However, chief executive Bob Keiller said the Scottish oil services giant remained well-placed for growth amid strong activity levels in areas like the North Sea.
Shares in Aberdeen-based Wood Group fell 8% after the company said it expected the engineering division to grow earnings before interest, tax, and amortisation by 10% to 15% for the full year. That compares with a previous estimate of around 15% growth in core earnings for the division, which helps oil and gas firms develop new facilities.
The disclosure of the change took the gloss off the announcement that total group EBITA from continuing operations increased by 19% to $243m (£156m) in the first half, compared with $205m in the same period last year.
The adjustment partly reflects the challenges that Wood Group will face maintaining growth at the division resulting from changes in conditions in the key Canadian market.
After expanding in Canada through acquisition, Wood has been hit by a slowdown in the market for new projects in the country. Mr Keiller said oil and gas firms have been deferring investment decisions amid uncertainty about issues like how to export oil from the country.
He noted continued uncertainty about using rail transport to ship crude in Canada, where a train carrying oil derailed in a disaster that killed 47 people last month.
Mr Keiller said Wood Group does not expect to see the situation in Canada recovering in the short term. But he added: "In the medium term we think it's inevitable that it will."
The engineering division will complete some big projects overseas this year, which will not contribute to earnings in 2014.
Wood Group noted the engineering arm and the division that works on existing assets has been benefiting from strong activity in the shale market in the US.
Some firms have used the controversial hydraulic fracturing - or fracking technique - to release oil and gas from rocks.
Asked if Wood Group would be interested in supporting firms that used fracking in the UK, he said: "It's not for us to decide whether it should or shouldn't happen here.
"I think people have got legitimate concerns that need to be aired and debated but ultimately if it was to go ahead and that was the settled view of society that it should then we would like to think the expertise that we've got from North America, from Australia and elsewhere could be used to service that market."
He said the group made good progress in the first half when Wood cashed in on record levels of spending by oil and gas in the North Sea. Investment in new fields in areas like West of Shetland generated work on subsea facilities for the engineering division. The division that helps firms boost output from existing assets continues to see "robust" activity levels in the North Sea.
Mr Keiller said:"In the medium term we see great opportunities to continue our business in the North Sea."
He said Wood Group continued to eye potential acquisitions, Asked if it might enter the bidding for the Kentz engineering business, which has rejected a £680m takeover approach from AMEC, he said: "It would be inappropriate to make any comment on that."
Shares in Wood Group closed down 72p at 831p. That gave it a market capitalisation of around £3.1bn.
Mr Keiller was chief executive of the Production Services Network business Wood Group bought for around £630m in 2010. He succeeded Allister Langlands as chief executive in November.