[GaryAitken (OP)]

I am trying to evaluate the design of some wells for a ground source heat pump, and would like to know how fast soil/rock temperatures change with depth from the surface -- in the first 500 feet / 150 m.  I know the first 2m or so is fairly rapid, but what happens below that?  In our area (western Montana, USA, elev. ~4200'), "ground temperature" is supposed to be about 43 °F, but it is unclear to me where that figure comes from, and there is an obvious non-linear change as depth increases.  I have an existing heat pump with 5 wells 200' deep, and another family member down in Colorado, at elevation ~8500', has one with 3 wells 350' deep.

[evan_au]

Around here, you want to heat in winter and cool in summer. The ground temperature is pretty stable more than 1m below the surface, and you can use a well of a few meters deep to draw on thermal inertia of the ground..

However, in Montana, you probably want to heat all year round, so you actually want to draw on geothermal heat sources - the fact that temperature rises with depth.
- To get a high temperature, you need to go to great depths
- But how quickly the temperature rises with depth depends very much on the local geology
- If the deep temperature is much higher than surface ambient temperature, you don't need to expend much energy to bring that heat to the surface. But there is a maximum rate that you can draw heat from the rock.

The average heat flow from the interior of the Earth to the surface is about 0.1 Watts per square meter (0.1W/m²).
- Obviously, it will be much higher near an active volcano!
https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget#Global_internal_heat_flow

Thermal resistance of rocks varies over a wide range:0.4 to 7 W m⁻¹ K⁻¹, so the rate that temperature increases with depth varies enormously with rock type.
https://link.springer.com/article/10.1007/s10973-018-7090-5#:~:text=Thermal%20conductivity%20of%20rocks%20falls,typical%20for%20felsic%20igneous%20rocks.

You could ask a local company that does this about the conditions in your area...

PS: I think Alan Calverd has a heat pump, but I think it might be the "shallow" variety?

[Petrochemicals]

To be honest I can't help you as you probably know more about your local area than I do.

https://ageconmt.com/montanas-geothermal-energy-potential/

[GaryAitken (OP)]

We only heat about 7 mos, and cooling is passive.  I understand there's a large variation, but was hoping there were some broad characteristics or a source of information for temperatures taken in some wells somewhere.  The people I have talked to are all working seat-of-the-pants without much information.  For example, Missoula supposedly has an average ground water temperature of 48°F, but that's well-water information and relatively (~50') shallow.  We are 65 miles from there; our well is 65' deep and ~44°F. But I would like to know what happens as you go deeper.
I did find a ground temperature predictor at groundtemperatures.com, which helped; at my location, it predicts the following:
      15 m (  49') 5.69 °C
      20 m (  66') 5.65 °C
      50 m (164') 6.51 °C
      75 m (246') 7.02 °C
    100 m (328') 7.37 °C
    125 m (410') 7.62 °C
I have no idea how accurate it is for our area, but probably about as good as I can get without an actual well test.

[evan_au]

      15 m (  49') 5.69 °C
      20 m (  66') 5.65 °C

Going slightly "backwards" between 15m & 20m is a bit odd?

[Petrochemicals]

We only heat about 7 mos, and cooling is passive.  I understand there's a large variation, but was hoping there were some broad characteristics or a source of information for temperatures taken in some wells somewhere.  The people I have talked to are all working seat-of-the-pants without much information.  For example, Missoula supposedly has an average ground water temperature of 48°F, but that's well-water information and relatively (~50') shallow.  We are 65 miles from there; our well is 65' deep and ~44°F. But I would like to know what happens as you go deeper.
I did find a ground temperature predictor at groundtemperatures.com, which helped; at my location, it predicts the following:
      15 m (  49') 5.69 °C
      20 m (  66') 5.65 °C
      50 m (164') 6.51 °C
      75 m (246') 7.02 °C
    100 m (328') 7.37 °C
    125 m (410') 7.62 °C
I have no idea how accurate it is for our area, but probably about as good as I can get without an actual well test.

Is Montana the area of very flat ground? Ground water will be a factor, the type of material, wet sand or chalk will allow a greater permiablility of surface water than clay or rock, the height of the site in relation to the local aquifer, the prominence of the site, a hill is obviously more exposed than a valley.

[GaryAitken (OP)]

      15 m (  49') 5.69 °C
      20 m (  66') 5.65 °C

Going slightly "backwards" between 15m & 20m is a bit odd?

I wondered about that as well.  Seems strange.  Since I had to enter the depth each time I may have mistakenly tweaked either the gps coordinates or the time of day by mistake.

[GaryAitken (OP)]

Is Montana the area of very flat ground? Ground water will be a factor, the type of material, wet sand or chalk will allow a greater permiablility of surface water than clay or rock, the height of the site in relation to the local aquifer, the prominence of the site, a hill is obviously more exposed than a valley.

We live on a small (~4 km x ~10 km) pretty flat area which is glacial outwash, at the abrupt foot of mountains on the west side of the continental divide; a river is within 10m and ~10m below the heat pump wells.  Our water well, ~50m from the river, is ~15m deep.  The "dirt" is glacial outwash cobble, from marble to 1m dia size; very little sand, at least in the first 3m; can't remember what the well log said, and I can't remember if the layer below the well water was clay or rock.  I think rock, and when punched through it was just more cobble.  The surface is extremely porous.

[Darlalarsen]

      15 m (  49') 5.69 °C
      20 m (  66') 5.65 °C

Going slightly "backwards" between 15m & 20m is a bit odd?

I wondered about that as well.  Seems strange.  Since I had to enter the depth each time I may have mistakenly tweaked either the gps coordinates or the time of day by mistake.

I feel the same.