Shell has been forced to burn off "significant" volumes of ethane because it cannot sell it to a firm that has temporarily shut down its plant with flaring issues in Fife.
Residents living near the Mossmorran site thought flaring would be reduced after Exxonmobil closed in August.
However, flares have continued to burn because Shell's only ethane customer is Exxonmobil, which shares the site.
Shell said it was "actively exploring alternative ethane outlets".
Exxonmobil chose to temporarily close its plant to undertake maintenance on its boilers.
Shell's Fife Natural Gas Liquids plant separates natural gas liquids into ethane, propane, butane and natural gasoline for storage and onward distribution.
It sells its ethane to Exxonmobil's neighbouring Fife Ethylene plant, which turns it into ethylene.
Since the Fife Ethylene Plant was temporarily closed down Shell said it "did not have the storage capacity for the significant quantities of ethane produced from North Sea gas".
Exxonmobil's plant at the site will be closed until at least November for work to be carried out to make the plant more "reliable".
A total of £140m of work will also be spent by Exxonmobil improving the plant.
ExxonMobil said it had started recruiting 850 temporary workers to carry out the work over the next 12 months.
The operator said the investment was on top of the £20m it spends annually on maintaining its Mossmorran site.
A Shell Fife Natural Gas Liquids spokesman said: "The (ExxonMobil) Fife Ethylene Plant is currently the primary customer for ethane supplied by the Shell Fife Natural Gas Liquids plant, and processes ethane into ethylene.
"Our ground flares are burning excess ethane as the Fife Ethylene plant is currently not available for receiving the ethane to process it into ethylene.
"We have taken measures within the North Sea (SEGAL) supply system to help to manage the situation and are actively exploring alternative ethane outlets during the temporary shutdown.
"However, the volume taken by the Fife Ethylene plant is significant and any solution is likely to be for some volume rather than the full volume of ethane the Fife Natural Gas Liquids plant produces."
James Glen, chairman of the Mossmorran Action Group, said: "I think it is ironic that Shell is being forced to flare off excess product because of the problems at Exxonmobil.
"Residents had hoped for some respite but they are having to continue to suffer from light and noise impact as a result of Shell's flaring."