Pilot-scale Study of Buoyant Settling of Immiscible Heavy Fluid in Mud – A Promising Technique to Stop Annular Gas Migration above Leaking Cement

Abstract

Abstract Irreducable casing pressure, also known as Sustained Casing Pressure (SCP), cannot be bled off permanently as it is caused by annular gas migration from the leaking cement. SCP poses environmental risk, and regulations demand its removal - particularly prior to well's plugging and abandonment operation. Bleed-and-Lube method, which is cheaper than the conventional mechanical removal methods, involves injecting heavy fluid into the affected annulus that would displace the annular fluid, balance the pressure at the top of cement and stop the gas leakage. Previous studies stated that the use of immiscible combinations of two fluids is more effective for the purpose; however, inattentive applications may result in excessive use of heavy fluid. In this study, a 20-foot carbon-steel pilot well annulus was manufactured and used for displacement experiments with various drilling muds and heavy fluids with different characters. Pressure change data was collected from four different levels of the annulus and volumes of fluids going in and out of the annulus were measured. Experiments indicated the formation of a mixture zone that would build bottoms up and expand during ongoing displacement. Proposed pressure build-up model suggests an exponential distribution of density of this zone, and shows its high depencency on fluids' properties and injection rate. The models were also converted into dimensionless process measures and proposed for the use in real well applications. The study clearly demonstrates the viability and recommends the correct application of the method.