The Operator of a subsea field in the UK North Sea studied the optimum process for the permanent plugging and abandonment of a number of subsea wells which included a campaign of downhole data gathering and safe suspension of selected wells using a Light Well Intervention vessel (LWI) prior to the arrival of a mobile drilling unit. The criticality of this phase of the operation was to enter already shut-in wells, and to establish access to wells which had not been accessed or worked over for over 20 years. This meant limited data was available for the condition of the casings and completions. In order to safely suspend the wells after the campaign, the wells had to be plugged above the reservoir and the envelope of the well had to be pressure-tested to confirm the integrity of the barriers. Once the well envelope and barriers were successfully tested in accordance to established industry criteria the wells could be safely suspended. It is to be noted that in this phase of the operation the tubing needed to remain in place and was not be retrieved, as the retrieval of the tubing would require a drilling rig. However, the main envelope of the well which had to be pressure-tested is located behind the tubing. In order to assess the condition of the casing behind tubing an Electro-Magnetic (EM)pulselogging tool was run in the well to determine the condition of the casing and to determine the level of the corrosion. This then assists the Operator to design the pressure test values for the operation. The major steps of the operation were as follows:
Multi -Finger Caliper (MFC) and EM pulse logging tool to be run inside the tubing using electric wireline to assess the integrity and corrosion condition of the tubing as well as the casing behind tubing.
Deep-set plug to be set at the bottom of the tubing to isolate the reservoir.
Deep-set plug to be pressure-tested to ensure the plug is set properly.
Tubing to be punched to circulate the tubing and A-annulus fluid.
The envelope of the well i.e. Completion Jewellery – Casing, Packer, Deep-set plug, etc. to be pressure tested. Prior to pressure testing the EM pulse logging data would be utilised to determine the pressure test values.
Successful pressure test would mean successful isolation and suspension of the well, however, in the event of a failed pressure test the root cause of the failure would be investigated using Spectral Noise Logging technique to detect and identify the leak point, e.g., casing leak, packer leak, plug leak etc. This specific logging tool was kept on board as a contingency service in case of a failed pressure test.
This paper describes briefly the physics of the measurements for EM pulse logging as well as Spectral Noise Logging in the context of this campaign followed by case studies which illustrate specific well data.
The paper also includes a description of additional sensors which were utilised in the campaign in combination with EM pulse logging and Spectral Noise Logging (SNL) to explain how these multiple sensors complement each other to assist with corrosion assessment and leak investigation.