Current understanding of subsea gas release: A review

Abstract

In this paper we review the current understanding of bubble plumes discharged from subsea sources related to oil and gas production, and show how CFD can be applied for risk assessments. A general introduction to causes and risks is given. This is followed by a discussion of the physics that need to be accounted for before giving a brief review of the different modelling approaches employed today. The empirical and experimental knowledge base is also summarized. An example of how CFD can be applied to study gas releases is given. At the end we outline what is needed to advance current understanding of such releases and model their interaction with the surroundings. The scope of the review is limited to the fate of the gas and the flow induced by the ascending bubble plume in the water column. Atmospheric dispersion of surfacing gas is not considered.

Major CO2 blowouts from offshore wells are strongly attenuated in water deeper than 50 m

“…Another factor favoring dissolution in the water column is the small bubble size predicted to occur for high‐flow‐rate CO 2 blowouts, which creates a high surface area to volume ratio favoring fast dissolution . We can also demonstrate this by looking at Eqn , which we can rearrange to roughly approximate the time it takes for a CO 2 bubble to dissolve where the approximate mass transfer coefficient for CO 2 in water is 1 × 10 −4 m s –1 for bubbles of diameter 0.5 mm as estimated by correlations for contaminated (i.e., dirty) bubbles .…”

“…The primary result that seawater is effective at attenuating offshore (subsea) CO 2 blowouts is consistent with what we know about attenuation of natural gas blowouts and seeps and the relatively larger solubility of CO 2 in seawater relative to CH 4 . Specifically, natural gas associated with oil well blowouts is well known to dissolve in the water column for cases of deep‐water blowouts such as the 2010 Macondo well blowout and from theoretical, modeling, and experimental studies . When one considers that CO 2 solubility in water is approximately 25 times larger than that of CH 4 and this factor carries over to seawater, the strong attenuation of CO 2 blowouts is not surprising.…”

“…23 Sintef researchers have also written a comprehensive summary of the state of understanding of offshore gas release that covers both natural gas and CO 2 systems. 10 The largest contributions to the science and technology of offshore well blowouts and pipeline ruptures come from the experiences with producing oil and natural gas including laboratory experiments of gas-release processes. 24 The vast amount of literature related to offshore oil and gas well blowouts includes the development, application, and comparison of simulation tools and models for oil and natural gas fate and transport in the water column often for deep-water (>150 m (500 ft) depth) environments.…”

“…However, localized leakage from a single well blowout or a few wells at GCS sites can cause health and safety hazards especially for localized high‐flow‐rate incidents (i.e., high CO 2 leakage flux). Note that while the term “blowout” in the oil and gas context is defined as any uncontrolled leakage of fluids, a major blowout is usually understood to imply large flow rates (>10 kg s –1 ) from a localized leakage pathway up a well . For pipeline failure resulting in a large leakage flow rate, the term “rupture” is used with a major rupture having flow rates larger than 10 kg s –1 .…”

“…Researchers at Sintef (Norway) have developed and demonstrated a sophisticated computational fluid dynamics (CFD) simulator of gas blowouts into the water column applicable to major CO 2 blowouts . Sintef researchers have also written a comprehensive summary of the state of understanding of offshore gas release that covers both natural gas and CO 2 systems …”