When forcing fluids makes quakes.

When forcing fluids makes quakes

Since the 1940s, petroleum companies have routinely enhanced oil recovery by injecting fluids into rocks surrounding reservoirs. This so-called hydraulic fracturing--which cracks the rocks and creates pathways for oil flow--is used in 27,000 wells every year.

Sometimes the injection or withdrawal of fluids causes earthquakes. And because there are detailed public records of fluid pressures in most wells, some scientists believe that oil and gas fields are ideal places to learn about the crustal stresses that induce seismicity. Wayne Pennington and Scott Davis, both at the University of Texas at Austin, used these field records to reconstruct histories of fluid pressures in a number of Texas fields, some of which had experienced quakes. They found that the pressures that triggered earthquakes were not what conventional thinking predicts.

According to Pennington, traditional models say that earthquakes are produced because injected fluids raise fluid pressure and weaken faults, which then slip. But those models predict that hundreds of Texas fields are seismic when in reality only about a dozen have earthquakes. "What we're beginning to conclude is that high fluid pressure in areas where there are weak stresses probably leads to fault creep [aseismic smooth sliding of a fault] and not earthquakes,' he says. The researchers believe that earthquakes result instead from very specific patterns of fluid injection and occur in regions of low fluid pressure that are suddenly overwhelmed with high stresses that migrate from nearby fluid injection spots. Pennington says the traditional theory is still valid for regions of high stress, but thinks their model is better for low-stress regions.