Abstract

Gas hydrates forming during well control events in deep water plugging kill and choke lines is a risk previously documented (Lage et al. 2002). In this paper a deep water well's temperature profile has been calculated to investigate the forming of hydrates in the wellbore casing and the riser annulus. The calculations show that water-based mud (WBM) has a large potential for cooling down the wellbore during circulation. The cold seawater surrounding the long riser act as a heat exchanger that gradually removes energy or heat from the drilling fluid. The relatively low thermal conductivity for cement will also result in the cement acting as an insulating layer against the formation, leaving the casing wall at a relative low temperature. The study shows that the temperature conditions for forming [BR1]hydrates are likely[BR2] present deep down inside the wellbore casing as well as in the wellhead, the BOP and the drilling riser.

The volume reduction when hydrates are forming may mistakenly be interpreted as loss of mud to the formation and not a result of gas influx to the wellbore. This may have safety implications. [BR3]Gas hydrates forming in the casing wellbore may also change the properties of the WBM since gas hydrates are formed using water from the mud.

The calculations also show that gas hydrates may stay stable in large part of the wellbore casing and the riser annulus and may be transported a long distance to the upper part of the riser and without any significant change in volume. If gas hydrates are transported up the riser, the released gas will expand rapidly upon the melting of the hydrates, resulting in a very limited time for the drill crew to perform the required actions.

Based on the findings in the study the authors recommend that the human factors implications of hydrates in drilling and well control should be further investigated in order to improve overall system safety.

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