Gas bubbles from dispersal valves 30/20″ casing annulus

In an offshore well, after cementing 20 inch casing, we start observing gas bubbles via 2 out of four dispersal valves on the low pressure wellhead housing(LPWH). We drilled pilot hole with no incidence of shallow gas. Also, we have drilled over 7 producing wells in this field without shallow gas recorded. Currently we're drilling 17½ hole section and still observing gas bubbles flowing through two of the valves on the LWPH on the sea bed. However, we optimistic that the gas bubble/flow we stop with time for their is know reservoir at that depth (between 500 to 1620ft-MD). Is there anybody that have experienced similar challenge in offshore drilling operations and if yes, how did you stop this gas flow or manage this challenge if the flow do not stop as expected?

Regards,
Clinton.
Wellsite Drilling Engineer.

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