Renewed vigor to tap into North America’s shales helped Schlumberger Ltd. to more than double its quarterly earnings, the oilfield services giant said Friday.

North American earnings in 3Q2010 jumped almost 90% from the year-ago period, despite a sharp decline in offshore activity that resulted in the deepwater drilling ban. The offshore work is expected to remain slow, but those losses will be more than made up from margin improvements on U.S. and Canadian land drilling, said CEO Andrew Gould.

However, the CEO warned during a conference call Friday that the increased drilling activity in North America’s liquids-rich shale plays may peak at some point in 2011.

Volatile gas prices are “being dampened by the liquids-rich shales where activity has been migrating over the last few months,” said Gould. “There is a ceiling to that. I’m not quite sure where it is. And the ceiling is governed by the fact that however much liquid you have in it, you still need to get some money for the gas. And that means that…not all the shales that are going to be drilled will be the liquids-rich ones.”

Price “volatility will be dampened, but fundamental economic principles have not gone away, and at some stage activity will moderate,” Gould said. “But please don’t ask me when it is because I don’t know,” he told an analyst.

Asked his view of prices going forward in dry gas shale plays versus liquids plays, Gould said “at some point, sufficient capacity will come on to bring down pricing across the hydrocarbon spectrum. It doesn’t matter whether it’s dry gas or liquids.

“It’s a question of at what point, in my opinion, the capacity in the market overtakes the demand, total demand…I’m going stick my neck out now and say I don’t think that’s going to happen until in the second quarter or third quarter of next year, but there is a risk it will eventually happen.”

Schlumberger, which restructured its onshore drilling business following the purchase of drillbit manufacturer Smith International Inc., has seen a healthy demand for its pressure-pumping services, Gould told analysts.

Schlumberger was able to capitalize on a renewed interest in tapping shale for oil and natural gas liquids, which are more service-intensive than gas shale plays. The oil shale and liquids jobs require the type of equipment the company acquired from Smith, he said.