Fluid injection and seismicity
I don’t use this blog much anymore to link to an article or post I find interesting — I mostly do that with Twitter these days — but I wanted to make sure this one gets wide distribution.
Mark Zoback, an earthquake expert at Stanford University, published a piece in EARTH Magazine yesterday (April 17, 2012) titled Managing the seismic risk posed by wastewater disposal. This article is a breath of fresh air. Just go read it now, it will only take a few minutes.
There has been quite a bit of kvetching the past few months regarding whether or not operations related to natural gas exploration and production, and specifically hydraulic fracturing (referred to as ‘fracking’), have caused earthquakes. There are numerous articles on the topic — many asking the question and others making proclamations.
Zoback’s article is certainly not the end-all-be-all on this issue, but it stands out for being rich in facts and technical details that are skillfully communicated. The bottom line is that these operations can and do induce seismicity. But, it’s not the hydraulic fracturing that’s doing it. Rather, it’s the injection of fluid into the subsurface:
[W]e have known for more than 40 years that earthquakes can be triggered by fluid injection. The first well-studied cases were earthquakes triggered by waste disposal at the Rocky Mountain arsenal near Denver, Colo., in the early 1960s, and by water injection at the Rangely oilfield in western Colorado in the late ‘60s and early ‘70s.
Such quakes occur when increasing pore pressure at depth caused by fluid injection reduces the effective normal stress acting perpendicular to pre-existing faults. The effective normal stress on a fault can be thought of as a force that resists shear movement — much as how putting a weight on a box makes it more difficult to slide along the floor. Increasing pore pressure reduces the effective normal stress, allowing elastic energy already stored in brittle rock formations to be released in earthquakes. These earthquakes would someday have occurred anyway as a result of slowly accumulating forces in the earth resulting from natural geologic processes — injection just speeds up the process.
And it’s not just the injection of fluid into the subsurface through a well that can increase pore pressure enough to induce seismicity. Geologists and engineers have known for decades that the accumulation of large volumes of water at the surface, behind newly constructed dams, can also create earthquakes. (Here are the results of searching ‘induced seismicity reservoir’ on GoogleScholar just to give you a flavor of that literature.)
The important conclusion at this point is that fracking is not triggering seismicity:
The concern about triggered seismicity associated with shale gas development arises after hydraulic fracturing, when wastewater that flows back out of the wells is disposed of at dedicated injection wells.
Emphasis mine. So, some might say: ‘Injection, fracking, whatever … the point is that these activities are causing earthquakes!’. Yes, these activities can and do cause earthquakes. But details matter, and here’s why: If we want to establish/improve regulations for these operations in the name of public safety we need to understand the mechanism. We need to do the basic science to address the problem. (This is an issue in geothermal and CO2 projects as well.) And I don’t care if it’s advocacy for or against natural gas extraction, either loses credibility if specifics about the science are distorted, cherry-picked, or omitted. Details matter.
I realize there’s a bigger-picture debate about whether or not to extract natural gas at all. This is a good debate to have no doubt. But, let’s make sure the current state of knowledge and understanding informs that debate.