| Summary: | Master's thesis in Industrial economics Scale build up is one of the leading reasons for production decline in oilfields worldwide. Scale can be formed whenever water is produced; either by direct precipitation from the water that occurs naturally in the reservoir rocks, or as a result of produced water becoming oversaturated with scale components when two incompatible waters are mixed downhole. The two most common scales formed in an oilfield are calcium carbonate and barium sulfate. Scale can be found in the reservoir, along the wellbore and in the surface process system. There are various methods for controlling scale. The methods shall either prevent scale formation or remove scale that has already occurred. Scale inhibition and choosing the correct injection fluid are two methods for preventing scale formation. Removal of scale can be divided into chemical dissolution and mechanical removal. For chemical dissolution, the alternative is to bullhead the well with chemicals to remove the scale. Mechanical removal includes the use of different equipment e.g. brushes, broaches, string shots and milling. This thesis focuses especially on mechanical wireline milling for removal of scale. Due to this, the wireline rig-up equipment for both platforms and riserless well intervention vessels, has been presented. Wireline milling is a mechanical method which uses a special tool string with a milling bit in the end, to remove scale. Four scale milling case histories from the Smørbukk field in the Norwegian Sea has been studied. They were all performed from a riserless well intervention vessel. Different plots with measurements from one of the case histories, taken during the milling operation have been looked into. The plots contain different measurements such as head tension, current and wireline tension and are plotted against time. The objective of the thesis has been trying to locate parts of the execution of scale milling operations that can be improved. Some of the key findings were: - When carrying out a scale ...
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