Operators are looking for well barrier solutions that are cost-effective, robust and in accordance with industry regulatory standards. An industry challenge when plugging and abandoning wells during exploration and at the end of a well’s lifetime is to remove annular fluid, such as oil-based mud, between two casing strings, when creating a well barrier that can be documented and verified.

To cut and pull the wellhead, the annular fluid between the casings must be circulated out, and a cement plug needs to be set as an environmental barrier. An environmental isolation plug functions to isolate the full crosssectional wellbore and to prevent the movement of wellbore fluids to the environment. A well barrier may also function as an environmental isolation plug, according to the NORSOK Standard D-010 on well integrity in drilling and well operations.

Traditionally this challenge was solved by cutting and pulling the casing during plug and abandonment (P&A) operations. For example, in a P&A scenario, a 133⁄8-in. casing can be typically cut and pulled at an appropriate depth inside a 20-in. casing in a well prior to setting a bridge plug. Thereafter, a cement plug can be placed on top of the bridge plug inside the 20-in. casing. In some cases, casing milling is required to set an environmental isolation plug.

Eliminating cutting and pulling

Cutting and pulling the casing can be time-consuming and costly. To address this challenge, Archer has developed the SPARTAN plug-and-perf (PNP) system. This downhole tool-based abandonment system eliminates the need for cutting and pulling a casing when removing fluid from the annulus and replacing it with a cement plug (environmental isolation plug or barrier) in the annulus and wellbore. Operations can then proceed and the wellhead can be removed.

The SPARTAN system consists of the SPARTAN ISO 14310 certified retrievable bridge plug and two separate activated single casing perforation guns. The guns’ design enables the first casing to be perforated in a controlled manner without compromising the integrity of the second casing.

With this system, the objective of circulating out annular fluid and setting the environmental cement plug can be met without cutting and pulling. The result is that a permanent barrier is set between two casings and in the wellbore.

Achieving efficiency

The operational time of establishing an environmental P&A plug by implementing a traditional cut-and-pull method is about 21 hours. A twotrip PNP operation takes about 15.5 hours. A PNP operation using the one-trip SPARTAN PNP system takes 9.75 hours, thus improving efficiency by more than 50%.

The plug does not require any weight/tail pipe to set, making it a choice for deviated wells. The plug is designed to hang perforation guns below, and the 3-in. ball valve enables activation balls for perforation guns to be dropped through the plug to fire the tubing-conveyed perforating (TCP) guns. Key features and capabilities include

  • ISO 14310 certified barrier plug;
  • Rapid set and retrieval;
  • No weight below to set;
  • Multiple sets without tripping;
  • Unrestricted 3-in. fullbore;
  • Ultrashallow, ultradeep or horizontal set 20 m to 6,550 m (65.6 ft to 21,489 ft); and
  • Designed to be run with TCP guns.

PNP method

The PNP method leveraging the SPARTAN plug can be summarized in six steps (Figure 1):

1. Run in hole and perforate the casing at planned depth;

2. Run in hole to just above casing shoe (or at planned depth) to set the plug and perforate;

3. Pump down the string to establish communication between the deep and shallow perforations. When pumping, the fluid will circulate out the lower perforations into the annulus, up and out the upper perforations. Circulate out and displace annular fluid to seawater to prepare for the cement job;

4. Displace the cement slurry through the drillstring and into the deep perforations. Returns are taken through the shallow perforations. By displacing the cement through the SPARTAN plug and into the perforations the B annulus is cemented. The integral ball valve in the SPARTAN plug is closed so the cement will stay in place without U-tubing back up the drillstring;

5. After the cement is placed and the ball valve is closed, the running tool is released from the plug, and a balanced cement plug is pumped through the running tool on top of the SPARTAN plug. This completes the barrier. At this stage, a barrier is complete in both the A and B annuli; and

6. After the cement is set, the barrier can be verified through pressure testing. A feature of the SPARTAN system is that as a result of the shallow perforations, both the A and B annuli can be pressure tested and verified. At this stage, the annular fluid has been removed and the environmental barrier is in place—all without removing the tubular. The wellhead can be removed safely, in accordance with NORSOK Standard D-010.

Case study

The SPARTAN PNP system has been implemented in several North Sea P&A operations. In one P&A operation, the system saved a major European operator nearly $3 million, or 11 days of rig time.

When drilling the well, the 95⁄8-in. casing became stuck and no circulation was possible through the shoe. The decision was made to plug and abandon the wellbore and facilitate a new sidetrack. The operator started recovering 4,577 m (15,016 ft) of casing and achieved limited progress. The operator then spent 21 days cutting and pulling 3,182 m (10,439 ft) of the casing. The openhole fishing operation with a well profile of 73 degrees inclination proved to be challenging. The operator needed to explore an alternative technical solution to complete the P&A operation.

The first step of the operation was to run down and set a SPARTAN plug at 4,852 m (15,918 ft) to provide a path for the cement and to isolate the bottom of the hole. The plug was run through approximately 900 m (2,952 ft) of open hole before it entered the 95⁄8-in. casing fish. The plug was set according to plan and procedure, and the running tool was pulled out of hole. The second step was to run the second plug with TCP guns, then perforate the casing between 4,838 m and 4,840 m (15,872 ft to 15,879 ft) with 12 shots per foot 0,5 holes. Part of the job was to set the SPARTAN at 4,824 m (15,826) and circulate the 95⁄8-in. casing annulus.

The third and last stage was to cement the 95⁄8-in. casing annulus up to 4,342 m (14,245 ft) through the SPARTAN ball valve and into the perforations and up. At the end of the cement displacement, the pressure was held inside the string. At that stage, the SPARTAN ball valve was closed to stop the cement from U-tubing from the annulus. Two cement plugs were set inside the casing up to 4,390 m (14,402 ft) to complete the P&A operation.

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