gas migration in the annular fluid column above the top of leaking cement. The leaking cement problem is widely spread as shown in statistics from Gulf of Mexico, Canada, Norway and other places where SCP has been regulated. The regulations require removal of severe SCP to continue well's operation and removal of any SCP prior to well's plugging and abandonment (P&A).Typically, SCP removal requires either downhole intervention or annular intervention methods. The latter method involves displacing the annular fluid above the top of the gas-leaking well cement with a heavy fluid to increase the hydrostatic pressure and stop the gas leak. Past field applications of the method failed – most likely due to incompatibility of the two fluids. In this study, a see-through scaled-down hydraulic analog of the well's annulus was designed based on a population of typical annulus sizes and used for video-taped displacement experiments with clear synthetic-clay muds and heavy (kill) fluids.The results show that miscible combination of the two fluids mix at the contact and show poor displacement. However, immiscible hydrophobic kill fluids settle rapidly in the annular fluid and provide more effective displacement. The study demonstrates importance of controlled injection of the kill fluid by adjusting the rate and nozzle size to set out efficient buoyant settling and prevent initial dispersion. The results also show that horizontal injection is superior to vertical injection as the impingement effect increases the efficiency of the displacement process. A side- (versus top-) injection geometry and the injection rate data are analyzed to develop empirical correlation of maximum injection rate for a given properties of the two fluids.