Isolation Barriers With Coiled Tubing in Plug-and-Abandonment Operations
Authors: Carpenter Chris 
Published: June 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 189956, “Successful Creation of Permanent Reservoir-Isolation Barriers Using Coiled Tubing on the Norwegian Continental Shelf,” by Iain Thomson, Halliburton, prepared for the 2018 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, USA, 27–28 March. The paper has not been peer reviewed.

Plug-and-abandonment (P&A) operations can be expensive, leading to negative net present value. Historically, P&A operations in the North Sea—estimated to hold some 3,000 wells of declining production—were performed with either drilling or workover rigs. In many cases, wells drilled from fixed platforms in the North Sea no longer have operational drilling rigs, requiring recommissioning of existing facilities or mobilization of workover rigs. The complete paper discusses an alternative approach to P&A using coiled-tubing (CT) processes and techniques to create permanent reservoir barriers.

Barrier Requirements

Permanent abandonment of a well can be defined as the activities involved in securing a well that will no longer be used, to ensure containment and no harm to the environment. Standard details of how permanent reservoir barriers are to be created are provided by the Norwegian petroleum industry through NORSOK Standard D-010. Section 9 of this standard details requirements for the creation of permanent reservoir plugs in abandonment activities.

The creation of these reservoir plugs typically is dependent upon the condition of the well, the formation type, the quality of cement behind the casing, and the number of potential inflow sources. Similarly, two cross-sectional barriers must be provided across any inflow source together with a surface well barrier. The permanent plug (typically cement) should have a length of 100 m (50 m in casing with a mechanical plug), be positioned as near the inflow source as possible, extend 50 m above the source or casing shoe, and extend to the full cross section of the well (i.e., rock to rock).

The cement should provide well integrity for the foreseeable future and should be impermeable; materials used should not have shrinking properties. Additionally, the cement must withstand mechanical impact and loads; be resistant to chemicals, such as carbon dioxide, hydrogen sulfide, and hydrocarbons; and be wet, to help ensure bonding to steel casings.

Field P&A

An operator determined that wells in an uneconomic field on the Norwegian Continental Shelf needed to be plugged in preparation for abandonment. The field, located off the west coast of Norway, comprises 22 wells from a 24-slot fixed-jacket production platform. Of these 22 wells, 17 were oil producers (five still producing/12 shut in), one well was an active water-injection well, two were active wastewater- disposal wells, and two wells were previous P&A wells that required removal of the conductor and surface casing. The P&A operation would be split into five phases, the third of which (setting primary reservoir plugs) would be performed with CT.

Well parameters varied, but primary reservoir barriers were generally set in 5½-in. or 65/8-in. completions at depths of between 2400 and 4400 m with inclinations between 85 and 90°.