Geological and Hydrological properties
When we are talking about geothermal resources, the first thing we have to look at is the ground we are standing on. In Calgary, the surficial deposits are mainly glacial tills. Tills act as good aquitards, and their thickness is around 13 m in the Calgary area (Meyboom, 1964). Below 13 m, the Saskatchewan sands and the Paskapoo Formation acts as confined aquifers. We will be focusing on the Paskapoo Formation.
The Paskapoo Formation
The Paskapoo Formation is Paleocene in age and is a mix of sands and mudstones (which acts as local aquifers and aquitards). As you go deeper into the formation, the higher the sand content is, meaning that it becomes a better aquifer (Meyboom, 1964).
The formation itself has a full thickness of around 800 m in the Calgary area, and a volumetric flow of about 18.2 L/day (Meyboom, 1964).
This formation is the best candidate for a shallow reservoir, but before we make this decision, some other things must be considered first, such as the hydrology of the formation.
Hydrology
The hydrology of the Paskapoo formation is an important first step in determining if this a good aquifer. Our calculations for the Paskapoo formation revealed that:
- The transmissivity of the formation ranges from 0.076 to 1840 m2 per day (Hughes, 2017; Meyboom, 1961).
- The specific storage ranges from 8 x 10-10 to 1.7 x 10-10 m-1 (Hughes et al., 2017; Augustine, 2015).
- Storativity of 6.4 x 10-7 to 1.4 x 10-5 (Meyboom, 1961; For more information on calculations, see appendix).
- This formation is also not exposed in a large area, meaning that recharge is very limited (Grasby et al., 2008; Meyboom, 1964).
However, this is not the only important information about the aquifer that needs to be taken into account. The Aqueous geochemistry is also important in determining if the formation is suitable for geothermal use.
Geochemistry
The aqueous geochemistry of the Paskapoo aquifer is an important part of the aquifer as well. The aquifer has a total dissolved solids (TDS) of around 800 ppm, which should not pose a big problem.
However, hardness (Ca+ and Mg+ ions) and sulfates (SO4-2) can cause scaling, which may, if left untreated, block extraction pipes. This is also made worse by high alkalinity (HCO3-, CO32-, OH-).
In addition to that, high chlorides may also cause corrosion (HCl; Regenspurg et al., 2015). Based on the graph presented, with everything combined, the aquifer has a total of 2040 ppm of dissolved solids.
The Paskapoo aquifer itself has relatively high hardness and alkalinity (Meyboom, 1964). On the other hand, it has a relatively low chloride and sulfates.
So, Can We Use the Paskapoo?
What's the difference?
So how does this compare to an average aquifer? For the hydrology itself, the range of normal values for a sandstone aquifer is highlighted below:
- Transmissivity = 10-4 to 10-3
- Specific storage = 10-6 to 10-5
- Storativity = 10-5 to 10-3
While our calculated Paskapoo Formation values are highlighted below:
- Transmissivity: 0.076 to 1840 m2
- Specific storage: 8 x 10-10 to 1.7 x 10-10 m-1
- Storativity: 6.4 x 10-7 to 1.4 x 10-5
Our Paskapoo aquifer has very high transmissivity but very low specific storage and storativity. This means that our aquifer has a high pump-ability with low storage capacity, coupled with low recharge. The problem with this combination is that we may run out of water as it is easy to pump but does not have a lot of water contained. High transmissivity also means that it is very easy for contaminations to flow, and this is really bad as the Paskapoo Aquifer is used as a major drinking source (Meyboom, 1964).
And the geochemistry?
For the aqueous geochemistry, the range of a normal TDS for all types of geothermal powerplant is from 1,000 to 300,000 ppm (Demir et al., 2014; Regenspurg et al., 2015; Scheiber et al., 2019).
In the case of the paskapoo aquifer which is a shallow, direct use plant type, a ppm of <1,000 is preferable. It is however, possible to use groundwater with higher ppm values than 1000, as there is less change in temperature and pressure compared to deeper and hotter geothermal plants. While this is not problematic, it is also not ideal as it will then require relatively high maintenance.