[Atomic-S (OP)]

What could currently be the main obstacle against routine tapping of geothermal energy via drilling into the hotter parts of the earth?

[Bass]

If you are referring to hotter parts at the earth's surface, active volcanoes are, at best, highly unpredictable.  If you are referring to drilling into the deeper earth (mantle/core), the technology isn't there yet.
Almost all geothermal fields are located in areas with higher than average heat flow- usually associated with either volcanoes or plutons.

Subduction causes orogeny.

[daveshorts]

A major obstical is how expensive drilling the hole is compared to the amount of heat you get out, in most parts of the earth - not iceland, or I guess hawaii. Especially as the places with good heat flow are normally associated with very hard rocks.

[JimBob]

There are also substantial geothermal reserves in Japan. I have worked on them.

But there are less dramatic geothermal methods in the development. Power Tube, Inc. ( http://www.powertubeinc.com/ ) have developed a method for using the small thermal gradient between the surface and any depth that has an ambient temperatur of 110 - 200 C. I have mentioned it before but not in the Geology section. If I had had any extra cash I would have taken part of the capitalisation of the start-up company. It really is a very well thought out, researched and designed tool that WORKS!




The mind is like a parachute. It works best when open.  -- A. Einstein

[Bass]

quote:Originally posted by daveshorts

A major obstical is how expensive drilling the hole is compared to the amount of heat you get out, in most parts of the earth - not iceland, or I guess hawaii. Especially as the places with good heat flow are normally associated with very hard rocks.

Not only cost of drilling, but also cost of the infrastructure to access geothermal fields and to deliver the power generated.
Hard rocks aren't much of a problem for drillers, especially with percussion drilling.  Conditions that make drilling difficult are broken, fractured and inconsistent rock- which is common in geothermal fields with rock alteration, breccia, faults and fractures.

Subduction causes orogeny.

[JimBob]

There is also an erratic water table in parts of a hydrothermal field due to gyser activity.



The mind is like a parachute. It works best when open.  -- A. Einstein

[Atomic-S (OP)]

So we would be talking, then, with respect to, let us say, a land location not a whole lot higher than sea level, of having to drill how far down to get meaningful heat? Of course here I guess the question is rather ambiguous because it depends upon what one calls "meaningful". The deeper you go, the hotter; whence we would expect more power potential per hole.

[JimBob]

The depth to the required temperature will vary greatly, depending where you are on the globe. The map at http://www.powertubeinc.com/product_info/current_geothermal.shtml shows potential areas where the geothermal gradient - amount of temperaature rise per 100 or 1000 feet of depth - is high enough to make the tools selected work. The Power Tube (no, I am not their shill, I know the people) is designed to work beginning at 110 C and there is ongoing research to bring this temp down to reduce costs.

The marketing strategy is to sell this to under-developed populations as the basic source for a village providing power for two way short-wave radio, computers, some lighting, etc.; just enough to get the people able to develope a better standard of living.

As well as traditional volcanic and thin crustal areas, sedimetary basins are also usable for power generation. The active compaction and resultant higher vertical compressional force creats higher than average temperature gradients. If you will note the study area in west Texas, in blue on the referenced map, this area is a Permian Basin  being primarily compressed by lateral force; the Mountains to the south are the result of Mexico and Texas colliding.



The mind is like a parachute. It works best when open.  -- A. Einstein

[Atomic-S (OP)]

Well, that is interesting; obviously there is enough heat available in one hole, in such sites, to permit useful, if not copious, quantities of power to be generated from within one hole. One would like to expand this idea to greater megawattage. Reminding me that according to the said web site, present common geothermal methods require injecting water into one well, then it travels through porous formations to another well, picking up heat from an extended region of rock, and then up, heated, to be used. It requires that an extended region of rock be tapped to provide the requisite heat. If we are talking, however, about a single hole, such as with the Power Tube, are we not limited intrinsically to the heat in the immediate vicinity of the well? Raising the question of how rapidly that heat can be replentished from the surrounding formations, a figure which will limit the sustainable power production from the well. Of course, the hotter the formation, the more rapid the heat flow between it and the cold side of the system (presumably room temperature supplied by cooling water from the surface), and therefore the more power potential.

[JimBob]

The small heat sink created by the well bore is negligable compared to the huge amount of heat contained in the rocks. The heat in place is from two sources - magmatic activity or pressure. In the Gulf Coast of Texas and Louisiana, as in other sedamentary basins, (the North Sea) heat is caused by sediment being dumped in the basin and the pressure of compaction generating heat. I have been on wells where it was necessary to have rubberized gloves to get a sample from the drilling mud, the mud had picked up so much heat from the well bore. Temperature in these wells approaced 350 degrees F at depth - 20,000 + feet. I have seen greater temperatures, too.







The mind is like a parachute. It works best when open.  -- A. Einstein

[Atomic-S (OP)]

quote:350 degrees F at depth - 20,000 + feet. I have seen greater temperatures, too.

Nice and toasty. 20,000 feet? That would be substantially deeper than the deepest underground mine where humans work, would it not?

[Atomic-S (OP)]

quote:The small heat sink created by the well bore is negligable compared to the huge amount of heat contained in the rocks. ... Temperature in these wells approaced 350 degrees F at depth - 20,000 + feet. I have seen greater temperatures, too.

But the issue is not the heat content of the general region, nor the temperature of it, nor even what percentage of the heat is disturbed by one well -- but rather the rate of possible heat flow through the geometrical bottleneck created by the small size of the hole, when only one hole is being used to access it, rather than a water flow between 2 holes. No matter how many billions, trillions, quadrillions, whatever of BTUs reside under the State of Texas, if you want to access them through one hole, then they all must flow through the relatively small surface area of that one hole, and must do so after traversing however many miles of rock that are between them and it to begin with. That limits how fast the energy can be gotten out of one hole. Once the heat in a hole starts to be tapped, the temperature of the rocks in the immediate vicinity of the hole starts to drop. At some point we would get to a steady-state situation, such as an effective bore temperature of 200 F, as heat is consumed by the generator at the same rate it is replaced by the surrounding rocks. So this basic thermal conductivity issue is what I was asking: what is the limit it imposes upon how many kilowatts or magawatts can be extracted from one hole? (Obviously we would have to have some parameters -- maybe we could assume, as in the above example, 20,000 feet deep and 350 degrees max. temp.)