CALGARY, Alberta (Reuters) - After two bleak years, shares of Canada's oil field service firms are rising again as buoyant natural gas prices get rigs back into the field.

Precision Drilling Trust, Ensign Energy Services Inc, Savanna Energy Services Corp, and their peers are now trading well above their early winter lows, backed by a new enthusiasm for natural gas as futures prices rise to two-year highs on cold winter temperatures and big drops in storage inventories.

"Natural gas prices on (the New York Mercantile Exchange) are now, on average, $10, the highest we've seen in a long time," said Kevin Lo, an analyst with FirstEnergy Capital. "Everything is shaping up for a pretty decent 2008."

Ensign was at C$16.87, racking up a 41-percent gain in less than three months.

"March has been a bit cruel, but that said, these stocks are up 20, 30 percent since bottoming in January," said John Tasdemir, an analyst at Tristone Capital. "The drillers are seeing some light at the end of the tunnel."

Drilling stocks are, by almost any measure, a bet on natural gas prices. Most of Western Canada's gas wells are quick to deplete and producers must keep drilling to find new reserves to replace tired wells. If gas prices are low, the industry steers clear, cutting back on drilling until prices justify the expense. That's been the case for more than a year, with drillers often fielding less than half their available fleet.

But this winter the tide changed. After two years of mild temperatures that boosted gas inventories to record levels, this year has been colder and storage levels have dropped, pushing prices higher. As well, Canadian production has sagged by almost one billion cubic feet a day.

Come the summer, gas could be in tight supply, especially if the weather is hot and power demand for the fuel rises while storage needs to be refilled.The falling supply and potential rise in demand could be a formula for higher prices.