* February WCS quoted at $37.55/bbl under WTI * February synthetic bid/ask is $0.50/$1.10 per bbl over WTI CALGARY, Alberta, Jan 15 (Reuters) - Canadian heavy crude oil prices strengthened on Tuesday on higher demand from Midwest refineries. Western Canada Select heavy blend for February delivery last traded at $37.55 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy Brokers. That compares with a settlement price on Monday of $39.25 per barrel under the benchmark and last week's lows of more than a $40 per barrel discount after Enbridge Inc imposed mid-month apportionment on three of its lines that export Canadian crude to the United States. When pipelines set apportionment, they reduce the actual volumes that shippers can move from amounts nominated. Because less crude can move through the pipeline, backlogs occur. The 796,000 barrel-per-day Line 4, between Edmonton, Alberta, and Superior, Wisconsin, and 450,000 bpd Line 67, between Hardisty, Alberta, and Superior, are apportioned at 10 percent. Line 6A, which can carry 609,000 bpd between Superior and Griffith, Indiana, is apportioned at 16 percent. Prices were also affected by refinery outages. Phillips 66 said on Tuesday that it returned a crude unit to service at the 365,000 barrel per day Wood River, Illinois refinery, though the unit is said to process light oil. As, well Genscape reported that Marathon Oil Corp had restarted an 86,000 bpd crude unit at its 212,000 bpd Catlettsburg, Kentucky, refinery. There were no trades for light synthetic crude from the oil sands for February delivery. Shorcan reported a bid/ask spread for the crude of $0.50/$1.10 per barrel over West Texas Intermediate. February synthetic settled on Monday at $0.65 per barrel over the benchmark.