Whether drilling water wells or for geothermal loop installation, many factors must be taken into account when grouting.
There are as many different reasons to drill a hole in
the ground as there are drillers that drill them, and each hole requires a
different grouting design. In some wells, such as oil wells, the grout not only
provides a pressure seal against formation pressure, but also provides a
significant portion of the physical strength of the well-bore and casing. For
this, cement grout is used almost exclusively.
The water well driller generally is concerned with a seal between possible
surface contamination of the producing aquifer, or commingling of different
aquifers. This may require just a small amount of grout at the casing shoe, or
a complete grout column to surface. Good drilling practices and local
regulations sometimes compete. For instance, where I live, regulations require
that grout be placed at the top 20 feet of a well. This will isolate the well
from surface contamination, but it will not assure a good seal at the casing
shoe, or prevent co-mingling of aquifers. The reason for this is simple, and
has nothing to do with the integrity of the well. It is much easier, and requires
much less time out of his air-conditioned office, for the inspector to see
grout at the surface, than to prove grout at the casing shoe.
In the past few years, geothermal loop installation has provided work for more
and more drillers, and the need for grout has risen. Grout in a loop hole has
some similarities – and many differences – from grout around a producing well.
In the first place, loops only produce or reject BTUs, not product. In order
for a loop to work, it must be able to transmit heat, either into or out of the
loop, to the surrounding formation. This requires the loop to be in intimate
contact with the formation. The simplest way to do this is to emplace grout.
The most common grout for this purpose is bentonite-based, but since bentonite
is a pretty good insulator, the grout manufacturers add a large amount of sand,
which has superior thermal transfer properties. This allows the grout
manufacturers to replace some of the relatively expensive bentonite with
cheaper sand, and charge more for the result.
The resulting grout bonds the loop to the formation, and facilitates heat
transfer at about the rate of the natural formation if it is a non-pervious
formation, like clay or unfractured rock. The loop must be in contact with the
formation to work properly. Voids will not work very well. This changes
dramatically in an aquifer.
Since an aquifer, by definition, contains water, and water conducts heat many
times better that dry formation, the better the contact between the loop and
the formation, the more efficient the loop will be. In this case, grout is an
insulator, and decreases the efficiency of the loop. It’s a little like putting
your loop in a thermos bottle. As an interesting side note, many loop
manufacturers now offer a little spring-loaded clip that presses the loop
against the formation to increase its efficiency. This effectively puts the
loop outside the grout. In a sand aquifer that can be induced to collapse
around the loop, the loop will transmit BTUs at least 25 percent better than
the most expensive thermal grout. Remember that sand is what “enhances” the
high-dollar geothermal grout. If you have 100-percent sand and no bentonite,
the enhancement is 100 percent.
I have proven this in the field many times in my area. In south Georgia and
the coastal geography generally in the top 200 feet is unconsolidated sand that
is saturated, and a pretty good aquifer. A loop can be installed, and the
formation induced to collapse by displacing the mud with fresh water, making
for very efficient heat transfer. The difference: In Georgia, the top 20 feet
of the hole must be grouted, and in Florida,
the hole must be grouted from bottom to top. I have installed systems within 2
miles of each other on both sides of the line. It generally takes 25 percent
more total feet of loops per ton on the Florida side because the grout
insulates the loops from the water in the aquifer.
Of course, grouting the top 20 feet of the hole will not isolate one aquifer
from another, but in our area, it is pretty much a continuous aquifer anyway,
so isolation is not an issue. This obviously would be a bad idea in area where
multiple aquifers are penetrated.
When choosing a bentonite grout, solids content always is a factor. In
non-permeable formations, the higher the solids content, the better the heat
transfer. Now 100 percent would be ideal, but is not pumpable by conventional
means, so we compromise to something we can pump. In a good aquifer, 0-percent solids
would provide the most heat transfer, but no sealing.
My point is that in any drilling application, many factors should be taken into
account. Formation (whether water-bearing or not), the purpose of the grout
(whether just for isolation or for mechanical strength), legal requirements,
good drilling practice, heat transfer, cost, pumpability and several others all are considerations
when planning a grout job. It’s not a one-size-fits-all deal.
The World According To Wayne: The Difference Between Geothermal Grouting and Well Grouting
February 1, 2011