08-28-2018, 06:16 AM
quote:
Originally posted by TomK
"The traditional geothermal drilling process has been to drill into the sandstone that has water in its pore spaces."
Now, the article is about geothermal drilling causing earthquakes. To me there seems to be a couple of major differences between this technique and the method PGV uses. 1) They drill through sandstone on the mainland (which is not present here) and 2) the new technique involves fracturing felsite.
So, what difference is there between drilling through sandstone compared to basalt and 2) is fracturing felsite even a technique used here? I'm not enough of a geologist to know if felsite is present underground in Hawaii in any significance (I know rhyolite is present but rare).
That article was referencing a specific project in Northern CA - they would have more accurately worded it: the traditional drilling process in the Geysers geothermal field is to drill into sandstone..."
The drilling process for geothermal steam production isn't substantially different and isn't what might generate earthquakes. In the Geysers production zone, the pore volume is filled with steam (referenced as a steam dominated system) with an underlying boiling zone below. I don't honestly know how dependent the production rate there is on bulk (interconnected) permeability for steam transport in the rock versus steam transport in the fracture permeability. Hawaii is generally considered to be a water dominated geothermal system where the rock pore volume is filled with liquid water at high temperature (and pressure). The interconnected pore volume in Hawaii's basalts is negligible, and fluid transport is pretty much exclusively by fracture permeability. The Geysers geothermal field has limited natural water infiltration into the system and several decades ago the production of steam, with minimal return of condensate to the reservoir, nearly killed the field (falling pressures, falling production rates, chemistry going sideways,...) and was only turned around by importing treated municipal wastewater from outside the field. Hawaii doesn't have issues with water availability but the production rate is limited by the fracture density in the rift zone - and that will be the trick in the future: finding those fractures. PGV had a couple of wells that encountered large fractures that were good producers; others not so much.
The Altarock project in N-CA intends to drill into the deep magmatic intrusive (the felsite) that supplies the heat for the geothermal field. It is expected that that rock will be quite dense and have few fractures and little to no interconnected pore volume. They will have to generate their own fractures by injecting water at high pressure to try and create a fracture network that will allow interconnection of wells - and allowing the operators to inject water into one well and draw steam from one or more wells nearby. (an often overlooked concept in geothermal is that the heat resource is the rock itself - water is, for the most part, only the transport medium). Typically, the hydro-fracturing is done by injecting a cocktail of water, chemicals to act as lubricants and reduce the viscosity of the water, and sand (called a propant) to hold the new fractures open once they're created. The intent is to generate fractures of a few millimeters or less in cross sectional aperture and meters to kilometers high and long respectively.
In Hawaii, we have dike complexes in the rift zones and PGV is drilling into one of those dike complexes; the felsite in N-CA is the underlying heat source; we don't have felsite (which chemically is related to more silicic volcanism) here, but we do have gabbro which is the basalt analog to felsite. That gabbro is created by the slow cooling and crystallization of basalt intrusions deep inside Hawaii's rift zones and calderas. The gabbros exist at depth in the Kilauea rift zones and are part of the heat source. But PGV has not, to my knowledge, made any attempt to drill into those gabbros and induce fractures in them for power production. As far as I am aware, they have relied on naturally occurring fractures within the rift zone for the required permeability to produce steam.
One of the challenges of inducing fractures - highlighted (and IMHO exaggerated) by the article, is that injection of water can also induce larger earthquakes because the injected water can act as lubricant for pre-existing faults that already have accumulated strain. (and this is an important point - if there is no pre-existing accumulated strain on the fault, there will be no earthquake...).
That is what happened in Basel, Switzerland - there was a pre-existing fault through that area that had not been active (i.e. generated a significant earthquake) in several hundred years that the project principals were unaware of. Their injection of water lubricated that fault and it slipped - generating a small earthquake. In an area that has (un-reinforced masonry) structures that were built well before earthquake codes were conceived of, there was significant damage even though, by Hawaii standards, the earthquake would be barely noticed. Whether injection of fluids in the felsic rocks underlying Geysers is likely to generate a larger earthquake is difficult to know. That part of California is seismically active - my sense is that where you have a lot of smaller quakes (which are strain-release events) occurring on a regular basis, the likelihood of generating a larger one is small. (I've often wondered whether injection of water into some of these large fault systems could be used to mitigate their ability to generate larger and more damaging events - strain is constantly accumulating on active faults at some rate and the longer the interval between earthquakes, the larger the energy release will be and the greater the damage they will do...).
That maybe was more than you wanted to know TomK, but I hope that it answered your question...