For matured offshore oil field in Abu Dhabi, lots of wells need to be permanent abandonment. According to the regulations on offshore permanent well abandonment, the wellhead shall be cut from 4 m below the mudline. The purpose of this paper is to introduce an abrasive waterjet technology to solve the challenge so that cut two or three casings around seabed which is cemented together with a high efficiency method.

Traditional operations for cutting multi-casing depend on milling cement and cutting single layer, which are inefficiency and time cost. Abrasive Waterjet technique utilize high water with abrasive material to cut multi-layer casing. This method does not limited to casing layer mount and size. The abrasive waterjet system is mainly composed of speed rotation control system, hydraulic anchoring system, cutting tools, ground supporting equipment, etc. The nozzle parameters were optimized by flow field simulation, and the corresponding tools were designed. The cutting tools includes several spray nozzles and transmit high pressure abrasive to casing surface to cutting. The cutting tools 360 degree rotation powered by speed rotation control system and fixed by hydraulic anchoring system. The abrasive waterjet is placed in cutting location in inner casing.

A two-caisng cementing inner casing 339.7mm and outer casing 850mm together is used as an experimental target. In order to closer simulate the actual working conditions on site, inner casing has 150mm off-center. The abrasive waterjet was inside the target to cutting. The system connects to a pump with 70MPa and 0.9-1.0m3/min flow rate. 40-60 mesh garnet is used as abrasive material. In order to simulate the cutting conditions on offshore site, the cutting tool is always submerged under the water during the test. After about 8 hours of experiments, the target cutting is successfully completed, and the cutting head and protective sleeve, hydraulic rotation speed control device and hydraulic anchoring device work smoothly, which proves that the abrasive jet cutting system has reasonable design and stable performance. The wear resistance of the nozzle during the experiment is analyzed. The nozzle does not change in the outlet diameter after 8 hours wear. Through this experimental research, it is proved that the reasonable selection of nozzle parameters and construction parameters can meet the overall cutting of eccentric multi-layer casing. This parameters can support for on-site construction parameter selection. If a traditional milling method to solve this problem, it usually takes several days. The advantages of this technology in time efficiency are proved. In addition, an offshore abandonment operation was applied by this technique which was conducted on a three-casing cemented casing.

This paper will provide a novel method to solve the multi-casing cutting challenge for offshore well permanent abandonment. The abrasive jet cutting system design, tools and parameters feasibility of field application are verified. Compare to traditional method, it can greatly reduce the operation time and project cost.

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