Let's Get Geothermal: Water Temperature and Pressure

Are the temperature probes responding to changes in water composition, during no-flow conditions? What type of temperature probes are used?

Temperature and pressure are directly related. An increase in pressure will definitely increase the temperature.

• Temperature is the measurement of how many collisions of molecules releasing heat in a substance. By compressing the water you increase the collisions and thus the temperature.

Also; If this is a drilling tool and water is flowing past while drilling; the movement of water will remove heat from the tool. When you stop the water movement, the water will continue to absorb the heat from the bit, however, it will remain in the area and increase in temperature until the temperature of the bit and water is the same. Later it will cool to the surrounding environmental temperature.

Interesting that at these pressures and temperatures that water is in a liquid form. Are you sure it hasn't turned to steam?

Never thought granite could be "impermeable". All the granite I have seen is made up of a mixture of crystalline structures that are full of micro holes. Besides sandstone and marble, I would think granite was the least impermeable of all rocks. It is hard but certainly not solid.

Check your steam tables. In pounds, the hole pressure has to be 6,000 psi hydrostatic, and probably another 1,000 or 2 to keep the water from flashing, even on the down leg. The dynamic pressure added at the top of the downhole brings the super-heated water up the up-hole. increased salination from extended contact probably increases the weight of the water in the pipes, or if your system is closed both sides, you are heating the trapped water causing expansion in a confined space, like a boiler on fire-up.

The increase in salinity says that you are expanding your pourous volume. Do you log water additions? What temperature is your return water? Must be lower to be pumpable, as the only pressure you have topside is the downpump pressure. Any good minerals in your water? Gold is fairly soluble in superfluid water, which is a probability. I haven't checked for the superfluid temperature of water.

Sorry, Wiki says water critical temp is 648 K, 375 C at 22Mpa. Superfluids are interesting. Maybe another 500m down you would have gotten the temp, and then you could figure that even gold would be disolved, dropping out when you chilled out of superfluidity. Your flow through the granite bed would probably increase 10 or 20 fold.

Ah, at 375C, the density is only 0.35. When flow stops, the water above your bed cools, increasing density(weight of water in the pipe, read as head, and thus pressure. The longer period of flow in the heating bed gives more heat transfer.

The local temp (bottom of the pipe) heats up more, but the cooling of the long standing water pipe drops in temp, but increases in weight, ergo, higher temp and higher pressure.

RichH

Agreed: it's most likely heat transfer from the surrounding geothermal heat source(s) into the now-stationary water column.

The pressures are too high for the water to boil to steam at those temperatures, and water being largely 'incompressible' does not behave like a gas.

The water can't heat up from the surrounding rock mass because the rock is actually cooler at the depth of the measuring tool (230 degrees C). The hot water that the measuring tool sees at 247 degrees C while the water is flowing past comes from fractures in the rock deeper down where the rock mass is 250 degrees C. The well has been drilled to the depth where the rock is 250 degrees C.

Agree that if one thinks on a molecular basis the temperature should increase.

Regarding your other comments. The drill bit has long been removed from the hole. Water is flowing from naturally overpressured fractures at the bottom of the hole and the temperature tool is located only part way down the hole at a depth where it should cool once the flow is stopped. After the initial heating the temperature does indeed start to drop.

The pressure is way too high for steam to enter into the equation even at 250 degrees C. Just have a look at your steam tables at 30-70 MPa.

Granite may be slightly permeable along grain boundaries. Unfractured granite has a permeability of around 10-100 microDarcy. This is much lower than sandstone.

May all so be due to the reduction in the flow. Fluid flow across a surface creates a barrier I believe due to adhesion. As flow increase this barrier increase reducing heat transfer to the surface. If the sensor is to the out side wall where it is 230 degrees C it may be dropping the reading some.

Well I am by no means learned in these areas so I limit my comments to only one. With all the energy problems and so called global warming that we have from the burning of hydro-carbons it looks like we could figure out a way to use some of this energy? I would think this is the same as super-superheated steam (- the salt) as we would call it in the paper mill boiler area (although at much higher pressures and temperatures). It seems to me that this would be great to run a big ole heat exchanger/turbine/generator combination to make electrical power.

pipewelder

The temp/pressure at the topside might be low, but the bottom end of the system has to be nearly 8,000 psi. Might not even need the downleg pump, as the density differential hot upleg/cool downleg X 12,000 ft. A pump couldn't produce that kind of flow.

RichH

So is there is no way of getting enough of this heat/pressure to the top surface to do any meaningful work because it will cool on on the long trip up? Or am I in over my head as usual or maybe both

pipewelder

Lots available, coming up like an artesian well, and though the granite won't etch out with acid, a moderately small explosive device could create enough of a gravel bed to increase the water flow pretty rapidly. 70 Mpa is roughly 700 atm, or 10,000 psi, so containment in a pressure system would be relatively easy, as about 4,000 psi should be the available 'geyser" force. more than enough to keep it hot water rather than steam. Doesn't sound like they are reinjecting the used water, which should be as important to do for the environment as using the water in the first place.

A closed topside loop would really be good. The cooler return water would fall thermally.

RichH

Hi RichH and other responders,

Thanks for your input. Just to get you into the bigger picture, yes we do have a down loop well, and we hope to generate large amounts of clean green electricity for the Australian electricity grid. We are drilling in a location where the rock temperature at 4-5km depth is the highest in the world outside of volcanoes, and we have this condition over an area of about 1,000 square kilometres. Our current concept is to have wells spaced at around 1km (that's 1,000 wells that will require drilling over the next few decades), but as we get better at this we should be able to sweep heat out of the rock more efficiently and decrease the well spacing to 500m (that's 4,000 wells over 1,000 square kilometres). If the rock fracture system is complex enough (rather than just a few direct pathways between injection and production well) then the circulation loop will last for many decades before we see cold water at the production well.

The impedance of the fracture system will dictate how much energy in re-injection pumping is required, and it is true that the density differerence of the water between the hot production well and the cold re-injection well gives you a buoyance drive of about 1,000 psi to help out here. If the fracture impedance is very low (<1,000 psi) no re-injection pump will be required, but this is unlikely. In fact the friction losses in the wells and surface pipes at the flow rates we require are almost 1,000 psi so that more or less counteracts the buoyancy drive.

On the cooling effect as the water rises to the surface, again this is dependent on flow rate. We have modelled this taking into account all well properties and rock conductivity properties to show that the steady-state temperature drop will be about 10 degrees C over 4.5km, so the water will reach the surface at about 240 degrees C if the rock temperature is 250 degrees C.

Would a larger central return be able to supply a ring of 6 or 8 source wells? The possibility of a belling tool bit to increase the size of the cavity and open more fracture lines would also be a possibility for both.

And in spite of several posts here, pressure does not increase the temp of relatively incompressible liquids. Release of the pressure, as in hydraulic systems, converts the potential of pressure to kinetic in the form of acceleration through the relief, which is then randomized into thermal energy as heat.

My guess would be that you wouldn't be able to efficiently and productively use more than 50° of drop for steam generation, but the surplus available heat could be used for drying, distilling, or processing. The second or third use of this resource would really add to the potentials, if you could add an ethanol plant, resulting in free distillation energy, plus drying of the distillers grain into supplemental animal feed or fuel pellets. Some pressurized water could be released for mechanical work (like pelletizing the dried grains).

As a quick guess, you are probably running the well with a casing, allowing the drill steel to be retracted and reused. 4,000 m of pipe is a lot to pull. Also, do you extend the returns to a depth several hundred feet below the risers? That might add a net longer loop downhole allowing greater flows before developing "short circuits".

The next really big potential would be a deeper well to the 375 or above temps to extract and use the supercritical water which would flow much easier downhole, and also transfer the energy far faster and more efficiently than vanilla water. The latest power plants use supercritical water for this reason. Once past the critical point, viscosity drops by a factor of 10. And thermal transfer rates nearly double. Plus, the supercritical water would have a lot of other uses. You would need to maintain about 220 Mpa to bring up the supercritical water in the same state, but it soundes like you have more than that available. If you got to that level, a ring of returns would be the way to return to a single supply. The flow near the supply would be of the SCW, whereas the sub SCW return would reachieve supercriticallity on it's way back to the supply.

Also, the distillation potential for drinking water is enormous. Again, mostly from second-use water, from it's own feedstock, or local brackish or mineralized water.

Lease the second-use processes requiring that the leasee pay the drilling costs, and a Caloric charge for the use. a 125° dry steam might also be able to be sold for remote ( 10 or 20 km) use, as our New York City does.

Waste not, want not. Par boil shrimp, sterilize canned goods, even dry lumber. So many options.

G'day

RichH