Soil vs. Rock: Rule of thumb for drilling costs

by **seattleboy** » Fri Oct 30, 2009 12:46 pm

Hello,
First post. I'm an architect trying to push geo-exchange systems with my large institutional projects around the country. I know that there are dozens of variables involved, but can someone hazard a range of drilling costs for different classes of soil and rock?

Much appreciated.

Seattleboy

by **waterpirate** » Fri Oct 30, 2009 4:49 pm

Hi and welcome,

The cost per foot for drilling is not as cut and dry and straight forward as we would like. There are a multitude of variables. I will try and address them as best I can.

Mud rotary drilling/ soil and shale with soft rock can be as cheap as 6.00 per foot in some places and as high as 16.00 others.

Air rotary/ most rock formations without the tough stuff as cheap as 10.00 and as high as 26.00

Dual rotary/ air and mud together with the tough stuff and and glacial till thrown in for fun 26.00 and up.

The short answer is to float a balloon in the area you are looking at for a budget number and you will find out what you need to know from the local talent. I would also ad that you need to talk with drilling contractors that are geo freindly or all your number, produvtion, and end results are going to be doomed from the start.

Hope this helps

by **sunnyflies** » Sun Nov 01, 2009 9:18 am

More than that, be sure they are experienced with drilling closed loops or the prices you get might be off. I have had estimates for installing them through saturated sand and gravel that ranged from $18,000 to $58,000 for exactly the same job. One was from an experienced geo driller, the high estimate came from an experienced well driller who was just beginning to get into geo closed loops.

by **Choice Mechanical** » Sun Nov 01, 2009 5:42 pm

In our area (limestone bedrock) air rotary closed loop drilling typically runs between $12-$15/LF for installation. Soil drilling's going to cost less per foot, but in most cases you need more linear feet per ton in soil than in rock, so keep that in mind.

by **seattleboy** » Sun Nov 01, 2009 7:00 pm

Thank you all for your responses. May I ask three follow-up questions?

1. I presume that your drilling costs relate somewhat to drilling speed. How does the speed (say, for a 300-foot hole) vary for different soil and rock types?

2. If a closed loop system for a hospital would require 300 holes but they don't have a year to wait around to get them all drilled, is it reasonable to expect that a drilling outfit would be able to run a few rigs simultaneously?

3. How much overhead clearance do you need to drill? If we were to drill a well field below a parking garage, could construction continue up above while the drill rigs were running below? I could perhaps provide 18 feet of clearance if we delayed pouring the lowest parking deck.

PS: Maybe the fact that we have almost nothing but glacial till up here would explain the high local drilling costs in the Seattle area.

by **Choice Mechanical** » Mon Nov 02, 2009 5:21 pm

We have to be firing on all cylinders, so to speak, to get two 300' holes done in a day in bedrock. I'd bank on 1.5 holes per day.

300 holes in a year is doable-for a drilling company that has several (more than 3) rigs. They can't neglect their other customers for a year, and I would imagine either using one big drilling outfit or several smaller ones, even from surrounding areas, to accomplish the task.

Our rig, and all those in our area, use 30' masts. Your local drillers could respond to the height issue, but 18' is pretty short headroom for the rigs I've seen. Might have to consider deeper bores if you're short on space.

by **down2earth geothermal** » Wed Nov 04, 2009 12:03 pm

The larger commercial projects (schools, hospital) are typically completed with a team of several larger drilling companies to get many (~300) boreholes done in a month or two. With the increase in scale comes a decrease in cost with a typical range, per IGSHPA-- being $6-15/ft. Seattle area should be primarily unconsolidated, saturated overburden so you are looking at mud rotary or sonic. 1,000' per day is probably reasonable in this geology using specialized rigs with automatic rod-handlers, hydraulic loopers and deep boreholes requiring few rig moves. The most I've seen reported is 2400'/day which would be crazy fast production.

18' of clearance would restrict you to a compact drilling rig with much lower production. You are probably better off drilling the loops prior to construction of the parking garage. Something to consider are specialized concrete footers with HDPE embedded that also serve as geo-loops that can be used which are more common in europe.

-Adam

by **Mark Custis** » Wed Nov 04, 2009 6:14 pm

Good looking out, I have seen such work in the Cleveland Ohio area. I am not sure why I am the only one noticing.

by **sunnyflies** » Mon Nov 09, 2009 10:05 am

Drilling costs for closed loops here on Long Island, NY is $20 a foot.

I've learned that the equipment needed for our sandy soil is different than what's needed for rock, which apparently is easier as it forms its own well casing. It seems some firm came down from upstate with a low bid for a job and after a week gave up because the hole kept caving in. Thankfully, it was not my job, which seems to be going well. I can hear them drilling outside right now.