Dr. Liane Smith
Wood Group Intetech
Many producing offshore oil and gas fields include a mixture of operating, suspended, and abandoned wells. One continuing challenge for the operators is to distinguish between those wells that have been suspended (i.e. for workovers) or abandoned safely, and those that are not correctly treated.
Suspended wells can encounter a variety of problems, and wells that have been permanently plugged and abandoned (P&A'd) can start to leak. There are numerous possible causes. Over time thermal and pressure cycling can generate micro annuli or cracks in the cement seal or other formations above the reservoir, while leak paths can develop from the inside of the wellbore outwards, or in the plugs themselves if they have not been positioned or tested properly.
Nevertheless, provided there is some structure left at the surface, there is scope for monitoring the condition of an abandoned well – even if it means using a single pressure gauge to ensure a reading of nil above a plug or wellhead. Best practice is to continue to monitor abandoned wells for a number of years, using a well integrity management system.
Well integrity management systems combine well operating and production data within a framework for decision-making, management processes and organizational structure. They also enable operators to maintain a full history of their wells. Analysis of historical data not only provides critical intelligence for managing abandoned wells safely, but helps operators get more from their assets.
Levels of suspension
There are many reasons why a producing well might be shut-in. Often shut-ins are necessary to perform routine maintenance of surface facilities, equipment or pipelines, rather than the well itself. The operator might also be experiencing lower than expected rates of recovery.
Meanwhile, integrity issues can arise from sources such as scaling, corrosion, failed well barrier equipment, or sustained annulus pressure. The latter is the number one killer of wells and can lead to an external leak or, at worst, a blowout. Even if no leakage occurs, when pressure within the well rises above its safe operating envelope, the resultant failure of well barrier equipment can be too risky or costly to repair and therefore the well must be abandoned.
If a well is being shut-in to discontinue flow for a short period, this is achieved by simply closing the surface safety valve (typically, the upper master valve). If the well has to remain suspended for a long period of time, a heavy liquid such as brine is pumped in to prevent fluid flow. This can then be removed later to re-start flow.
Should permanent abandonment be required, the well is plugged by setting mechanical or cement plugs in the wellbore at specific intervals to prevent fluid flow. The P&A process normally requires a workover rig and pumping of cement into the well to restore the natural integrity of the formation penetrated.
Properly plugged wells can save the operator substantial sums through avoidance of lost production. If a well is not properly abandoned, it may provide pathways for brines, hydrocarbons, or other fluids to migrate to surface, leading to the technically demanding and costly task of re-entering the abandoned well to plug a leak.
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|Overview of well types and states for different reservoirs with integrity status color coded. (Image courtesy Wood Group Intetech)|
In regions such as theNorth Sea and Asia/Pacific, the growing number of aging assets has meant operators must adopt more robust approaches to managing well abandonment. Britain's Department for Energy and Climate Change (DECC) estimates that as of 2012, there were 740 suspended wells on the UK continental shelf with around 5,000 wells to be decommissioned.
Oil and Gas UK has guidelines to steer operators on the considerations that need to be taken when suspending operations in a well for a limited time, re-entering a well safely, and abandoning a well. The guidelines provide minimum criteria for ensuring full and adequate isolation of formation fluids both within the wellbore and from surface or seabed. The Well Suspension and Abandonment workgroup of the association's Well Life Cycle Practices Forum has also produced guidelines on qualification of materials for the suspension and abandonment of wells.