Full-Scale Testing of Gravity Displacement with Heavy Fluid in Well Annulus to Stop Gas Migration and Reinstate Operations Safety

摘要

Casing Pressure (CP) and Annular Casing Pressure (ACP) is a common well integrity problem, and if cannot be permenantly bled down, it may lead to failure of casing head or casing shoe causing operational risks and environmental pollution. CP or ACP can be caused by gas migration from the top of leaking cement or shallow gas formations, and its removal is necessary to continue well's operation. The conventional mechanical removal methods require a rig and long lasting field applications, and are considered expensive. Alternative method involves displacing the drilling mud in the well with an immiscible heavier fluid to increase the hydrostatic pressure on top of leaking zone and stop the gas leakage. To date, pilot tests provided a useful insight for the method, though, effectiveness of the method remained unknown for real well applications. For the purpose, a full-size test was conducted in a pressurized 2750- foot well. The operational parameters (i.e. injection rate, duration, heavy fluid volume) were designed based on the learnings from the pilot tests and a numerical fluid transport model that would predict the velocity of heavy fluid column moving downwards in the mud column was developed. As a result, average density in the well could be increased from 8.5 to 9.05 ppg. The analysis of the results shows that high injection rates, especially where pump pulsation is present, may lead to heavy fluid dispersion that, forms two fluids emulsion and stops the displacement process. In addition, gradual bleeding- off of surface pressure invokes more gas release from the cement top that leads to flotation and reversal of heavy gravity settling of droplets. For a successful displacement, injection rate and well-head pressure must be controlled over the whole operation. The paper discusses the full-scale experiment design, operational problems, and provides analysis of the process performance described by a simple model of gravity settling and pump pulsation effect. The presented study contributes to the development of a novel well-intervention technique that is considerably cheaper than mechanical methods.