Oil wells cause earthquakes.

Oil wells cause earthquakes

In 1958, a geologist calculated that injecting fluid into the ground increases the chance of earthquakes. Thirty-one years later, another geologist has shown the reverse: pumping gas or oil out of the ground can also trigger earthquakes.

Pumping out underground crude contracts the rock in oil reservoirs and sets up large pressure changes over short distances, Paul Segall of the U.S. Geological Survey in Menlo Park, Calif., calculates in the October GEOLOGY. Vertical contraction makes the ground above the reservoir sink, while horizontal stresses pull surrounding rock inward. If the pull becomes strong enough to shear the rock, an earthquake results.

Although geologists have reported mild, shallow earthquakes near gas and oil fields since the 1920s and have long suspected the wells as the cause, Segall's mathematical analysis shows specifically how and where the ground slips, says geologist C. Barry Raleigh of the University of Hawaii in honolulu. Segall's ground-breaking work, Raleigh adds, represents "a messy problem neatly tied up."

"This is the first time that anyone has shown in any kind of analytical way that withdrawing fluid causes earthquakes," confirms John D. Bredehoeft of the USGS.

Because such quakes seem limited in magnitude, petroleum engineers probably won't change their techniques, Raleigh says. But they may apply Segall's work to squeeze more fuel out of wells. Underground reservoirs often consist of fractured rock surrounded by fluid. Pumping the fluid can collapse the fractures, sealing off the remaining reservoir. Segall's analysis could be used to forestall this collapse, Raleigh says.