Using a one-size-fits-all solution when it comes to geothermal grouting will provide disappointing results. While grouting may have historically been seen as a means for maintaining low costs, it’s something that should never be sacrificed in a bid to be competitive.
In an earlier column, I talked about the importance of grouting vertically or horizontally bored geothermal loops as soon as the pipes are inserted into the holes. Proper grouting at the right time and with the right material provides an all-important environmental surface barrier and thermal connection between the pipes and the ground surrounding it, while also protecting the quality of the groundwater.
Grouting techniques within the geothermal industry need to be of the highest calibre. This starts with effective planning at the design stage. There are a few things to keep in mind when determining which grouting material will be most effective in an installation:
- What is the anticipated geology of the land on which you will be drilling?
- Where are existing wells on adjacent properties in relation to where you will be drilling?
- Are there flowing conditions or naturally occurring gas in the area where you will be drilling?
When it comes to determining which grout material will be most effective in terms of thermal conductivity and long-term sustainability of the loop system, the geology of the landmass you are drilling is very important. Bedrock is porous and can act like a sieve, so it’s vital to select a grout mixture that will prevent the grout from running off the bedrock. This is particularly true when drilling in areas like the Niagara Escarpment in Ontario, where the geology is largely limestone and porous rock. This usually calls for a thicker grout in zones that are highly fractured.
A standard mix grout eventually disappears into the bedrock, leading to less thermal conductivity in the loop and possible holes in the ground where the grout has given way. One of the options when grouting in porous rock conditions is to use granular bentonite. It will do a good job of filling the hole and won’t drain away.
We recently completed a geothermal installation where the middle of the drilled hole was porous, but the geology above and below that point was denser. In this instance, we elected to grout the bottom of the hole, add coarse bentonite in the porous zone, and then regrout from that point to the surface. While the material costs were higher, we recognized that the geothermal loop would function at its optimum level because we used the grouting solution most effective for this particular installation.
In areas like Toronto, on the other hand, where the geology is primarily shale, more compact and less porous, the type of grout used doesn’t raise the same challenges you face with bedrock.
It is wise to review existing well records to see where neighbouring property wells are located. If the ground in the area you are drilling is fractured and porous, the grout you are using could potentially leak into an adjacent well. Knowing where these wells are in advance, as well as the geology of the landmass in the area, may help determine the depth of the holes and the type of grout material.
Naturally occurring gas pockets flowing conditions
If you are drilling in an area prone to, for example, natural gas pockets, you should be using neat cement with bentonite to grout the holes around your loop. Neat cement grout cures to a hard, chalky consistency and is ideal for preventing erosion due to groundwater seepage and for preventing natural gas from percolating out of the ground. Neat cement has a slightly higher conductivity than 20 per cent solid bentonite grout, and can be thermally enhanced by adding sand much the same.
Over the years, the geothermal industry has introduced grouts with higher conductivity that enable them to transfer more heat than the standard 20 per cent solid grout largely used by the water well industry. Geothermal grouts tend to be a blend of bentonite and sand or bentonite and graphite. The goal of these grouts is to generate higher conductivity and protect the quality of the surrounding groundwater.
Regardless of which grout you use, the material you select should be based on the geology of the land, the presence of adjacent water wells, the presence of natural gas or oil, and its ability to protect the quality of the groundwater and generate efficient heat transfer in the loop system.
Stan Marco is a well-respected and highly sought after geothermal knowledge expert and educator. He is an active member within the ground water community, a board member with the Canadian GeoExchange Coalition and cofounder and CEO of GeoSmart Energy & GeoSmart Drilling Services.