Did you know... geothermal wells can be highly deviated too?
Just as in the oil industry, the first geothermal wells were all vertical, which remains common practice mainly because it is cost-effective. The maximum depth is typically about 10,000 feet (3 km). Deviated geothermal wells have been drilled too, extending laterally over horizontal distances up to about 5,000 feet (1.5 km) and dipping at angles of less than 45° as measured from the vertical. In most geothermal fields, the rock formations are made up of volcanic and sedimentary rocks.
New groundbreaking developments are now happening in the geothermal industry borrowing methods used for unconventional hydrocarbon development. Recently, a well having a long horizontal leg was drilled for the DEEP geothermal project in the Williston Basin, Saskatchewan, Canada.
The type of rocks being drilled into for geothermal development are relevant because up until now very few have reservoirs hosted by granitic rock, which is abrasive and hard on wear and tear of downhole equipment. Examples of such wells include 14-2 at Roosevelt Hot Springs, WD-1A at Kakkonda, Habanero 1 in the Cooper Basin, 33A-7 at Coso and OTN-3 in Finland. Of these, WD-1A has the hottest bottom hole temperature (~500°C), and OTN-3 is the deepest, but for this rock type, highly deviated wells are absent.
The new deep well at Utah FORGE, 16A(78)-32 is thus notable. It shows that the drilling of sub-horizontal well trajectories in granitoid are achievable. Such highly deviated wells are required for EGS wells in order to intersect a large number of sub-vertical fractures and to maximize energy production.
Figure showing the geothermal well profiles, host rocks and deviation angles: conventional wells in red; sedimentary basin wells in green (Saskatchewan, Canada; Groß Schönebeck, Germany); metamorphic-plutonic well in blue (Helsinki, Finland); granitoid wells in black (Roosevelt Hot Springs, Utah; Kakkonda, Japan; Cooper Basin, Australia; Soultz-sous-Forêts, France; Coso, California); granitoid well in pink (Utah FORGE).
References
Ayling et al. 2016, Geothermics 63, 15-26. https://www.sciencedirect.com/science/article/pii/S0375650515000395
Kwiatek et al. 2019, Science Advances,5 https://www.science.org/doi/10.1126/sciadv.aav7224
Ledésert & Hébert 2012, Heat Exchangers - Basics Design Applications, 447-504, https://doi.org/10.5772/34276
Muraoka et al. 1998, Geothermics, 27:507-535. https://www.sciencedirect.com/science/article/pii/S0375650598000315
Sabin et al 2016, Proceedings Stanford Geothermal Workshop (https://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2016/Sabin.pdf)
Zimmerman et al. 2010, Geothermics, 39, 59-69 (https://www.sciencedirect.com/science/article/pii/S0375650509000674)
