Well-Abandonment Solutions Use Bismuth and Thermite Effectively and Safely
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Traditional methods of abandoning a well by use of cement and bridge plugs are still common practice among most operators globally. Although resins have been used as an alternative, no major developments have been made in well-abandonment materials for nearly a century. This paper demonstrates a new way to create gas-tight seals during well abandonment, overcoming the limitations of traditional methods and reducing the operator’s liability and potential environmental impact after decommissioning has been completed.
Statement of Theory and Qualification Testing
Bismuth has multiple commercial uses outside of the oil and gas industry. The idea of using bismuth for sealing in downhole environments is not new; the earliest patents were filed in the 1930s. Bismuth possesses many unique qualities, including
- Relatively low melting point (273°C) compared with other metals
- Viscosity very similar to water when in liquid form
- High density, with a specific gravity of 10
- Noncorrosive and unaffected by hydrogen sulfide or carbon dioxide
- Expands approximately 3% upon solidification
- Nontoxic (used in place of lead in some commercial applications for this reason)
- Eutectic metal, converting from liquid to solid state almost instantaneously when it cools below its melting point, bypassing the gel phase
The challenge in creating a seal with bismuth has always been how to deploy it downhole—melting it and then causing it to solidify and expand where the seal is required. Previous attempts were made with electrical heaters. These heaters required large amounts of power (480 V and 11 A to generate 540 kJ of energy) for hours to melt a relatively small amount of bismuth alloy. Voltage drops in the electric line limited the depth at which the tool could be run in the well. After melting, the electrical heaters could not keep the bismuth in the liquid phase long enough for it to reach the sealing area and fill the void it was intended to seal.
To obtain the energy necessary for this application, thermite was considered as a heating element. Thermite is a combination of iron oxide and aluminum powder. When these are combined in the correct chemical composition and activated, a reaction occurs that results in byproducts of aluminum oxide, iron, and large amounts of heat. To activate the chemical reaction, thermite must be heated to a temperature greater than 2000°C; the resulting output is in excess of 10 000 kJ. The temperature at which the thermite burns can reach as high as 2000°C. For the thermite heating element to be used with the bismuth, the burning temperature would have to be controlled and consistent. To accomplish this goal, binding and damping agents were added to the thermite. The binding agents ensure that the chemical composition of iron oxide and aluminum powder remains consistent without separating, resulting in a repeatable reaction. The damping agents control the amount of heat generated and the speed at which the reaction takes place.
After researchers combined bismuth with the modified thermite heaters, it became evident that a 273°C-melting-point material was not ideal for all well environments. To control the solidification process and ensure the bismuth will fill the sealing area, solidify, and expand, the wellbore fluid must be used as a cooling agent to extract the heat from the bismuth. If the wellbore fluid is too cool, the bismuth solidifies too quickly and a seal will not be achieved. To combat this, bismuth alloys were developed. When small amounts of other metals are added to the bismuth, the melting temperature can be altered. Alloys with melting temperatures ranging from 93 to 263°C were created. Through liquid/solid tests and polish tests aimed at analyzing their grain structure, these alloys were found to exhibit consistently the same unique qualities of bismuth that were mentioned previously. Combining these alloys with modified thermite heaters results in gas-tight seals in wellbores with temperatures ranging from 4 to 150°C.
When the theory of putting bismuth and thermite together was first explored, the idea was to create a gas-tight bridge plug in tubings in which conventional well-abandonment materials had not been successful. During well abandonment, stopping the flow of gas as close to the source as possible is important. This is accomplished by either squeezing the producing formation with cement or placing a bridge plug in the tubing directly above the producing interval. With that in mind, and with the development of modified thermite heaters and bismuth alloys, the concept was first tested inside a length of 4½-in. tubing. The result was a seal that was successfully tested and qualified to ISO 14310 V0 at 65°C and 5,000 psi.
Well-Abandonment Applications of Bismuth and Thermite
After qualification was complete, the tool was ready for field deployment. It was first deployed in 12 test wells in Oklahoma. Each of the tools run was the same as the one tested as described previously, but each was run under different well conditions. They were set inside 4½‑in. tubing at depths ranging from 2,000 to 8,000 ft and deviations of up to 50°. After setting, each was pressure-tested to 5,000 psi from surface. After pressure testing, the tubing was retrieved from each well to evaluate the seal.
After successful field trials in Oklahoma, the tools were run in two wells in Alaska. These tools were run under the same conditions as those in the Oklahoma wells.
Three more tools then were run for zonal isolation during well abandonment. The first two were set in the Norwegian sector of the North Sea. In each of these two wells, the producing zone had been isolated with a V0-rated mechanical bridge plug and cement. After placement of the cement, the wells continued to bubble gas up the tubing and build surface pressure up to 1,200 psi. These bismuth tools were set in a 5-in. liner and pressure-tested to 4,000 psi. A 24-hour leak test was conducted on each with no pressure or bubbles seen at surface (Fig. 1).
The third tool was set in the UK sector of the North Sea. On this well, the 5-in. tubing experienced a kink that restricted the inner diameter of the tubing above the setting depth to 3.1 in. The operator had previously set two V0‑rated high-expansion mechanical plugs, but gas continued to leak past to surface. The bismuth tool was set inside the 5-in. tubing, pressure tested to 1,400 psi, and negative-pressure-tested for 1 hour with no bubbles or pressure seen at surface.
With successful tool deployments inside of the tubing, the idea of sealing with bismuth in multiple annuli was the next step. The first bismuth tool to be deployed for sealing not only inside tubing but also in tubing by casing annulus took place in the Norwegian sector of the North Sea. This well, with 4½-in. tubing and 7-in. production casing, had been temporarily abandoned by setting a V0-rated mechanical bridge plug in the tubing. The tubing was perforated above the bridge plug and a cement-balance plug was placed in the tubing and production casing annulus. The cement was underdisplaced, leaving cement in the tubing higher in the well than in the annulus. As with the well abandonments mentioned previously, this well continued to build surface gas pressure in the tubing, as well as in the annulus. The tubing was perforated above the cement in the tubing, and a bismuth tool was run in the well across the perforations. Once the heater was activated, the alloy melted and solidified at the bottom of the tool. Once the remainder of the liquid alloy inside the tubing reached the perforations, it flowed out into the annulus. In the same manner as seen in the tubing, when it reached below the heater and was cooled by the wellbore fluid, it created a base in the annulus as well. The remainder of the liquid alloy pooled in the annulus, solidified, and expanded to create a gas-tight seal in the well. This seal was pressure-tested to 4,000 psi and a negative pressure test was completed for 1 hour with no pressure or bubbles seen at surface.
As well abandonment has received more focus and the need for gas-tight seals grows, the applications for bismuth and thermite to create these seals has grown as well. This tool was recently gas-tested to 500 psi with nitrogen for 12 hours with no bubbles. A repeat test was performed 10 days later with the same results, which led to its first deployment in the Norwegian North Sea. Following the success of this field trial, development for 20×30-in. casings is ongoing. Additionally, developments to seal in three annuli in one single deployment (9⅝×13⅜×20 in.) to be used as an environmental plug are ongoing in many regions throughout the world. Bismuth and thermite also can be used for through-tubing operations. Running them through tubing and setting inside the production casing below the production packer allows for a gas-tight barrier as close to the source as possible in cased-hole completions.
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Well-Abandonment Solutions Use Bismuth and Thermite Effectively and Safely
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