How Does Geothermal Drilling Trigger Earthquakes?
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About a million years ago, there was a magmatic intrusion (protovolcano) that didn't make it to the surface. Under the surface is a rock called felsite—you can think of it like granite; it's the heat source for the sandstone.
The new project is going to exploit the felsite directly. But there's no water in the felsite, so they drill, then they pipe water under strong pressure and flow rate, to fracture the rock. They'll be using earthquake-monitoring equipment and will send cameras down the hole to see which direction the fractures were occurring. Then they drill a second hole to intersect the new fracture.
So the potential is to extract much more heat, but you have to create your own fractures and you have to introduce water.
If some of this sounds like an engineer doing geology, maybe it is because it is an engineer doing geology
What are the chances this deep geothermal drilling near The Geysers could set off one of the larger faults, like the San Andreas?
Here's what we know: You can think about The Geysers—the upper three miles (4.8 kilometers) of crust—as a sponge, and the sponge is wet. Now we're taking fluid out of the sponge, and we're taking heat out of the sponge. When you dry out a sponge, it contracts. The Geysers is contracting. From the data, we can see it pulling in, which means that it's changing the stress field around it.
Surrounding the field are some active faults, which have the capacity for some larger earthquakes. So one day one of the tectonic faults is going to move. People are going to ask the question: Did the shrinkage of The Geysers cause the movement of the fault? If that's the case then we have a larger issue.
See here.
Engineers are even worse at political science than geology. No need at all to wonder if geothermal power will be blamed for earthquakes, drought, famine, disease. It will be.
The threat of removal of heat from magmas is like worrying that spooning water into a teacup will dry up the oceans.
On the other hand the draining of aquifers or introduction of water may well be the proverbial straw that did in the camel.
It is not difficult to understand why AltaRock would want to carry out EGS experiments at The Geysers. The lengthy history of power production and even terrible mismanagement of the vast resource offer an unparalleled object for bench testing of technology.
In my view it is mindless to do so at such a location.
Best, Terry