LONDON (Reuters) -Shell on Monday flagged impairment charges of up to $4.5 billion for the fourth quarter, mainly related to the Singapore refining and chemicals hub the oil major is looking to sell. Ahead of fourth-quarter results on Feb. 1, the company also said gas trading would be significantly higher than the previous three-month period, while oil trading results were expected to be significantly lower over the same period. Shell’s liquefied natural gas (LNG) production volumes were expected to come in at 6.9 million to 7.3 million metric tons, a slightly higher range from its previous gu…