Abstract

The world's first offshore methane hydrate production test was carried out in March 2013 in the Eastern Nankai Trough. The dissociation of methane hydrates in sediments was produced by depressurization. In this situation, flow assurance (FA) and prevention of hydrate re-formation in the wellbore is an important issue because water mixed with methane is transported through the well and seabed facilities, and subject to the well bore flowing pressure while being cooled-down by the surrounding seawater (around 4°C near the seabed).

A particular concern results from incomplete downhole gas separation of methane and water: this leads to the methane production line containing a small amount of water, and the water production line containing a small amount of free methane gas. In this water dominant flow line there is then the possibility of flow blockage due to gas hydrate re-formation and accumulation, but knowledge about behavior of hydrates under such water dominant conditions was not well known.

To investigate the potential for hydrate re-formation downstream of the downhole separator (including subsea pipes and equipment), Japan Oil, Gas and Metals National Corporation (JOGMEC) and Oilfield Production Technologies (OPT) developed a unique flow loop maintained at low temperature and high pressure. The flow loop was some 20 m in length. Notably, 6 m of the flow loop was constructed in optically clear cast acrylic, which allowed image capture in horizontal and vertical flow lines, and at flow stagnation points, of

  1. methane/water flows outside the hydrate region

  2. the formation, development and flow characteristics of hydrate slurries

In this paper we describe the flow loop design, show images of flow taken by a high speed camera, and our measurements of temperature and pressure in this water dominant system, and the potential of FA problems caused by hydrate re-association. The results and observations were used in the design of the test plan and test procedures implemented in the East Nankai production testing.

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