Liberum Capital is advising investors to 'sell' Wood Group amid concerns about falling oil prices and demand next year.Wood on Thursday forecast an annual performance in line with hopes and ahead of 2013 thanks to growth in its PSN production services arm and US shale gas.Liberum said it wanted indications from the group's management about how falls in the price of West Texas Intermediate crude oil below $90 a barrel would affect the business.The broker has also voiced concerns about how a potential slowdown in oil industry demand for services would affect suppliers like Wood."We remain sellers, worried about the environment in 2015," Liberum said in a note.Shares in Wood rose 27.5p or 4.05% to 706p in mid-morning trading in London.