Abstract
The key word for the production development of offshore methane hydrate reserves is "Low-cost". In this article, we propose the design of a subsea production well that could be deployed at the Nankai Trough, offshore Japan. Instead the traditional drillship, we intend to use a combination of three specialized vessels: (i) top-hole drilling vessel, (ii) well intervention vessel equipped with Coiled Tubing Unit and (iii) subsea equipment support vessel. These three specialized vessels have cheaper daily rates than a single last generation drillship. Keeping the limitations of such vessels in mind, we propose a well with a top-hole composed only by the surface casing. Further, we intend to use the "coiled tubing drilling" technology to drill-in, what shall reduce the operational time. Then a liner could be used as interface with the reservoir. In terms of completion, the methane shall be produced by the annular and a tubing shall be installed to remove the water from the well bore using a pump that is installed outside the well. Such arrangement makes the installation and maintenance of the pump easier because the equipment can be replaced without the need of a heavy workover.
Summarizing, the use of specialized vessel in distinct phases of the well construction may reduce the cost in 2 ways: cheaper daily rates than a traditional drilling and reduced operational time due to specialization itself (reduced time for rig up/down of equipment for distinct phases). Further, the use of coiled tubing and a simple completion shall reduce the operational time and equipment costs.