Page 154 - Geothermal Energy Renewable Energy and The Environment

140 Geothermal Energy: Renewable Energy and the Environment

environment. The extent to which this slurry penetrates the wall rock will depend on the local per-
meability and can be highly variable (Figure 8.3). The mechanical strength of the gel is sufficient to
hold in place loose material in the wall rock, as well as reduce the incidence of wall collapse.
Occasionally, zones of very fractured rock are encountered. If the extent of the zone is large
enough and its permeability high enough, the drilling mud can preferentially flow into it rather
than down the hole. To lose circulation of the drilling mud in such a way can be a serious problem.
Drilling cannot proceed until such zones are sealed. This is attempted via a variety of means, most
commonly by pumping large volumes of mud into the hole or changing the mud formulation. If the
lost circulation zone cannot be sealed, there is no choice but to abandon the hole. This is, of course,
a costly consequence.

well compleTion
Once drilled, the hole must be finished in a way that facilitates its use for the application being pur-
sued. If the hole is to be used for a closed-loop ground source heat pump system (see Chapter 10 for
a detailed discussion of these systems), the loop or loops of pipe are placed in the hole and the hole
is then grouted. Grouting is a process in which a permanent material is placed in the hole to keep it
sealed and stable. Grouting also performs the function of isolating the borehole from the surround-
ing environment, thus preventing exchange of chemicals or contaminants between the borehole and
any local aquifers, as well as preventing cross communication between aquifers. In a ground source
heat pump application, it is also important that the grout have high thermal conductivity in order to
facilitate heat transfer between the closed-loop pipe and the geological environment, and that the
grout forms a strong bond with the borehole wall in order to prevent water circulation along the wall
that would reduce the heat transfer efficiency.
If the purpose of the hole is to obtain geothermal water for use in aquaculture, spas, green-
houses, or other direct use applications, a liner or casing is put in place that will assure the per-
manence of the hole and protect equipment that may be placed in the hole, such as piping, pumps,
and sensors. The casing is grouted into place to assure the same environmental and structural
standards are satisfied as in the case of the completely grouted holes for the ground source heat
pump applications.
Grouting is composed of a mixture of various compounds that can be pumped when initially
mixed together but then will harden in place. Various cements or special bentonite-polymer mix-
tures are commonly used. The advantage to these grouts is that they have much lower permeabilities
than the surrounding rock, which thus assures that the hole will be physically isolated from the
environment. They also have high strength, which helps assure the integrity of the hole.

environmenTal issues
We have discussed the importance of assuring that, as a well is being drilled and once it is com-
pleted, it remains isolated from the surrounding environment, particularly if it penetrates an aquifer.
Accomplishing this requires careful attention to assuring that the permeability immediately adja-
cent to a well wall is sealed and that a membrane of grout or cement plus casing, if appropriate, is
sound and complete.
Additionally, drilling that requires use of a drilling mud requires construction of a pit in close
proximity to the drilling rig. The pit is used to hold the mixed mud and it is from this pit that the
mud is pumped into the well to cool the bit, remove cuttings and seal the well wall. Mud that returns
from circulation carrying the cuttings must also be dumped in a pit. At the completion of drilling,
the pits should be cleaned up and the local landscape restored. How this is done, and the require-
ments for meeting local regulations will depend upon the regulatory agency that has jurisdiction
over such matters. In some instances this expense can be significant and must be factored into cost
projections for a project.