Over the past 3 years, an operator in Malaysia has developed several large-bore gas wells with horizontal trajectories. The completion team was faced with several challenges in designing the completion of these wells, particularly in the area of deploying a plugging device to test the completion string and to set the production packer. Traditionally, two conventional plugging methods have been used ? slickline-deployed plugs or ball-activated pump-out plugs. Due to the deviated trajectories and associated intervention risks, both methods were deemed unfeasible. A coiled-tubing intervention could have been considered, but this method would have added significant cost to the completions. To actualize these wells, therefore, an operator needed to find an alternative method that would provide a low-risk, cost-effective tubing-testing and packer-setting device.
An option that would eliminate the problems discussed above, a novel "disappearing plug" plug concept, was presented. This plug is unique in that after usage, the plug material dissolves, leaving full tubing drift. The operator chose this option.
The paper discusses the plug design, how it reduced completion costs and risks by eliminating extra trips into the wellbore for running and retrieving plugs or test tools. The discussion also covers the factors an operator considered before choosing the disappearing plug option as well as details of the first installation. This installation provided experience into specifics required for successful usage of the plug as well as best practices.
To date the operator in Malaysia has successfully installed 27 disappearing plugs, and no failures to date have occurred.
The field in Malaysia consists primarily of two shallow gas bearing reservoirs, at approximately 2,650 ft TVD. These reservoirs are laterally extensive, covering an estimated area of 200 square km with estimated gas-in-place (GIP) in excess of 2 Tscf. The reservoirs are made up of a sequence of highly laminated sand and shale deposits with significant sand-size variability and high fines content.
In late 2004, the operator planned to complete the well, which had a measured depth of approximately 6500 ft with a maximum deviation of 80 degrees. The completion design was to incorporate a 7-in. tubing string with a hydraulic-set packer. A primary completion concern was the need to run a plugging device below the packer so that the packer could be set. A wireline-set plug had been considered, but it was deemed as too risky because of the extreme deviation. When considering the risks associated with running a wireline fishing job in a big-bore completion-string design, the operator decided to look for another option. A ball-activated pump out plug was reviewed; however, in a horizontal big-bore completion, placing a ball on seat presents another tricky challenge, especially when there might be debris in the well. Furthermore, in a big-bore completion, the energy released when the plug expends is very high due to the high fluid volume of the completion string. This usually results in a violent hydraulic impulse which can damage sensitive completion jewelry. Ball-activated pump-out plugs will also introduce debris into the well, as when the ball and seat is expended, the items will remain in the well.
Hence, the operator decided to use a different type of plug technology ? an interventionless disappearing plug (Larimore 2000). At that time, the plug had had a 100%-success-rate track record with 25 out of 25 installations having been successful.