There are three critical factors to create a commercially viable geothermal project: very high-water flow rates (e.g. 300 l/s to generate 5MW net electrical power @ 120°C); temperatures generally above 110°C; and drilling depths of 4,500 m or less. In Canada’s Western Sedimentary Basin (WCSB), especially in Alberta, these conditions are met in a localized number of places, mostly in the central to northwestern parts of the province.  Hot water (less than 110°C) is available over a much broader area, and could provide significant energy resources for direct-use applications at relatively shallow depths.

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In a sedimentary basin, the key factor is water flow and this is what makes geothermal drilling and development significantly different from oil and gas drilling. To exploit the subsurface heat resources there are four main factors:

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1. Well bore diameter (drilling);

2. Pump size (capacity);

3. Pump setting depth (parasitic load); and

4. Power plant efficiency.

 

Some of the important aspects related to well bore diameter (drilling) are the following:

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• Mass flow and energy: The lower the temperature, the more water (mass) that must be pumped to get the same energy value. This means large diameter wells, big pipe, and large pumps (see the figures below).

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• Completions: “Completions” are how a well is constructed, or in another way, how they are drilled and lined with pipe to recover fluids.  Large diameter wells are required for the massive quantities of water pumped out of the well and to do that very large “Electrical Submersible Pumps” (ESP) are needed.

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Schematic representative (not to scale) of an O&G completion (left; note the tiny ESP) and a geothermal completion (right; note the much large ESP set is a wide diameter well bore) (Teodoriu and Falcone 2008).

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A schematic showing the actual differences in well bore sizes commonly used in O&G and Geothermal wells.

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•  Well Longevity: Because resource extraction can last for decades, the wells must be drilled and constructed in a very robust manner. This includes more cement to hold in the pipe in place over decades, made more challenging because of the large diameter holes.

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• Well Testing: Because geothermal wells are much larger and are drilled with long sections of very porous rock standard Oil and Gas testing procedures responds in different ways.

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• Fluid Production: As can be seen in the diagram (above), in a geothermal well, water is produced through an “open hole” whereas O&G is produced through much smaller tubular completions. This impacts a number of different aspects of the well drilling and completion.

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As a final comment, due to their size, challenges in cementing, lost circulation, and other factors, deep geothermal wells will typically take 3 to 4 times longer to drill and complete than oil and gas wells.  They also take significantly longer to test for reservoir characteristics, but when complete, they can provide low cost energy on a generational time scale.