Removal of Sustained Casing Pressure by Gravity Displacement of Annular Fluid

Sustained Casing Pressure (SCP) is the undesirable casing head pressure of a well annulus that rebuilds when bled-down. As the conventional methods for SCP removal using rigs are expensive, there is a need for improvement. Annular intervention for replacing the fluid above the leaking cement with a heavier fluid to stop gas migration is a solution for SCP removal; however, previous attempts failed due to miscibility of injected fluids. Using hydrophobic heavy fluids for the purpose is a newly proposed technique to the technology. Potential of theoretically selected and produced immiscible heavy fluids are investigated in characterized annular fluids. A transparent laboratory scaled-down hydraulic analog of well’s annulus provided visual evidence for displacement geometry and did the first stage testing of heavy fluid injection into clear synthetic-clay muds. A 20-foot physical model then tested the performance of the displacement process. Settling of various heavy fluids with densities from 11 to 23 ppg in drilling fluids with densities from 9 to 13 ppg provided quantitative bottom pressure data. Finally, a full-size test in 2750-foot well examined the viability of the technology. Visualization experiments proved that the counter-current flow in annulus leads to up-lifting of heavy fluid droplets and must be minimized for a desirable displacement process. Selection of injection geometry and rate are also essential to maintain a controlled transport of heavy fluid downwards. Pilot experiments developed mathematical correlations relating the process performance to fluid properties and rate. Full-size test shows that hydrophobic heavy fluids are able to slip in long columns; however, bridge-over of buoyant settling may occur due to high injection rates and/or flotation effect of migrating gas that was entrapped in annular fluid. The findings in this research present solid support to the viability of immiscible gravity displacement of annular fluid for remediating a well annulus affected with SCP. For given fluid properties and in confined annular space, injection rate is the key to a successful displacement. Finally, the research proved that the duration of a complete displacement process and required heavy fluid volume are inversely correlated. For any operation design; time and killing material restrictions must be considered

Design and field-scale demonstration of the buoyant-kill process for restoring integrity of wells with sustained casing pressure

Sustained Casing Pressure (SCP) is the persistent casinghead pressure resulting from the well’s annular integrity loss and migration of gas above the leaking cement sheath. Downhole—intervention methods for SCP removal employ workover rigs to access and plug off the leaking annulus at depth from within the well. The inexpensive rig-less method of Buoyant Kill involves surface injection of immiscible heavy fluid at the casinghead into the well’s annulus to replace the annular fluid column above the gas-leaking cement and stop gas migration. Laboratory and pilot-size testing have shown feasibility of the method. Presented here, is a field-scale demonstration experiment conducted in a pressurized 2750-foot well with water displaced by an immiscible brominated organic fluid over 20-h long operation. Discussed is experimental design supported by fluid selection study, benchtop and pilot tests, and fluid transport model. Also analyzed is the process performance and operational problems. The results confirm feasibility of the Buoyant Kill technique for SCP removal. The overall change of the wellhead and downhole pressures follows the expected patterns indicating partial removal of the simulated SCP. The removal is incomplete due to pumping pressure pulses and wellhead pressure variations resulting in some emulsification of the two fluids and partial loss of the injection fluid into the overflow. The results show that wellhead pressure control and injection pump operation are critical for successful SCP removal using the Buoyant Kill technique

Pilot-Scale Experimental Study and Mathematical Modeling of Buoyant Settling of Immiscible Heavy Fluid in Mud To Stop Annular-Gas Migration Above Leaking Cement

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.</jats:p

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

Abstract 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.</jats:p

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

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\u27s operation and removal of any SCP prior to well\u27s 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\u27s 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

Laboratory visualization of gravity fluid displacement in well annulus affected by sustained casing pressure

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\u27s 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

Laboratory Visualization of Gravity Fluid Displacement in Well Annulus Affected by Sustained Casing Pressure

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.</jats:p

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

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.</jats:p

DataSheet1_Design and field-scale demonstration of the buoyant-kill process for restoring integrity of wells with sustained casing pressure.pdf

Sustained Casing Pressure (SCP) is the persistent casinghead pressure resulting from the well’s annular integrity loss and migration of gas above the leaking cement sheath. Downhole—intervention methods for SCP removal employ workover rigs to access and plug off the leaking annulus at depth from within the well. The inexpensive rig-less method of Buoyant Kill involves surface injection of immiscible heavy fluid at the casinghead into the well’s annulus to replace the annular fluid column above the gas-leaking cement and stop gas migration. Laboratory and pilot-size testing have shown feasibility of the method. Presented here, is a field-scale demonstration experiment conducted in a pressurized 2750-foot well with water displaced by an immiscible brominated organic fluid over 20-h long operation. Discussed is experimental design supported by fluid selection study, benchtop and pilot tests, and fluid transport model. Also analyzed is the process performance and operational problems. The results confirm feasibility of the Buoyant Kill technique for SCP removal. The overall change of the wellhead and downhole pressures follows the expected patterns indicating partial removal of the simulated SCP. The removal is incomplete due to pumping pressure pulses and wellhead pressure variations resulting in some emulsification of the two fluids and partial loss of the injection fluid into the overflow. The results show that wellhead pressure control and injection pump operation are critical for successful SCP removal using the Buoyant Kill technique.</p