Case studies show strong differential activation
Despite the best efforts of equipment manufacturers, service companies, and operators, a certain percentage of seals, connections, fittings and other components of wellheads, downhole tubulars, or other equipment weaken or fail over time. When any of these components fail, the pressure integrity of the downhole barrier systems is compromised.
If the pressure integrity of the well is compromised, the release of pressure through the failed components can lead to inoperable subsurface safety valves, casing pressure, environmental pollution, loss of production and, in extreme cases, blowouts. As such, it is essential to safe and environmentally sound operation of a well that the sources of the pressure leaks be cured.
The most common means of curing sources of pressure leaks has been use of risky and expensive rig operations to cure the leak. To date, a major component of the total cost of curing leaks in wellheads and downhole tubulars and equipment is the cost of mobilizing expensive equipment and numerous personnel to the well location.
Separate but intangible costs of such wellhead and downhole operations are the risks associated with such operations including personnel injury, environmental, wellbore damage, and the risk that the operator may not be able to re-establish production from the well.
Pressure activationTo date, the difficulty with attempting to cure the leak in-situ without requiring rig operations has been to find a sealant mechanism that seals the leak without clogging or damaging critical components in the wellbore or hydraulic system. Most downhole leak sealants have had the potential of sealing, not just the leak, but also other components of the wellbore.
A method of repairing wellhead and downhole leaks in situ - without having to mobilize expensive and risky rig or wireline operations - was the impetus for the development of a pressure-activated sealant by Seal-Tite International of Mandeville, Louisiana.
The sealant is designed to seal leaks in tubulars, wellbores, and in severe environment hydraulic systems. What distinguishes this sealant from other products and processes is that the sealant is pressure activated. The sealant will remain fluid in the wellbore or any hydraulic system until released through a leak site. Only at that point of high differential pressure, through the leak site, will the sealant reaction occur and bridge across the leak.
Blood analogyThe sealant is analogous to blood coagulating at a cut. Blood only seals at the location of the cut. The Seal-Tite sealant only "heals" the point of differential pressure, that is, the leak site. The remainder of the sealant stays fluid. It will not clog or plug the hydraulic system or well components. The sealant can be left in the system or flushed out without plugging the system or having any detrimental effect on any components.
The release of pressure through the leak site causes a chemical reaction between the monomers, short-chain polymers and other components of the formula. As the reaction proceeds, the catalyzed sealant plates out on the edges of the leak site and, simultaneously, links with itself across the leak site to seal the leak.
Flows through filtersThe components of the basic sealant either are in solution or are so small that the sealant can be flowed through a 25-micron filter at low-pressure drop. If the flow of sealant through the filter is increased, resulting in an increase in the pressure drop through the filter, the filter will be sealed by the sealant mechanism within five minutes.
Because each sealant formula is custom blended to the particular conditions of the leaking hydraulic system - whether it is tubing, casing, wellhead or SCSSV - the pressure activated sealing mechanism is effective regardless of the temperature (to 350°F), pressure (to 17,000 psi), or delay in reaching the leak site.
This sealant technology has successfully performed leak-sealant operations in a number of different applications:
- Subsurface safety valves
- Wellhead tubing and casing hanger seals
- Casing and tubing pinholes and connections
- Umbilical lines
- Subsea well control systems.
Case historiesThe product has been used successfully in the Gulf of Mexico, Alaska, North Sea, Malaysia, and Abu Dhabi. The following representative case histories outline the capabilities of the sealant, the procedures used and the results of operations.
The best illustration of the capabilities of the sealant was in sealing a leak from a chemical injection line fitting into the annulus without plugging the actual chemical injection port.
- Injection fitting: The chemical injection fitting on a subsea well was leaking into the annulus. A custom-blended formula of 15 gallons of Seal-Tite sealant was prepared and delivered to the subsea wellhead by remote operated vehicle (ROV). At the wellhead, the ROV injected the sealant into a hot tap of the chemical injection line. The sealant was pumped into the chemical injection line while releasing pressure at 7000 psi through the annulus, but maintaining pressure on the tubing. As such, the differential pressure of 7000 psi was established through the leak site while there was little or no differential pressure through the injection port.
After the leak into the annulus was sealed, the remainder of the sealant was displaced at low-pressure drop through the injection port and into the tubing. When the well was brought on production, the unused sealant was produced without any detrimental effect to downstream production and separation equipment.
By selectively creating high differential pressure through a leak site while maintaining low differential pressure through essential flow paths, Seal-Tite is able to cure isolated leaks without effect on other flow paths.
- Valve leaks: Leaks in subsurface safety valves are experienced in every producing region in which SCSSVs are used as a safeguard against blowout. In the North Sea, a well had a severe leak of 1,000 ml/min in the SCSSV control line, causing the well to shut-in. Using data on temperature, pressure and leak rate, a custom-blended sealant formula was prepared. A technician ran troubleshooting diagnostics at the wellsite.
Diagnostics indicated leaks, in both directions, between the V-packing of the wireline retrievable SCSSV and polished bore nipple. The operating company was unable to retrieve the valve due to ongoing operations on an adjacent well, which prevented access to the well with wireline equipment. Using a custom-designed injection system, the technician pumped sealant into the hydraulic control line of the safety valve until the pressure-activated sealant - after only 10 minutes of pumping - had repaired the leak. The normal valve-operating pressure of 6,000 psi was maintained. After four hours, the technician performed function tests that verified full capability of the valve, and the well was returned to production. Leaks in SCSSVs have been cured in systems operated at 16,000 psi and 310°F.
- Casing leaks: In Alaska, artificial-lift gas was leaking from an inner annulus through a connection leak to the outer annulus. The operator was unable to bleed outer annulus pressure below 800 psi. A custom-blended sealant formula was atomized into the artificial-lift gas and injected into the inner annulus, while bleeding outer annulus to atmosphere. By this procedure, sealant entrained in the artificial-lift gas was carried to the leak site. After sealing the leak, the outer annulus pressure dropped to 14 psi. The sealant allowed a successful casing integrity test to 3,000 psi, and the well was put back on production.
- Tubing leaks: In the Gulf of Mexico, gas was leaking from the tubing through connection leaks to the annulus. The leaks were in joints between the surface and the safety valve. A temporary plug was set above the safety valve and a custom-blended sealant formula was atomized into nitrogen and injected into the tubing. As with the casing leak, sealant in the nitrogen was carried through the leak site. After sealing the leak, the well was put back on production and the annulus pressure stabilized within normal limits.
- Wellhead leaks: Wellheads - In Alaska, an operator was experiencing pack-off leaks in wellheads. High viscosity greases had failed to seal the leaks and the operator did not want to use cements or epoxies that would "set up" throughout the wellhead. Seal-Tite injected the pressure-activated sealant through one test port of the wellhead while releasing pressure through the second test port. Once sealant had filled the void area, the void area was "pressured up" to 80% of the rated wellhead pressure. By this procedure, the sealant was extruded through the pack-off leak sites. The pressure on the void area was then cycled between 0 psi and 80% of the rated wellhead pressure to create a two-way seal through the leak sites.
Backpressure valve - A similar success occurred in tubing and casing hangers by installing a backpressure valve into the
tubing hanger, dumping the sealant into the wellhead and pressuring the sealant from the interior of the wellhead and out through the leaking hangers.
The technician filled the wellhead with custom-formulated sealant, and installed the tree cap. Using a pump connected to the crown valve, the wellhead was pressured to 5,000 psi and held there until the leak was sealed. The technician cycled pressure between zero and 5,000 psi to force additional sealant into the hanger seals, until the bleed-off rate subsided to zero at 5,000 psi differential pressure. After the seal was verified, the backpressure valve was removed, excess sealant was flushed out and the well was put back online.
Leaks in wellhead and wellbore equipment are sources of dangerous casing pressure problems that need to be addressed. It is essential to safe and environmentally sound operation of a well that the sources of the pressure leaks be cured. The use of pressure-activated sealant is a safe, low-risk, economical alternative to use of rigs or wireline operations to repair or replacement of the leaking wellhead, wellbore or hydraulic systems.
Copyright 1999 Oil & Gas Journal. All Rights Reserved.