There is a very interesting article in NYtimes by James Glanz on the nascent geothermal industry in the U.S. and recent efforts to start projects to recover this energy. The article focusses on the efforts of one company AltaRock, who is currently in the process of getting through various permission to start a project north of San Francisco. They have received permission to drill from the Bureau of Land Management but are awaiting permission to fracture the rock.
And that is what the controversy and fears are about, that hydro-fracturing of the subsurface rocks might trigger damaging earthquakes.
This has happened once before near the town of Basel, Switzerland in 2006 and the article has quite a dramatic description of the events:
The opening of each fracture is, literally, a tiny earthquake in which subterranean stresses rip apart a weak vein, crack or fault in the rock. The high-pressure water can be thought of loosely as a lubricant that makes it easier for those forces to slide the earth along the weak points, creating a web or network of fractures.
Mr. Häring planned to use that network as the ultimate teapot, circulating water through the fractures and hoping it emerged as steam. But what surprised him that afternoon was the intensity of the quakes because advocates of the method believe they can pull off a delicate balancing act, tearing the rock without creating larger earthquakes.
Alarmed, Mr. Häring and other company officials decided to release all pressure in the well to try to halt the fracturing. But as they stood a few miles from the drill site, giving the orders by speakerphone to workers atop the hole, a much bigger jolt shook the room.
“I think that was us,” said one stunned official.
Analysis of seismic data proved him correct. The quake measured 3.4 — modest in some parts of the world. But triggered quakes tend to be shallower than natural ones, and residents generally describe them as a single, explosive bang or jolt — often out of proportion to the magnitude — rather than a rumble.
Triggered quakes are also frequently accompanied by an “air shock,” a loud tearing or roaring noise.
You can just imagine the panic in the nearby community after this sequence of events. The project was shelved.
AltaRock claims it has learned from Basel and has a developed superior method that minimizes risk. They say that the area they have chosen in Lake and Sonoma counties has a history of only small earthquakes and their project will steer clear of large faults.
Its not clear though how they or any other company could stop small tremors from cascading into a larger earthquake. The situation is not at all similar to say capping a well if you want to stop production. I am not a seismologist but I don't think we know enough about stress initiation and propagation through fracture networks to be confident about thresholds. Stop hydro-fracturing if earthquakes reach say 1.5 on the Richter because anything above that is a potential trigger for a larger one. I doubt if we are confident about the science to use arbitrary thresholds to minimize earthquake risks.
Another point I wanted to put out is that such geothermal projects will necessarily be located in areas of high earthquake risk. Geologically active regions will be the ones with enough heat nearer the surface and it makes economic sense to take advantage of this shallow natural heat and avoid drilling too deep. Which mean that the geothermal industry if it has to grow rapidly without getting tangled in litigation and protests must have a transparent and honest dialogue with various stakeholders about the current state of the science of earthquakes and the risks hydro-fracturing entails. For the people living near these project areas, this is not just a Not In My Back Yard because it spoils my view kind of a protest situation.
In this regard AltaRock has not made a good start. At least going by the NYtimes article the company has been less than forthright about the risks involved. For example in the seismic risk report it filed it did not give due importance to the connection between the Basel earthquake and the geothermal drilling despite Swiss seismologists agreeing that the drilling and fracturing of the rock did cause the earthquake. Even senior BLM engineers involved in giving permissions were unaware of the Basel incident and acknowledged that this information should have been disclosed.