Sustained Casing Pressure (SCP) in petroleum wells poses environmental risk and needs to be removed using 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 and used for video-taped displacement experiments with clear synthetic-clay muds and heavy (kill) fluids. The results show that only immiscible hydrophobic kill fluids provide effective displacement. The study demonstrates importance of controlled injection of the kill fluid to set out efficient buoyant settling and prevent initial dispersion. 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.
Summary Annular casing pressure (ACP) is defined as the accumulated pressure on the casing head. If pressure returns after bleed-down, then the casing annulus is said to be showing sustained casing pressure (SCP). SCP is caused by late gas migration in the annular-fluid column above the top of leaking cement and may result in atmospheric emissions or underground blowouts. Removal of SCP is required in places where SCP is regulated, particularly before the well-plugging and abandonment operations. Annular-intervention methods for SCP removal, which are less expensive than the conventional downhole-intervention methods, typically involve injecting heavy fluid into the affected annulus that would displace the annular fluid (AF), 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-ft carbon-steel pilot-well annulus was manufactured and used for displacement experiments with various water-based drilling muds and heavy fluids with different properties. Pressure-change data were 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. The proposed pressure-buildup model suggests an exponential distribution of density of this zone, and shows its high dependency on fluids’ properties and injection rate. The mathematical models were also converted into dimensionless process measures and proposed for use in real-well applications. The study demonstrates the viability and recommends the correct application of the method.
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.
Sustained Casing Pressure (SCP) is a well integrity problem and its removal is required. Techniques that involve replacing the fluid inside the annulus with a heavier fluid (kill fluid, KF) to stop gas migration have so far failed due to issues resulting from fluid incompatibility. This study aims to develop an intervention fluid compatible with water-based annular fluids. Based on the theory of buoyant slippage, brominated organic fluids have been produced and tested to assess compatibility and performance with multiple physical models. Results showed that the KF was able to settle down in water-based fluids, build up and exert pressure at the bottom. Experiments also exposed the formation of a mixture zone just above the building-up KF column. Lower injection rates and/or larger nozzle sizes decrease KF dispersion, prevent mixture zone formation and increase KF recovery. Intervention fluids developed in this study may revive the defunct bleed-and-lube (B&L) technique that would dramatically reduce the cost of SCP removal or may be used in an alternative process of continuous displacement that would significantly reduce the time of well intervention. Presented in the paper is also a road map for testing the SCP removal process that would lead to development of this technology.
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