Put a plug in it

ConocoPhillips is working to plug and abandon 130 wells in the southern North Sea, reducing the number of days it takes as it works through the program. But, it wants to improve further, Elaine Maslin reports.

The GMS Endurance jackup. Photo from GMS.

ConocoPhillips has been learning by doing as it gets through some 130 well plugging and abandonment (P&A) operations in the southern North Sea.

The firm has been making incremental improvements as it works through the Viking area and the Lincolnshire Offshore Gas Gathering System (LOGGS) field wells, some dating back to the 1960s.

Over 18 months it has abandoned 24 wells. In that time it has reduced the average number of days per abandonment from up to 32 on the first three wells, to a 22 day average across all 24 wells, by learning from each operation and applying those lessons to the next. But, it wants to make further gains and reduce the average time to under 10 days. “To do this, we will need more than incremental improvements – we will need transformational technologies,” said Andrew Hutchison, well abandonment technology team lead for ConocoPhillips UK, at the ITF Energy Showcase in Aberdeen earlier this year.

Image from ConocoPhillips.

ConocoPhillips has a large portfolio of mostly unmanned satellite platforms in the southern North Sea, off the east coast of England. The company’s P&A campaign in the area started in June-July 2014 in the Viking area, which comprises of late 1960s through to 1980s wells, including production, exploration and appraisal wells from seven normally unmanned platforms plus one manned complex. The first Viking field was discovered in 1968, with first production in 1969. Typically, they are dry gas wells, with 9-5/8in casing with 7in liner, with completion string 5½ x 4½in.

Today, ConocoPhillips’ P&A operations are ongoing in the LOGGS area, with wells dating from the 1980s, and once they are completed, the company will move on to the Caister Murdoch System (CMS) area.

An initial problem on satellite platforms is that deck space for equipment is limited and it is hard to P&A wells without using a jackup drilling rig. This means the focus has to be on how many days it will take to P&A a well, thus reducing the number of the days the drilling rig is needed.

“To complete an abandonment, and prevent fluids coming to surface, there needs to be permanent barriers from “rock to rock” across the full cross-section of the wellbore,” Hutchison says. “Just pumping cement down the tubing isn’t deemed adequate because you cannot guarantee a good cement bond all the way around the tubing, which could create a potential leak path up the ‘A’ annulus.”

The GMS Endurance jackup. Photo from GMS.

ConocoPhillips has been punching the tubing and bull heading the “A” annulus and tubing content into the reservoir, then setting temporary tubing barriers. This allows the Xmas tree to be removed, the blowout preventer (BOP) set and the completion to be pulled from the well. Once the completion is out, a logging string is run inside the casing to analyze the quality and quantity of the “B” annulus cement.

“At the moment, this cannot be done without first recovering the tubing,” he says. If adequate cement is found, a permanent barrier can be set by using a mechanical plug and placing a large cement plug on top of it, adjacent to the reservoir cap rock. But, if the annulus isn’t adequately isolated, i.e. to 100ft in length as per guidelines, a 100ft window has to be section milled to remove 100ft of casing adjacent to the cap rock. This allows poor cement and the inadequate isolation to be repaired by exposing the formation and setting a cement plug from “rock to rock.”

“To mill a 100ft window in 9-5/8in casing you are not going much more than 3-3.5ft/hr if you are lucky,” he adds. “So you are 30 hours on the bottom and that doesn’t include swarf management. Typically, you are looking at 4-5 days to mill each window.” It’s a problem that the industry is seeking a solution for (See story on page 38 about plasma milling).

ConocoPhillips has substantially reduced the amount of time taken to perform these operations. The company has gone from 26-27 days per well on the first, three-well satellite, which was abandoned, to an average of 42 days per well on the second three-well satellite. There was one troublesome outlier that ended up pushing the average up much higher However, the rate then steadily improved to 15 days per well on the last satellite, setting an average of 22 days per well overall, including conductor recovery and pulling the tubing.

But, they want to reduce this time per well even further. “We can continue to apply lessons learned, and we are doing that well, but a transformational technology would radically change the way that wells are abandoned,” Hutchison says.

One transformational technology the company is searching for would allow analysis of the “B” annulus cement, while leaving the tubing and tree in-situ.

“Some companies are looking at a solution to the ‘A’ annulus problem, by cementing the tubing inside the casing without leaving a channel, but we still don’t have a product to see through two tubing strings to see if we have isolation in the ‘B’ annulus,” Hutchison says. “Even if we did, if we logged before the rig arrived and found it to be poor, we would still have to mill the window,” an operation which involves rigging-up into the well with intervention equipment, removing the tree, installing the BOP, etc.

“If we could find a solution that would allow us to keep the tree in place until we have to cut the conductors beneath the mud-line, this would be a game changer for the industry.”