An industry first was achieved by successfully dissolving 200 kg of barium sulphate scale from the upper part of the tubing in a well on the Alwyn North field in the North Sea. This well experienced rapid and very severe sulphate scale build-up resulting in the tubing retrievable safety valve being stuck in the open position. A new scale dissolver was used for the first time in this operation and successfully removed enough scale to reinstate the well. A full workover was prevented with a total cost saving of about El million.
This paper describes the well history, the investigation, the scale removal alternatives and the laboratory tests performed on the scale dissolver. It discusses the operation design, the operation itself and the scale dissolving results and concludes with a description of the wireline reinstatement operations. It also highlights the continuous advancement in success and capability of the through tubing workover.
Well N5 on the North Alwyn platform was drilled and completed in 1987. The well is an oil producer, producing from the Brent North West reservoir. During February 1992 seawater breakthrough occurred and the watercut increased from 5% to 65%. A standard scale inhibitor squeeze was performed in the following month and since no production decline was noticed further action was not deemed necessary.
During the preparation for the summer shut-down in the beginning of June 1992, it was found that the TR-SSSV would not close. After several attempts it was decided to set a wireline plug in the wireline insert safety valve nipple. However, it was not possible to set the plug in the nipple. When back on surface, large scale deposits were found inside the plug body. The Xmas tree was also found to be lined with a 1/4" layer of scale. Laboratory analysis of the scale showed it to be of the following composition:
76 % Barium Sulphate 13 % Strontium Sulphate 5.2 % Calcium Sulphate Trace Calcium Carbonate
For removal purposes this is one of the 'worst' scale compositions possible.
Since the upper and lower master valves of the Xmas were passing and a wireline plug could not be set, the well was killed before any further investigation took place. Accurate measurements of the control line volumes (pumped volume and returns) and the indications of the 'flapper positioning tool' run on wireline, confirmed that the flapper and/or the flow tube of the tubing retrievable sub-surface safety valve (TR-SSSV) was stuck in the fully open position.