Sorry for the long winded post.  I didn't realize how much I had typed until I submitted.

I will be doing my geothermal wells here in the next month or so.  I've got a well driller lined up for the job.  Cost is $6/ft for a 4.5" geothermal well and includes grouting with standard BH-20 non-thermally enhanced bentonite.  Thermally enhanced grout and silica sand aren't available locally and it is cost prohibitive to have it trucked in for one small job.  I will be responsible for putting the pipe in the hole and running the lines to the house.

My plan is to install a Climatemaster TEV038 3 ton 2-stage package unit.  My heating and cooling loads could be satisfied by a 2.5 ton unit (~26,000 BTU cooling, ~30,000 BTU heating), but that size isn't offered in the Tranquility 30 series.  The 3 ton unit should operate in first stage >90% of the time.  I've run the Climatemaster Geodesigner software and came up with 3 vertical wells each 230' deep.  These will be located 20' from my basement wall penetrations.  I will be installing Centennial Plastics pre-made 3/4" x 510' (255' + 255') HDPE U-bend loops.  These are available locally for ~$150 per loop.  The header for the loops will be in my basement mechanical room.  I'm planning on installing valves (full flow brass ball valves) for each loop to aid in flushing and constructing the header using Propex fittings and 1" Uponor Pex for the ~8' to the unit.  Valved fill and air purge vents will be installed at the loop system high points.

I have a 100' deep water well on my property that has been in service for ~20 years.  The well report documented red clay 0'-30', black shale 30'-100'.  Static water level in the well is ~10' below the surface.  Water flow rate is approximately 9-10 gpm.  The water has high TDS and isn't suitable for open loop in my opinion.  My geothermal wells will be located ~100' from the water well and on the same elevation.  I've researched USGS data and it shows that I am located solely in the Western Interior Plains confinement zone (considered a minor aquifer in my area with a depth in the thousands of feet and generally low well flows of <10 gpm due to tight structure).  I've researched other well reports in the immediate area and wells up to 600' deep have encountered similar black shale for the entire bore except for the overburden and intermittent bands of sand stone at >300'.

Assuming the water level in the geothermal wells comes up to ~10' from the surface, how much steel/rebar is typically needed to sink the 3/4" pipe in the 230' well?   Is 20' of 1/2" rebar sufficient?  This is assuming the pipes are filled with water and pressurized to ~100 psi prior to lowering in the well.  How is the rebar typically attached to the pipe U-bend?  Tape, wire, or a combination of the two?

Rather than pump the entire well bore full of low conductivity bentonite grout, I would like to back fill the holes with coarse washed river sand available locally and typically used for concrete.  Assuming the well remains full of water, this should provide much better heat transfer than straight bentonite.  I would stop the sand ~30' from the surface and the driller would then seal the top 30' of the well with BH 20 bentonite grout.  He said this was legal and acceptable in my area.  To help ensure that the sand doesn't bridge while backfilling, I've come up with an idea to get a 5' piece of 1/2" black iron pipe and cap with 3/8" hole drilled and 300' of 3/8" nylon rope run through the pipe and cap and knotted on the end to keep it from pulling through the cap.  I will lower the iron pipe down the hole with the loop.  As the sand is backfilled, I will periodically pull the rope and pipe back out of the hole and "tamp" the sand to insure it is compacted around the pipe.  The iron pipe will have the threads cut off the lower end and edges ground smooth.  Does anyone have comments regarding this approach?

Geodesigner indicates a minimum winter entering water temperature (EWT) of 44-48 F and maximum summer EWT of 86-94F depending on borehole thermal conductivity assumptions (lower conductivity for saturated silt/clay, higher conductivity for average stone, saturated shale should be somewhere between the two).  At the higher minimum EWT, no anti-freeze is needed per the Geodesigner program.  Is there some type of corrosion inhibitor recommended if no anti-freeze is used?  My plan is to initially forego anti-freeze and leave the low temperature cut-out jumper installed and monitor the system to see how it performs.

What to fill the loops with?  The options I have are 1) my well water that passes through an aeration sand filter (removes sulfer and iron) with PH neutralizer (crushed oyster shell) and then passes through a water softener to remove the calcium carbonate hardness, or 2) municipal treated water supply (surface water) available a few miles down the road and hauled to site in a 55 gallon barrel, or 3) distilled water purchased at the grocery store.  Per the Climatemaster Pressure Drop calculator spreadsheet, I will need 54 gallons to fill the loops and inside piping.

Regarding loop pressure drop, the Climatemaster spreadsheet shows 21.0 ft hd total for my system with 100% water.  I have selected the Magna 25-140 variable speed high head pump internal to the geothermal unit.  At the recommended 9 gpm, this pump has 43 ft hd capability.  The low head pump option only has 20.6 ft hd capability at this flow.  The Magna 25-140 has a maximum power consumption of 230 W, but at 9 gpm and 21 ft hd, it should only consume ~100 W or less per the manufacturers curves.  this compares to a maximum consumption of 85 W for the low head pump option.  The spreadsheet shows a Magna 25-140 maximum pump capability of 12 gpm given my loop design with a circuit Reynolds Number of 11,230 at this flow.  The smaller pump showed a Reynolds Number of ~7000 with its maximum flow capability of 8 gpm.   Minimum recommended is 2500.

Any thoughts or comments on the above would be appreciated since this is my first time at doing this.

I would review this document (ignoring any MI specific info). Note that cementitious grout is a recommended grout for all boreholes.

1) I am always specifying that I want the whole borehole grouted. I have seen gas, contaminated water coming up, contaminating other aquifers etc.
2) Certainly the variable speed pump is a good choice. For my taste the pressure drop is a bit too high, but I guess without antifreeze it is OK.
3) We always bring in our fill water.

Grouting is a requirement in enlightened enforcement zones imho. ground or cross contamination of aquifersshould be gaurded against with every effort and tool available.

Good Luck getting the loop down the hole....I would not describe this as a DIY project !!!

Are there examples of getting sand down a hole and well compacted without a tremie and pump?

Posted By jonr on 06 Apr 2015 07:35 PM
Are there examples of getting sand down a hole and well compacted without a tremie and pump?

This is nothing that I have ever done nor would attempt.

But DIY'ers can be pretty resourceful, I hope he tells us how it turns out.

I appreaciate all of the comments including concerns with groundwater protection because my home water well is ~100' away from the geothermal wells. But I feel pretty comfortable based on my research and geology of the area that I'm only dealing with a single minor aquifer at 230' deep. I've reviewed numerous well construction reports for the area. There is a 6" water well not too far away but slightly higher on the ridge that is 360' deep. The well went through 15' of clay then black shale the remainder of the well according to the report. The well was cased for 20'. All of the water in the zone from -20' to -360' can freely intermingle.

But given all of that, I have considered some changes to my well backfill plan. Now rather than using sand for backfill, I am considering rock crusher dust from a local rock crusher. This material is hard rock fines up to 1/4" minus. This material is used for road base and foundation backfill in the area. It sets up tight due to the nature of the fractured rock particles and has a very low permeability compared to sand. Another thing I am considering doing is inserting a bentonite plug ~7' long (50# of bentonite pellets) at -100' using PDSCo Pel-Plug coated bentonite pellets just to provide additional assurance that water from below -100' doesn't migrate up into the zone where I get my well water.

Note also that this is in a rural area and I am in the middle of 52 acres. My nearest neighbor is 1/4 mile to the east of me. To the west and north of me is >1000 acres of state owned heavily wooded wildlife management area. To the south is a river and >5000 acres of federal wildlife refuge (bottom land). I am located on a ridge on the high bank side of the river and all of the river alluvial deposits are to the south of me per state geology maps. I'm not aware of any gas wells within 20 miles.

Edit:  scratch the idea about using rock crusher dust/fines for backfill.  I went and looked at it and it clumps too bad when moist/damp and would likely never make it to the bottom of the hole.  I think I will stick with the coarse washed concrete sand and insert the bentonite plug at -100' as mentioned above and then BH-20 bentonite grout provided by the driller for the final 20'-30' at the surface in the clay region to block surface water.  Note that nothing is set in stone at this point.  I'm just putting ideas down. 

Does anyone have any experience with the bentonite pellets mentioned above or similar products such as Baroid Holeplug?  If using the bentonite pellets, are the time-delay coated pellets needed or would just standard non-coated pellets be sufficient to get down 100' through standing water?  I've seen it written where the coated pellets are good for 500' wells.

Posted By Palace Geothermal on 06 Apr 2015 05:22 PM
Good Luck getting the loop down the hole....I would not describe this as a DIY project !!!

What is the concern?  I talked with my well driller (geothermal wells is his primary business) and he didn't have any concerns with getting the pipe in the hole.  He has several drill rigs, but the rig he has in my area is a Gardner Denver 1000 series air rotary.  He said that if all we encounter is consolidated clay/shale/sandstone, there shouldn't be anything in the hole except water assuming we hit water.

To put the pipe in the hole, I planned to put the pipe reel on my tractor front end loader forks, fill the pipe full of water and pressurize the pipe, raise my front end loader to ~10' high, attach 2 10' sticks of rebar to the pipe u-bend with wire and heavy duty duct tape, drop in hole, push to bottom.  What am I missing?  That was my original plan, but after talking to my well driller again today, he said he would put the pipe in the hole.  All I have to do is provide the pipe.

Posted By docjenser on 05 Apr 2015 02:09 AM
1) I am always specifying that I want the whole borehole grouted. I have seen gas, contaminated water coming up, contaminating other aquifers etc.
2) Certainly the variable speed pump is a good choice. For my taste the pressure drop is a bit too high, but I guess without antifreeze it is OK.
3) We always bring in our fill water.

Regarding the loop pressure drop, I ran the Climatemaster loop design using 1" x 510' loops and 1-1/4" piping inside from the header to the unit.  With this arrangement I had 14 ft hd at 9 gpm using 100% water.  With this arrangement I could use the low head variable speed pump with the geothermal unit, but it doesn't save much electricity wise over 3/4" loops at 21 ft hd and the high head variable speed pump, maybe 20 watts at most.  Plus, the 1" u-bend loop pipe would cost ~$250 more for 3 loops than 3/4" and be a lot harder to get in the 4.5" well hole.  Loop temperatures were ~1 degree better with the 1" pipe vs. 3/4" pipe.

The fill water you use, is that just municipal tap water or something with higher purity?  I can get 5 gallon jugs of purified (reverse-osmosis) water for a reasonable price locally and I only need 10 or 11 of them.

Soory, my mistake, I assumed 2x510', total of 1020' per loop. With 9-10 gpm your are good with 3 boreholes x 510' total pipe length, 3/4" pipe. No reason to go crazy on the water, municipal tab water should be fine, each way is good.

Regarding borehole grout, does anyone have any experience with cement based grouts?

I found a grout mix online (google cement mix 111) as follows:

One 94# bag portland cement
200# sand
6.2 gallons water
~1.35 pints superplasticizer
~1 # bentonite
Yield = 19 gallons

Per online documents, this has a higher thermal conductivity than high solids bentonite grout. The components are also readily available locally. Is this type of mixture pumpable with a typical grout pump? I haven't asked my driller yet if this is something he can work with.

For my three (3) 230' deep boreholes with 3/4" loop pipe at 0.7362 gallons/ft of borehole, it will take 505 gallons to completely fill them. At 19 gallons per batch, that is 26.6 batches of the above mix.

Ignoring any contamination issues, I don't think that any grout will come close to the thermal performance of water that is allowed to move freely (ie, convection in a no grout case). For those few inches, it's like a lake loop.

But if you ASSUME your getting water contact and then down the road a few years that changes your dry loops will not work well.

Chris

Cement based grouts are not approved for geothermal loops by IGSHPA. Engineers in my area say that are not to be used on the commercial projects that we do.

>Cement based grouts are not approved for geothermal loops by IGSHPA.

In their 2014 standard, section 2B.1.2, the IGSHPA specifically lists cement based grouts. And grouting is qualified with "when penetrating more than one aquifer", suggesting that in some cases no grout is required. What's the document/section where this was changed?

Posted By jonr on 08 Apr 2015 01:46 PM
Ignoring any contamination issues, I don't think that any grout will come close to
the thermal performance of water that is allowed to move freely (ie, convection in
a no grout case).

First, "Ignoring any contamination issues" is criminally irresponsible -- and risky.

Next: thermal conductivity of water is DISMAL. It's only a little better than bulk
HDPE -- and vastly worse than HDPE if you're comparing an inch or three
of water-filled annular cavity to a pipe wall that's only a few millimeters thick.

So, if the water isn't moving at all (or not quickly enough to strip off the external
surface boundary layer), thermally enhanced grout will be several times better
than water -- and many, many times less risky.

I should have said that cement based grouts are not recommended by IGSHPA

Posted By jonr on 08 Apr 2015 10:01 PM

In their 2014 standard, section 2B.1.2, the IGSHPA specifically lists cement based grouts. And grouting is qualified with "when penetrating more than one aquifer", suggesting that in some cases no grout is required.

Thanks for pointing that out.

Here is the IGSHPA link for anyone else interested in this topic: 

Closed-Loop / Geothermal Heat Pump Systems, Design and Installation Standards, 2014 Edition

Here is the exact quote: