Publication Date:
2023-01-27
Description:
Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub‐seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi‐phase flow model, we test plausible gas sources for pockmarks in the south‐eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre‐existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm‐induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm‐induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure‐sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas‐rich pore fluids, wave‐induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear.
Description:
Plain Language Summary:
Thousands of pockmarks, circular depressions in the seafloor, were reported in North Sea, presumably formed in response to wave motions during major storms. It has been hypothesized that these pockmarks formed as pre‐existing shallow free‐gas pockets were mobilized by pressure changes of the waves. However, mechanisms that could have mobilized free‐gas are not yet constrained. Moreover, large scale free‐gas accumulations have not been reported in this region, and therefore, commonly invoked mechanisms like tensile failure and breaching of capillary seals are hard to justify as they rely on the presence of pre‐existing gas pockets. Here, through modeling studies, we tackle the question of the source of the observed free‐gas. Our study consists of two parts: First, assuming that some hitherto unknown shallow free‐gas pocket is indeed present, we test whether storm‐induced pressure changes could breach capillary seals. We find that free‐gas damps pressure changes due to its high compressibility, making the mobilization of pre‐existing gas unlikely. In the second part, we propose an alternative mechanism where free‐gas spontaneously appears due to exsolution from pore‐fluids. We test the feasibility of this mechanism and show how periodic pressure changes can lead to a persistent gas phase, that could explain the elusive gas source linked to these pockmarks.
Description:
Key Points:
Storm‐induced pressure changes can lead to spontaneous appearance of free gas phase near the seafloor.
This process is driven by pressure‐sensitive phase instabilities.
This mechanism could help explain elusive gas sources in recently observed pockmarks in the North Sea.
Description:
Aker BP (AkerBP)
http://dx.doi.org/10.13039/100016998
Description:
Deutsche Forschungsgemeinschaft, DFG
Keywords:
ddc:550
;
pockmarks
;
storm related pockmarks
;
spontaneous free gas
;
gas source
;
modeling
Language:
English
Type:
doc-type:article
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