Plugging wells

person by Finn Harald Sandberg and Kristin Øye Gjerde, Norwegian Petroleum Museum
Operators have a duty to ensure that all the wells are securely sealed when a field ceases production or when platforms and any wells tied back to them are removed.
— "Greater Ekofisk Well Plug & Abandonment Scope" from a ConocoPhillips presentation, 2016.
© Norsk Oljemuseum

Known as plug and abandon (P&A), this procedure aims to prevent hydrocarbons leaking to the surface or fluids migrating between different formations to pollute them.

The process involves the installation of several barriers down inside the well, with “in perpetuity” as the time frame for their duration.[REMOVE]Fotnote: Simen André Aarskog, A Method for Probabilistic Time Estimation of Plug and Abandonment of the Wells on the Brage Field, MSc thesis.

This can be accomplished using a number of methods. The relevant Norwegian standard describes six possibilities, depending on conditions in and around the reservoir.[REMOVE]Fotnote: Standards Norway, Norsok standard D-010 – well integrity in drilling and well operations, revision 4, 2013.

A total of 660 production wells have been drilled on the eight fields making up the Greater Ekofisk Area – Albuskjell, Cod, Edda, Ekofisk, Eldfisk, Embla, Tor and West Ekofisk.[REMOVE]Fotnote: Norwegian Petroleum Directorate, Fact Pages, 1 January 2019.

ConocoPhillips has already implemented the necessary safeguarding of more than 400 of these. That includes about 60 related to the four fields which have been shut down with their platforms removed.[REMOVE]Fotnote: Phillips Petroleum, Epoke no 5, April 1999: 3.

The Norsok standard D-010 specifies plugging at three locations:

  • down in the actual well (blue) – primary barrier
  • the area between the two lowest casings (red) – secondary barrier
  • right up at the seabed – barrier between well and seawater.

Up to five plugs are used in the Ekofisk wells. According to the standard, primary and secondary barriers must be set against any hydrocarbon-bearing formations, plus a surface plug.

On Ekofisk 2/4 A, for example, the five seal used included two against the reservoir, two against the formation higher up and one at the surface.

Each plug must be 55 metres (165 feet) long and composed of special plugging cement. It must extend to the sides of the formation – in others words, from inside the casing to the well walls.

Procedure

The well is logged to obtain data about its condition before the plugging operation begins. Based on the results, decisions are taken about the most suitable method.

Milling involves removing the metal casing – a time-consuming operation. Where possible, other and quicker methods are preferred.

If the log shows that the borehole wall butts against the casing, the desirable approach will be to use the formation itself as a barrier.

The casing is then perforated at the top and bottom of the section where the plug is to stand, followed by installing a packer between the perforated areas.

Throughflow in the annular space between casing and borehole wall is then tested. If no leaks are found, the formation can be used as the barrier and a cement plug set inside the casing.

Some leakage is normally experienced, and a perforation, wash and cement (PWC) job will then be done. This involves cleaning out the annular space before a cement plug is anchored in the well wall.

Optimisation

Valuable time can be saved by carrying out several assignments simultaneously each time equipment is to be run in the well – which can take eight to 18 hours.

Confirmation must be obtained that the cement plug is not leaking. This gets tested by tagging (striking) the top of the plug and pressure-testing the well.

Today’s standard can be challenged here, avoiding a separate tagging run by implementing compensatory measures. These include maintaining an overview of all material quantities used for the plug and taking samples of the cement to document its strength. Systematising experience and assessing what is necessary has cut the number of runs per well on Ekofisk, thereby reducing rig time and costs.[REMOVE]Fotnote: Pioner, no 2, 2016.

In 2016, each well was plugged in less than half the anticipated time – around 20-25 days rather than 58 – without any decrease in quality.

Published 29. July 2019   •   Updated 22. October 2019
© Norsk Oljemuseum
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