Abstract
The 3.1- and 3.7-km-deep FFC-1 and DGE-1 geothermal explorations wells drilled into the Precambrian crystalline basement on the southern margin of the Fennoscandian Shield are evaluated regarding experiences from drilling, geological conditions, and thermal properties. Both wells penetrate an approximately 2-km-thick succession of sedimentary strata before entering the crystalline basement, dominated by orthogneiss, metabasite and amphibolite of the (1.1–0.9 Ga) Eastern Interior Sveconorwegian Province. The upper c. 400 m of the basement is in FFC-1 severely fractured and water-bearing which disqualified the use of percussion air drilling and conventional rotary drilling was, therefore, performed for the rest of the borehole. The evaluation of the rotary drillings in FFC-1 and DGE-1 showed that the average bit life was very similar, 62 m and 68 m, respectively. Similarly, the average ROP varied between 2 and 4 m/h without any preferences regarding bit-type (PDC or TCI) or geology. A bottomhole temperature of 84.1 °C was measured in FFC-1 borehole with gradients varying between 17.4 and 23.5 °C/km for the main part of the borehole. The calculated heat flow varies between 51 and 66 mW/m2 and the average heat production is 3.0 µW/m3. The basement in FFC-1 is, overall, depleted in uranium and thorium in comparison to DGE-1 where the heat productivity is overall higher with an average of 5.8 µW/m3. The spatial distribution of fractures was successfully mapped using borehole imaging logs in FFC-1 and shows a dominance of N–S oriented open fractures, a fracture frequency varying between 0.85 and 2.49 frac/m and a fracture volumetric density between 1.68 and 3.39 m2/m3. The evaluation of the two boreholes provides insight and new empirical data on the thermal properties and fracturing of the concealed crystalline basement in the Fennoscandian Shield Border Zone that, previously, had only been assessed by assumptions and modelling. The outcome of the drilling operation has also provided insight regarding the drilling performance in the basement and statistical data on various drill bits used. The knowledge gained is important in feasibility studies of deep geothermal projects in the crystalline basement in south Sweden.
Highlights
Enhanced Geothermal Systems (EGS) are defined as geothermal reservoirs that are created to be able to extract commercial amounts of heat from tight sedimentary formations or crystalline basement (Tester et al 2006)
The aim of this study is to evaluate and compare the results from the drilling operation and measurements conducted in the crystalline basement in FFC-1 and the results from the neighbouring DGE-1 well in Lund (Rosberg and Erlström 2019)
Our study evaluates the drilling performance, rock mass composition and physical properties as well as fracturing and thermal properties, which all play a significant role in assessing the bedrock prerequisites for EGS in south Sweden
Summary
Enhanced Geothermal Systems (EGS) are defined as geothermal reservoirs that are created to be able to extract commercial amounts of heat from tight sedimentary formations or crystalline basement (Tester et al 2006). Already in the 1970s research with hot dry rock projects as precursors to the EGS concept started (e.g., Armstead and Tester 1987; Brown 2009). Numerous EGS projects have since been performed mainly in relatively hot crustal regions from which knowledge has and is developed regarding establishing deep geothermal reservoirs in tight rock. The concept of creating a deep EGS in relatively cold crust for direct heat exchange is challenging but will, if successful, enable the use of geothermal heat in areas which, so far, have been disqualified for high-enthalpy geothermal systems. Knowledge on the EGS-related properties of the crystalline bedrock at greater depth in these areas and in Sweden is constrained and every new deep drilling project as the ones presented here provide invaluable new information and knowledge
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