Mohr Circles and Human Induced Earthquakes
Years before fracking became a controversial extraction process; again, why are we letting oil companies take their waste water and inject it into the ground? I took a rock mechanics class that introduced me to hydraulically induced earthquakes, usually as a result of very large man-made reservoirs. Whenever I hear of fracking companies shrugging their shoulders over the increase in earthquakes near their operations, I see a Mohr Circle chart in my mind:
As pore pressure increases, the effective pressure decreases and the circle shifts to the left and closer to the failure envelope.
The graph of the circle represents the shear stress, tau on the vertical axis and stress, sigma, on the horizontal axis. The circle diameter is the difference between the normal and perpendicular effective pressures at the location you’re interested in. The line (defined by the friction angle of the material with a cohesion component to place it above zero) represents the failure envelope of the rock/earth site specific environment, the system will rupture if the circle touches or crosses the line.
Again this is not just a puddle or hose, this is an immense amount of water changing the pressure underground. For reservoirs, the ground water level rises when the reservoir fills and areas that were above the water table now are under and experiencing pore pressures. The normal and perpendicular stress is reduced by the pore pressure shifting the circle closer to the failure line and a potential earthquake.
And then there are fracking operations, Oklahoma and Texas are known for their earthquakes, right? No. But now we have injection wells trying to increase pore pressure in the rock to open up pathways for oil to be extracted, and more significantly waste water disposal in other injection wells. Clean or dirty water, if you are increasing pore pressures in geology with faults or areas where the Mohr circle is close to the failure envelope, then you have the very real potential to induce an earthquake.
The more technical aspects of the Mohr Coulomb Theory can be researched on your own, this was just a quick explanation of what is going through my mind when people are surprised that earthquakes seem to be increasing near deep injection wells.
ChaSch 