The idea of drilling single diameter wells has been debated for a number of years. And in recent years, the dream of mono-bore wells is taking form thanks to the development of new technologies. The use of expandable tubulars and the drilling with casing system are the most favoured processes.

This paper discusses the possibility of coagulating these two technologies in an attempt to achieve maximum economic and productive efficiency. The system consists of the usual assembly for drilling with casing, which uses the top drive mechanism, and an expansion cone (mandrel) installed above the drill lock assembly. The expansion cone is operated from the surface through the rotary table. The operation involves drilling to the casing depth using the drilling with casing assembly, and then bringing the expanding cone into action. The expansion procedure starts by conditioning the string and pumping in the expanding fluid through the expansion mandrel. It is caught in the area between the mandrel and the drill lock assembly by closing the bypass valves in the latter. Thus the pipe joints above the drill lock assembly are expanded by the normal procedure, till the mandrel reaches the surface. The mandrel is pulled out, and subsequently the drilling with casing BHA is retrieved by a wireline.

This procedure makes use of the economic savings achieved in case of drilling with casing and the productive maximisation which is achieved for expandable tubulars. By using the expandable tubulars at shallow depths, more casing strings can be accommodated resulting in production of deep formation(s). This procedure also involves a reduction in time related to tripping operations which was earlier associated with the drill string assembly.

More importantly this new technology finds it major application in extended reach deep water drilling, where ultraslim wells need to be drilled to reach the formation. This technology requires a class three or four drilling vessel instead of a conventional class five vessel, thus reducing the drilling costs substantially. These two technologies are currently being used in independent fields and there is no published evidence about the combination of these two operations. Now as this method is researched further, new and more improved versions of the same can be developed.

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