How Do Drilling Fluids Affect a Geothermal Installation?
Before the homeowner turns up the thermostat, before the HVAC contractor installs the equipment, and before you place the loops and grout the borehole, there is Step 1: the drilling of a good hole that stays open and allows for the loop to be installed to the required depth.
To many, that sounds simple. But recent geothermal conferences focus so much on the geothermal aspect of the system that we sometimes forget about the borehole. It does not matter if we are drilling a water well, environmental well or a geothermal hole — the basics of keeping the hole open are the same.
A case in point was a driller who called me shortly after I started with CETCO Drilling Products. He was drilling geothermal holes in a sandy area along the coast in the Northeast. His crew had been on site over the allotted time for the job and they were fighting the soil conditions on every hole. They would get the hole to 100 feet and then only get 50 feet of loop down before they hit a collapsed zone. This was not their normal drilling area and the owner of the drilling company asked me to see if I could help on site. When I arrived, the first thing I noticed was that the bentonite drilling fluid was very thin. It was a good mix for clay, but not for coastal sand. In short order we doctored up the mud pan and we were off to the races!
When we went over the mix, the driller explained that he thought that, because they were just installing loops and not water well casing, they could get by with a straight bentonite mix and wrestle the loops in. Did I mention they were new to geothermal drilling? I have told this story while teaching recent geothermal classes and at least one person will fess up to the same experience.
Another reason for this back to basics article is that at recent geothermal shows I’ve met a lot of HVAC contractors who had recently purchased rigs to “do the complete job.” Many were straightforward about their knowledge or lack of knowledge about the drilling segment of the job. Many states and water well associations are dealing with the licensing and limited licensing of these contractors. That is a whole other article. The fact is that a borehole is a borehole no matter what its purpose — period.
Keeping it really simple for this text, you need to follow the commercial bentonite company’s mixing instructions for the soil you are drilling in. In sandy conditions, adding a PAC polymer to your drilling fluid can tighten up the filter cake and stop the fluid from seeping into the surrounding soil. In clay, depending on the type of clay, you might need to add a synthetic polymer to inhibit the swelling. In recent years some commercial bentonite producers have developed specialized polymers to cut sticky clays down.
Another issue to think about is saltwater intrusion and the effect it has on sodium bentonite clay. Brine water is not a friend of sodium bentonite, but there are ways to correct for small levels of intrusions and other drilling fluid products that can be used in high saltwater areas. Ask your distributor or drilling products representative for more information on their particular products.
To summarize, the drilling operation is as important as the installation and proper loop grouting. Cutting corners or not planning for the job’s soil conditions can waste time. As my driller found out on the coast, time is money! So drill it right and keep it tight.