Eliminating annular gas migration

A section of a solid bismuth alloy plug, showing zero porosity, that has been removed from a 20-in. test plug under trial for an AkerBP P&A project.

BiSN is transforming downhole sealing for the oil and gas industry with its innovative Wel-Lok M2M technology. The sealing system utilizes a ground-breaking modified thermite heater, in conjunction with bismuth-based alloys to create downhole sealants that are superior to traditional elastomer seals, resins and cement. The tools can be run downhole on electric line, slickline, coiled tubing or workstring. Once on-depth, the modified thermite chemical reaction heater is activated to heat the alloy until it melts. The liquid alloy flows into the area that it is intended to seal, and solidifies and expands to create an impermeable gas-tight seal.

In contrast to earlier electrical heat-based technology, BiSN’s thermite-powered Wel-lok M2M technology dramatically reduces the heating time required to melt the alloy and create an effective seal—from hours to minutes, reducing downtime and costs, and eliminating the need for special wireline equipment. With tools suitable for completion, intervention and abandonment stages of a well’s lifecycle, the company has developed, tried, tested and proven bismuth alloy as an effective sealing solution for a number of common problems that occur downhole, providing an effective alternative to traditional methods. The company’s completions tool provides operators with what has become known as a “downhole insurance policy.”

GAS MIGRATION

Gas migration is a common issue across the industry. It occurs when gas migrates from production intervals or shallow gas zones and can cause potentially catastrophic problems if left unchecked. Gas migration may occur very early in a well’s life, but it can occur months or even years later.

There are thousands of wells throughout the world with sustained annular pressure, due to gas migration, and all these wells have one thing in common—cement is the primary sealing barrier. When gas migration becomes evident, companies have two options; they can shut it off immediately or they can monitor the pressure, evaluate the risks and continue to operate the well for a limited time while it is still safe to continue production. Determining this time depends on a number of factors, including the region’s operating regulations and/or the well’s location.

In most cases, many of these wells are shut in, as operators determine this the best way to alleviate pressure in a safe, cost-effective manner. If a successful method cannot be achieved, then these wells are abandoned, and the remaining reserves may never be recovered. Others continue to operate, producing hydrocarbons until it is no longer feasible. These wells are finally shut in and abandoned. In either instance, gas migration must be dealt with eventually, and a gas tight solution is required to maintain seal integrity, long after the well has been abandoned. The industry requires new technology to efficiently address these issues.

NEW APPROACH

BiSN, a UK- and Houston-based business, has been developing solutions for various downhole issues, including gas migration, by utilizing bismuth alloys and thermite reaction heaters, with technology addressing problems at completion, intervention and abandonment stages of a well’s lifecycle.

Developing reliable solutions to solve common problems. The company has developed an innovative alternative to address some of the limitations and difficulties in setting cement barriers. Harnessing the energy available from a thermite-powered chemical reaction heater, and delivering an expanding, metal alloy, has created a far-superior solution to the traditional methods of downhole sealing.

Bismuth is very unusual amongst metals, as it expands when it solidifies, like water when it turns to ice, and has a low viscosity (similar to water), and high density (10 times that of water). An idea first explored in the 1930s by another service provider, this alloy had never been successfully implemented as a sealant until BiSN introduced thermite, rather than electrical heat, to the process.

The thermite reaction heater is used to melt the heavy liquid alloy, which then flows, fills and molds into downhole leak paths, eliminating the need to pump it into place. It is able to flow through the smallest gaps, including sand screens, perforation tunnels and corrosion holes, pushing wellbore fluids out of its path as it goes. As the bismuth cools, it expands, creating a gas-tight seal on solidification. Furthermore, the alloy is corrosion-resistant (including to H₂S, CO₂ and acid).

Fig. 1. The Wel-lok M2M TDAP tool was successfully tested in 2014.

The thermite-powered technology rapidly increases heating, reducing the time required to create an effective seal. The process generates over 10,000 kilojoules in a few minutes (depending on the size of tool), dramatically reducing the time required to create an effective seal from hours to just minutes. The heat generated through the chemical reaction is non-hazardous and non-explosive, and does not require any permits or special handling. Removing the need for specialist wireline equipment, the technology also reduces downtime and costs.

TESTING AND TRIALS

The first field trials of a tubing plug were carried out in a test well during 2013, proving the technology concept. The first successful commercial run of a casing leak seal (CLS) was made just 18 months later, after the lessons learned had been developed and implemented. In this case, a cementing port in 7-in. casing was successfully sealed and tested to 7,000 psi. In addition to the successful field trials and commercial deployment, the technology was independently tested and qualified to ISO 14310 V0.

Building on the initial success, the company began analyzing ways to utilize bismuth alloys in newly drilled wells with the first prototype Well-Lok M2M thermally deformable annular packer (TDAP) tool, built and tested in 2014, and then commercially deployed in 2019, Fig. 1. Input was given from the cementing subject matter expert from a major oil and gas producer, to ensure the design was practical and suitable for use in standard operations.

Utilizing bismuth for completions. During the completion phase, it is critical to ensure that the well is ready for production (or injection) after drilling has taken place. It is important to ensure the sealing integrity of the well before the operation begins.

Fig. 2. A shipment of TDAP tools leaving the workshop.

Traditional methods of sealing the annulus with cement and packers is often unreliable, due to elastomers failing or micro-annuli and channeling forming in the cement. This can lead to gas migration and sustained casing pressure that will require remediation before the end of the well’s life.

This issue can be resolved by running the Wel-Lok M2M thermally deformable annular packer, as a preventative tool, as part of the original casing string. It consists of bismuth alloy cast on the outside of a standard joint of casing. It can be used on any size of casing, and the string is run and cemented in place, as normal. The TDAP tool can seal in either cased or open-hole wells, Fig. 2.

After cementing, the TDAP tool is set by running a heater inside the casing to melt the alloy and create a gas-tight seal in the annulus, eliminating the possibility of future gas migration, which may occur, due to cement degradation over time. When utilized in the design of the well, this tool can eliminate future expensive interventions and unreliable cement squeezes or alternative remediation measures that may be required.

Similar to other tools in the company’s service line, the Wel-Lok M2M TDAP has no mechanical or moving parts, is available in a range of sizes to suit API and non-API casings or open hole, creates a gas-tight seal without the use of elastomers, and produces an impermeable, gas-tight seal as the bismuth alloy solidifies.

Fig. 3. A TDAP tool being deployed in the DJ basin, Colorado.

FIELD APPLICATION (DJ BASIN)

Gas migration is a common problem across the global industry, affecting both onshore and offshore operations. Likely causes can range from geology of the well to the well construction techniques applied. TDAP provides what is effectively a future “insurance policy” for a problem that is likely to be encountered at some point in the well’s lifecycle. Bradenhead, a local term for surface casing pressure, is a common problem for wells in Colorado’s DJ basin. This issue is caused by cement channeling in the production by surface casing annulus, and can lead to costly interventions and abandonments over the lifecycle of a well.

One operator in the region decided to address this issue up front during the original well construction to avoid any disruption to the well or damage to the casing during future interventions. They took the decision to implement a proactive solution on their new wells. In this instance, Wel-Lok M2M TDAP was cast onto the outside of a 5½-in. joint of casing and run in the well as part of the production casing string in 10 newly drilled wells, Fig. 3. The joint was run in to a depth above all producing zones in the well, to act as a final barrier in the well, if or when gas migration occurred. To ensure that the bismuth alloy could seal against the open-hole formation when the casing was cemented in place, top of cement was left below the TDAP.

If surface casing pressure is seen at any time throughout the life of the well, a modified thermite heater can be run on wireline, inside the casing, and activated to melt the alloy and create a gas-tight seal in the annulus. Pre-placing the alloy in the annulus allows a seal to be created without the need to perforate the production casing, and the depth of the alloy sleeve can be found by simply running a CCL. Another operator in the DJ basin has deployed the TDAP tool in 40 additional wells and continues to utilize a TDAP in every newly drilled well.

VERSATILE ANNULAR SEALING TECHNOLOGY

BiSN’s engineers developed the unique, core technology to be specifically applied to the completion stage of a well. This is not a one tool-fits-all solution, but a bespoke innovation, adapted for annular sealing. Traditionally, elastomer seals and cement have been used for decades to seal the annulus between casings. Many field engineers will attest that elastomer and cement are known to have a high failure rate, having to be remediated using cement or resin squeezes. These are expensive, difficult and often unreliable workovers.

The Wel-lok M2M TDAP is the new generation of annular seals (or packers), providing an ongoing insurance policy to the operator throughout the lifecycle of the well.

Cement repair tool. The company also has developed an intervention tool to remediate sustained casing pressure in existing wells. The Wel-lok M2M cement repair tool (CRT) has been designed to seal a cemented annulus. The casing is perforated to open up the annulus and break up the cement, which then allows the alloy to seal the annulus through the perforations, eliminating the original leak path. The CRT tool can be run into a well at any stage via electrical line, providing a more cost-effective intervention option than traditional perf and squeeze techniques, and are proven to be more effective. The difference between the CRT device and the TDAP tool is that TDAP is already set in place within the well, reducing the requirement to plan for the intervention stage, as this has already been done at the completion stage.

Operators have reported cost-savings between 50% and 80% over traditional methods when a TDAP tool is run during well construction, depending on the complexity of the well and the effectiveness of the traditional method.

Limitations of traditional products. An alternative application, identified by BiSN, uses the TDAP technology as a replacement to a swell packer, that creates a seal using rubber that swells when exposed to water- or hydrocarbon-based fluids. However, the swelling of the rubber reduces its density and strength, and consequently there is a possibility that the swell packer will fail. Furthermore, if well fluids are not as expected, or change over time, this can result in a failure to, or a reduction in, expansion of the elastomers. This situation can create a leak path, resulting in a loss of isolation of the intended zone leading to surface casing pressure, failure to shut-off water production, or cross-flow compromising well performance.

For shallower applications, the TDAP tool will replace a swell packer and be run into the well, cast onto the outside of the intermediate or production casing. The heater will be deployed and activated immediately after landing and cementing the casing string, melting the bismuth alloy and providing a gas-tight seal in the casing annulus.

For more traditional open-hole zonal isolation applications, often at high deviations, the TDAP tool would be run on the production tubing in tandem with a swell packer, where the swell packer can act as a base to the alloy seal. If significant water production occurs from a specific zone, the sliding sleeve will be closed to prevent the water from entering the production string, as normal.

By this stage, the swell packer elastomer will have expanded, but it may not achieve a seal, preventing isolation of the zone effectively. The modified thermite heater will be deployed in the same operation as the sleeve is closed, melting the TDAP tool and isolating water production in the annulus. This will prevent the water from coming into production from another zone, higher in the well, and entering the production string.

REDUCING P&A COSTS

The Wel-lok M2M TDAP is also enabling operators to plan their abandonment at the design stage of the well. The key to a successful well and rig-less well abandonment is the selection of seals that will secure the well, long after abandonment is complete, reducing corporate liability and preventing any environmental impact.

Currently, the only accepted barrier for plugging and abandoning (P&A) wells is cement plugs. They have been used with varying degrees of success as—due to cement shrinkage on curing—abandonment plugs have been found to leak. Over time, they may deteriorate, particularly in corrosive environments, due to the material from which it is made. Conventional plugs rely on elastomers for sealing and are unlikely to provide gas-tight seals in perpetuity and are not approved as barriers.

Installing a Wel-lok M2M TDAP at the beginning of a well’s life, with careful consideration of the placement of the seal, allows operators the assurance that they have a reliable gas-tight downhole sealing solution in place for the life of well—including P&A. While bismuth hasn’t been approved as an abandonment solution yet, BiSN is working with operators in various regions, to go through the process of meeting the regulatory requirements for bismuth to be used as an alternative to cement plugs.

Fig. 4. A cross-section of 13 ³⁄₈-in. x 20-in. from an STC application, demonstrating zero porosity.

Some operators, which have had historical problems with cement barriers, recognize the value of the gas-tight, lifetime sealing ability of bismuth alloy and consequently use BiSN to seal off their wells and then spot cement plugs on top to meet regulatory requirements. The Wel-lok M2M seal thru casing (STC) tool is particularly beneficial for P&A, as it can seal off the tubing/casing and annulus in a single run with wireline.

Offshore Norway. In 2018, working with Altus Intervention and AkerBP, BiSN deployed the world’s largest bismuth plug via E-line in AkerBP’s Valhall A-30 well in the Norwegian North Sea. The tool was set to evaluate the effectiveness of bismuth as a permanent sealing solution for P&A operations, in lieu of cement. With 3,500 kg of bismuth alloy, a gas-tight surface seal was created inside 18 ⁵⁄₈-in. casing through a section-milled window cut in the 13 ³⁄₈-in. casing, entering previously unchartered territory regarding weight, size and precise setting procedure, Fig. 4.

This plug has been monitored for the past 19 months, and no pressure or bubbles have been seen above the plug. Had a TDAP been installed on the 13 ³⁄₈-in. casing during well construction, the need to section mill, to place the plug in the annulus, would have been eliminated, further reducing the cost of the abandonment operation.

CONCLUSION

Since 2015, BiSN has 130 deployments of its Wel-lok M2M technology, including 50 TDAP tools in newly drilled wells. An additional 50+ tools have been deployed during well abandonment, successfully eliminating annular gas migration. In one U.S onshore well, the Wel-lok M2M TDAP tool has been activated, using a modified thermite heater, after 3,200 psi. of annular pressure was seen in the well. Following the successful melting of the alloy, a 24-hr inflow test was conducted, and the pressure remained at 0 psi with no bubbles or flow observed.

While the TDAP tool is deployed at the completion stage of the well, this tool has benefits to the operator throughout the entire well lifecycle, reducing the need for costly and inevitable interventions, and providing a solution to expensive abandonment.

Wel-lok M2M TDAP is transforming how operators think about the design and construction of their wells and allows businesses to forward-plan in ways they’ve never been able to before. With benefits to the operator’s bottom line and its environmental impact, this tool is set to change the way that the industry thinks and works.