The new Oil and Gas Authority and the Oil and Gas UK have set out clear guidelines for the decommissioning of oil and gas infrastructure and wells following the effect of low oil prices on our maturing North Sea reserves. There have been conferences and presentations on the subject of decommissioning but little referencing the regulators requirements to isolate and seal wells in perpetuity.

Many different figures exist offering an estimation of the cost of decommissioning however the estimate for plugging and abandoning wells was recently estimated at 44% of the overall cost by Dr Angela Seeney of the OGA. Topside removal seems to be clearly understood and clients can expect to maintain some control on costs. Subsurface conditions may however be more difficult to estimate. Liabilities for wells remain with the operator indefinitely including the liability for leakage after decommissioning is complete.

Plugging and abandoning wells requires firstly the isolation of distinct permeable zones and secondly the isolation between hydrocarbon bearing zones, distinct pressure regimes and water bearing zones. The Geoscientist will be called upon to pull together the relevant information to inform the operator of the geology, the well structure, stress state and the pressure regimes. To do this he will require an understanding of the virgin conditions and the depleted conditions. He will also be required to establish whether the reservoir is likely to return to the virgin state or remain depleted. The Geoscientists assessment will significantly affect the cost of plugging and abandoning wells but will also considerably impact on the long term liability should a plugged well leak.