Drilling experiences
The Rate of Penetration (ROP) acquired in FFC-1 when drilling in the crystalline basement illustrate that high penetration rates, up to 15 m/h, were obtained during the attempts with percussion drilling using air, between 2150 m and 2242 m depth (Fig. 3). Cost consuming trips to inspect hammer, as well as difficulties to monitor downhole hammer function, and hole cleaning due to heavily fractured and inflow of formation fluid led to the abandonment of the percussion drilling method. The Caliper log also verifies that the basement down to c 2500 m included several zones of weak and unstable rock that resulted in significant break-out and greater borehole diameter (Fig. 3). These zones were also associated with inflow of water, which rendered difficulties to clean the well as well as getting the hammer to properly operate. However, it is not concluded if the method could have worked at greater depth with less fractured rock and less inflow of water.
The ROP for the conventional mud rotary drilling varied between 1 and 4 m/h, the values are similar for the ROPs acquired during the drilling of the crystalline basement in DGE-1 (Rosberg and Erlström 2019). Larger bit dimensions and initially different drilling method, air rotary, was used in the crystalline basement in that well. Cardoe et al. (2021) report average ROPs between 2.3 and 2.6 m/h for the sections drilled with rotary techniques in the two deep wells in Espoo, Finland.
In total, 15 drill bits were used in FFC-1, seven PDC and eight TCI roller cone bits, see Fig. 4. Two of the used drill bits, one TCI roller cone and one PDC, are shown in Fig. 5. Drill bit manufacturers often use their own product names and nomenclature and to be able to compare the drill bits presented in Fig. 4 the IADC code (International Association of Drilling Contractors) is used. Information about the IADC code can be found in e.g., IADC Drilling Manual (2000) and there is also so called IADC calculators available on the web. In average the drill bits drilled 58 m before being changed or 66 m if the two bits that only lasted for 2 and 3 m are excluded. Three drill bits lasted more than 100 m and where all used in combination with a downhole motor. The longest distance drilled with one bit was 146 m and it was achieved using a PDC drill bit. The operational parameters varied within the following intervals during the bit run: WOB: 2.3–10 ton, Bit RPM: 165–215, ROP 2–6 m/h, Torque: 3–10 kNm(E.ON 2021). Three of the PDC bits also resulted in the highest average ROP obtained during the different bit runs, 3.1–3.3 m/h.
The drill bit summary from the drilling in the crystalline basement in DGE-1 is also included in Fig. 4. The drill bits used before and after the whipstock installation are presented separately. Air rotary drilling was used before and mud rotary after whipstock installation, and in both cases larger drill bit sizes than in FFC-1 were used. Detailed information about the used drilling methods, drilling dimensions and the whipstock installation can be found in Rosberg and Erlström (2019). Totally, 21 drill bits were consumed before the whipstock installation and in average the drill bits lasted for 68 m. Three drill bits lasted more than 100 m and the longest distance drilled with one of the TCI roller cone bits was 152 m. It can also be seen in Fig. 4 that there is a learning curve, after around 175 m drilling into the crystalline basement the meters drilled by each drill bit increased. In this case it is more likely due to experienced gain during the drilling operation than due to geological changes. In total nine drill bits were used after the whipstock installation and in average the drill bits lasted for 56 m or 62 m if the water hammer prototype bit is excluded. Two of the drill bits lasted for around 110 m. The lessons learnt from comparing the drill bit consumption during the two deep crystalline basement drillings in Skåne are that the drill bits lasted in average between 62 and 68 m. Quite similar results, despite different bit dimensions and drilling fluids were used. This is somewhat unexpected since a larger dimension TCI bit normally lasts longer than a smaller one, because of larger and more robust bearings. The rate of penetration is also quite similar when comparing the two drillings as well. It seems like PDC bits were a better option than TCI roller cone bits in FFC-1, but to make this evaluation more correctly information about the bottom hole assemblies (BHAs), operational parameters and the rock composition must also be included in the evaluation. It can for example be noted that the best bit performance was achieved using a downhole motor in the FFC-1 drilling. In this paper, the drill bit consumption in the two wells have been compared but not the drill bit performance in respectively well. One way to do this is to compare the mechanical specific energy (MSE), initially presented by Teale (1965), for the different bit runs. The MSE is ratio the between the mechanical energy input from the drill rig and the responding ROP and it can also be explained as the energy required to remove a unit volume of rock. That different drill bit dimensions were used could be considered if the MSE is calculated.
There are room for improvements to decrease the drill bit consumption when drilling new deep wells in the Fennoscandian Shield Border Zone. Longer bit life and a lower drill bit consumption will markedly decrease the total well cost or expressed as footage cost/cost per meter. Cardoe et al. (2021) reported that the average rotary bit life in the first deep well drilled in Espoo, Finland was 69 m, which is like the values obtained in FFC-1 and DGE-1. The same paper also describes that a special bit design was made before rotary drilling was applied in the second deep well, which increased the average bit life to 116 m. Recently, Energy and Geoscience Institute at the University of Utah (2021) reported exceptional results for a 9145 ft (2787 m) deep well, where three 8.75” (222 mm) PDC bits were changed after drilling 332 m, 368 m and 376 m, respectively in granodiorite. The longest bit life, with almost the same dimension 8.5” (216 mm), of the TCI bits used in DGE-1 well was around 110 m. Despite it is not the same bit type and used in a much younger rock, it is still at least three times less drilled in comparison to the PDC bits used in Utah.
The evaluation of the percussion drilling using air is omitted from this paper, but still one observation can be made that the drilling method cannot be applied if the water influx is high. Percussion drilling using air has been successfully used in the deep EGS-drillings in Espoo, Finland (e.g. Think Geoenergy 2019) and during the drilling of DGE-1 both air rotary drilling and the testing of percussion drilling were applied down to 3365 m, which is described in Rosberg and Erlström (2019). The main difference if the two drillings are compared with FFC-1 is that problems with water influx is only reported from FFC-1.
Bedrock composition
The description of the cuttings (examples are shown in Fig. 6) and correlation with the Spectral Gamma Ray log (Fig. 7) gives that the crystalline basement in FFC-1 is composed of two rock suites. The relative distributions of the various rock types in FFC-1 are compared with DGE-1 in Table 1.
The bulk part (80.6%) of the penetrated rock mass in FFC-1 is composed of different gneisses, gneiss granite and gneiss diorite. These silica-rich rocks (> 70% SiO2) are dominated by quartz, feldspar, and minor amount of minerals such as mica and hornblende. The colour varies from dark red to light red and grey. Darker red banded varieties with higher amount of dark minerals such as biotite are predominantly found in the upper part of the FFC- 1 borehole down to c. 2300 m depth. The varieties occurring below are less foliated and relatively quartz-rich and greyish. There are also scattered thin intervals with very quartz-rich and mica-rich (muscovite) gneiss at 2630–2640 m, 2756–2686 m and 2835–2841 m.
Table 1
Relative distribution of rock types in FFC-1 and DGE-1
|
FFC-1 Relative distribution, %
|
DGE-1 Relative distribution, %
|
Amphibolite
|
19.4
|
16.2
|
Light red-grey gneiss
|
33.1
|
32.6
|
Grey quartz-rich gneiss
|
32.3
|
-
|
Dark red-brown gneiss
|
15.2
|
40.0
|
Granite
|
|
7.4
|
Dolerite
|
|
3.8
|
The amphibolite/metabasite/amphibolite-gneiss rock suite represents a group of metamorphic silica-poor mafic rocks dominated by hornblende and plagioclase. The low content of potassium feldspars is also portrayed in the Spectral Gamma Ray log. These rocks are characterized as fine- and medium-crystalline, mostly dark grey and black and often with a white spotted texture of light-coloured plagioclase in a matrix dominated by hornblende and biotite. Several of these rocks can also be classified as amphibolite-gneiss since the quartz content is relatively high and they show a weak foliation.
The mafic rocks constitute 19.4% in FFC-1 and occur both as thick bodies in the upper part and lowermost part of FFC-1, and as meter-thick and thinner bands/streaks within the gneiss- dominated intervals. The density ranges between 2.9 and 3.2 kg/dm3, which is significantly higher than gneisses with densities between 2.6 and 2.7 kg/dm3. This difference is also significantly pictured in the PhotoDensity log (Fig. 3), which furthermore helps to differentiate these rock types in the borehole. Larger bodies of amphibolite also provide conditions for delivering strong seismic reflectors, which is verified in the older seismic data in Malmö where the thick amphibolite at the bottom of the borehole (3025–3133 m) links up to a seismic reflector (Hammar et al. 2021). Likewise, strong seismic reflectors in the basement in DGE-1 were confirmed to originate from changes in the acoustic impedance at the gneiss-amphibolite boundaries (Rosberg and Erlström 2019).
The two dominating rock suites in FFC-1 and DGE-1 correspond to the Sveconorwegian gneiss-dominated terrain in SW Sweden exemplified by the outcrop areas on the Romeleåsen Ridge and in northwest Skåne, e.g., Kullen and Söderåsen (Fig.1).
The gneisses are quite similar in both wells. The main difference is that there is a greater proportion of dark red-brown, foliated and banded gneiss in DGE-1 and that the quartz-rich variety in FFC-1 is not distinguished in DGE-1.
Another notable observation is the absence of Permo–Carboniferous dolerites in FFC-1. These, rocks are a common part of the rock mass in DGE-1 as well on the Romeleåsen Ridge and within the Sorgenfrei-Tornquist Zone in Skåne. This difference could be explained by that the main phase of Variscan rifting and emplacement of NV–SE dolerite dykes in Permo–Carboniferous time was mainly within the Sorgenfrei-Tornquist Zone and not as frequent outside this zone to the southwest.
There is also a notable difference in the overall thorium and uranium content in the rock mass in FFC-1 in comparison to DGE-1 (cf. Spectral Gamma log in Fig. 7). The significantly lower values in FFC-1 are interpreted to represent the relatively U-Th-depleted rock mass in the Eastern interior Sveconorwegian segment in comparison to the eastern boundary and transition segments (data from the radiometric map of Sweden; Geological Survey of Sweden).
Fracturing and structure of the rock mass
The upper ca 500 m of the crystalline bedrock is found to be severely fractured in FFC-1. The Caliper log shows over-sized and very poor borehole conditions (Fig. 3), which affects the quality of the geophysical logs in this section. Below 2500 m the fracturing is less significant to the borehole which provided good operational conditions for the CXD and the SCMI tools. The results from these gave unique and novel data on the fracturing of the rock mass at these depths. The first study of the fracturing, performed by Weatherford on commission by E.ON, shows that there is conclusive evidence of a conductive fractured zone between 2562 m and 2695 m (Badulescu et al. 2021). The fracture volumetric density as well as the fracture frequency in this zone is significantly higher (average 3.39 m2/m3 and 2.49 frac/m) than below where it is generally less than 2 m2/m3 and 1–2 frac/m (Fig. 3, Table 2). The fracture frequency is commonly > 4 frac/m for the most fractured part of the zone where also individual fracture apertures reach up 12 mm (Ciuperca et al. 2021).
A calculation of the linear fracture frequency for three main borehole intervals including the fractured zone (2562–2695 m) and the interval above (2450–2522 m) and below (2695–3106 m) is shown in Table 2. No values could be calculated above 2450 m due to poor borehole conditions. The average linear fracture frequencies in these three major intervals ranges between 0.85 and 2.49 frac/m and average volumetric fracture densities between 1.68 and 3.39 m2/m3.
Borehole data on the spatial distribution and orientation of fractures are, besides in situ stress and hydraulic data, essential parameters to build Discrete Fracture Network (DFN) models. However, it is not a straightforward process to rate the linear fracture frequency and the volumetric fracture density in FFC-1 with respect to the rock mass feasibility to be hydraulically stimulated and suitable as a geothermal reservoir. There are also few reference values on the linear fracture intensity for the Fennoscandian crystalline basement at greater depths in Sweden. Existing data comes primarily from the Swedish Nuclear Fuel and Waste Management Company (SKB) borehole investigations down to c 1000 m depth. Their data from boreholes in granitoid rocks at Laxemar, on the east coast of Sweden, give an open fracture frequency generally below 3 frac/m and a volumetric fracture density between 1.4 and 4.6 m2/m3 (La pointe et al. 2008, SKB 2009). Another reference data set comes from image-log data of the spatial distribution of fractures in the crystalline basement in the deep geothermal projects in Basel in Switzerland, Soultz-sous-Forêts in France and the Rosemanowes site in the UK. These give fracture spatial distribution signatures for depths ranging between ca 2000 to 5000 m which are mostly below c. 1 frac/m (Meet 2019). In addition, < 0.3 frac/m are reported between 6900 and 7135 m depth in the KTB deep borehole (Zimmerman et al. 2000).
Comparable reference values on the fracture volumetric density are even more scarce. A study by Rogers et al. (2015), even if this is on relative shallow rock masses, indicates that the transition from a massive rock mass to a more kinematically governed blocky rock mass occurs at c. 2–2.5 m2/m3.
With respect to the scarcity of data and difference in geological setting between the various reference values, it appears that the FFC-1 data fall within what is generally observed for other deep drilling projects in crystalline rocks.
Table 2
Calculated linear fracture frequency for three intervals in FFC-1.
Borehole depth
|
Interval thickness
|
Number of fractures
|
Linear fracture frequency,
frac/m
|
Volumetric fracture density, m2/m3
|
2150–2450 m
|
300 m
|
No reliable data
|
--
|
--
|
2450–2562 m
|
112 m
|
181
|
1.62
|
2.67
|
2562–2695 m
|
133 m
|
329
|
2.49
|
3.39
|
2695–3106 m
|
411 m
|
353
|
0.85
|
1.68
|
The dominant strike of the SCMI identified conductive borehole cross-cutting fractures, between 2154–3106 m is N–S. There is also a less dominant NE–SW strike noted. Remarkably there is no significant NW–SE strike, which is otherwise the general direction of the main faults in Skåne. (Badulescu et al. 2021). The conductive fracture sets are interpreted to be open and correspond to the NW–SE strike-slip and associated N–S extension in Skåne since the Permo-Carboniferous (Bergerat et al. 2007). These initial interpretations will, however, be further scrutinized in an ongoing in-depth analysis.
Scattered iron oxide coatings on the fracture/fissure planes, are identified on the cuttings, especially for the section down to c. 2500 m. Only few findings of these were noted in the deeper part of the borehole. Besides this, a white, soft non-calcareous clayey material is found in most samples from the gneiss dominated intervals. XRD and chemical analysis give that it is mainly composed of feldspars, quartz and mixed-layer clay minerals with chlorite. A possible explanation is that these are low temperature alteration of feldspars, a process that is common in granitoids (cf. Plümper et al. 2009; Morad et al. 2010). Noticeable is that the same type of material also occurs frequently in the DGE-1 cuttings (Rosberg and Erlström 2019). But it still needs to be clarified how these likely hydrothermal alterations are found in the rock mass. Are they primarily found in association to fractures or are they more evenly dispersed in the rock?
There are also frequent occurrences of calcite fracture fillings in the cuttings, especially in the metabasite and amphibolite intervals. Beside these mineralisations there are also greenish undulating fracture fillings with epidote and chlorite found in the gneiss. The overall fracture fillings found in FFC-1 and DGE-1 agrees well with the fracture mineralogy in rocks from the Dalby quarry (Halling 2015).
The overall structure, foliation, banding and folding, of the bedrock in Skåne is interpreted by Ulmius et al. (2018) and by Wikman et al. (1993) to be steeper in the boundary zone and gradually more horizontal in the interior part of the Eastern segment in the Sveconorwegian Province. A dominant low angle and horizontal foliation is identified in the CXD and SCMI logs in FFC-1 fits with the overall structure of the interior part of the Eastern segment. The seismic reflectivity of the upper part of the crystalline basement around FFC-1 also displays a dominant sub horizontal signature that overall fits this model.
Thermal conductivity, density, specific heat capacity and calculated in situ thermal conductivity
Thermal conductivity, density and specific heat capacity measured on cutting samples from FFC-1 are presented in Table 3, based on analysis by Klitzsch and Ahrensmeier (2021). Results, except specific heat capacity, from similar bedrock samples from Dalby quarry on the Romeleåsen Ridge are also included in Table 3. The measured thermal conductivity values on cuttings dominated by gneiss varied between 3.85 and 3.91 W/(mK) and between 2.54 and 2.59 W/(mK) for cuttings dominated by metabasite/amphibolite. In addition, the thermal conductivity was measured to 3.1 W/(mK) on cuttings dominated by gneiss-granodiorite. The values are used in Eq. 2 for calculating the in-situ thermal conductivity. The calculated average kin−situ is 3.57 W/(mK) for gneiss, 2.35 W/(mK) for metabasite/amphibolite and 2.85 W/(mK) for gneiss-granodiorite. Based on the geological classification the thermal conductivity for the gneiss-granodiorite is later used for calculating the heat flow in the open hole section down to 2693 m and the higher value for the gneiss is used, below this depth. In addition, the thermal conductivity value for metabasite/amphibolite is used for intervals dominating by these rock types.
Table 3
Compilation of thermal conductivity and density measured on cutting samples from FFC-1 and bedrock samples from Dalby quarry on the Romeleåsen Ridge. Specific heat capacity measured at 30°C and 100°C on the cuttings from FFC-1 is also presented.
Sample
|
Rock type
|
Thermal
conductivity (W/mK)
|
Density
(kg/dm3)
|
Specific heat capacity
(J/kgK)
|
FFC-1
|
|
|
|
at 30°C/100°C
|
2771–2774 m 1)
|
Grey-light red-pinkish, quartz-rich gneiss, with < 5% of basic rocks
|
3.85 ± 0.05
|
2.657 ± 0.02
|
770/877
|
2873–2875 m 1)
|
Grey and light red gneiss with < 10% metabasite and diorite
|
3.95 ± 0.03
|
2.656 ± 0.02
|
765/865
|
2954–2956 m 1)
|
Grey and light red gneiss-granodiorite/ with < 15% metabasite
|
3.10 ± 0.03
|
2.694 ± 0.03
|
770/877
|
3098–3100 m 1)
|
Amphibolite/metabasite with < 10 % amphibolite gneiss and grey-red gneiss
|
2.54 ± 0.02
|
2.994 ± 0.03
|
778/880
|
3104–3106 m 1)
|
Metabasite/amphibolite gneiss < 5% grey-red gneiss
|
2.59 ± 0.02
|
2.969 ± 0.02
|
776/879
|
Dalby quarry, 2) outcrop sample
|
Red granitoid/gneiss, fine, foliated
|
3.55 ± 0.10
|
2.617 ± 0.05
|
n/a
|
-“- 2)
|
-“-, massive, fine, granitic
|
3.23 ± 0.10
|
2.622 ± 0.05
|
n/a
|
-“- 2)
|
-“-, medium, foliated, biotitic
|
3.46 ± 0.10
|
2.619 ± 0.05
|
n/a
|
-“- 2)
|
Light red–grey, orthogneiss, fine
|
3.58 ± 0.10
|
2.616 ± 0.05
|
n/a
|
-“- 2)
|
Red orthogneiss, medium, weakly foliated
|
3.52 ± 0.10
|
2.628 ± 0.05
|
n/a
|
-“- 2)
|
Amphibolite
|
2.38 ± 0.10
|
2.960 ± 0.05
|
n/a
|
-“- 2)
|
Amphibolite
|
2.16 ± 0.10
|
2.908 ± 0.05
|
n/a
|
1)Cuttings sample analysed by Klitzsch and Ahrensmeier (2021), 2) rock sample from outcrop analysed by the Geological Survey of Sweden
|
It can be seen in Table 3 that samples dominated by gneiss has a significantly higher thermal conductivity and lower density than samples dominated by metabasite/amphibolite. There is also a good agreement between density values obtained from the cuttings and the values obtained from the outcrop samples. The thermal conductivity values measured on the cuttings from FFC-1 is relatively close but higher than the values measured on the outcrop samples.
The specific heat capacity is an important parameter for future thermal modelling of an EGS-system. Klitzsch and Ahrensmeier (2021) measured the parameter using a calorimeter for different temperatures. Unfortunately, there are no measurements on the outcrop samples. In Table 3 the values for gneiss dominated samples are quite like the values obtained from metabasite/amphibolite. In case the values are expressed as volumetric heat capacity, the average value for metabasite/amphibolite, 2.32 and 2.62 MJ/m3K is greater than the average for the gneiss dominated samples, 2.05 and 2.33 MJ/m3K, measured at 30°C and 100°C, respectively. The values on the FFC-1 samples correlate to what is known for cored boreholes in granitoid rocks at Laxemar on the west coast of Sweden, thus, further in on the Fennoscandian Shield. Sundberg et al. (2009) present heat capacities for these rocks that are between 2.16 and 2.23 MJ/m3K.
Thermal gradient
The bottomhole temperature in the FFC-1 borehole is 84.1°C. In the upper part, above 2610 m, the mean temperature gradient is 23.5°C/km and in the lower part, below 2880 m, the mean temperature gradient is 17.4°C/km, see Fig. 3. The zone in between seems to be thermally disturbed since the temperature gradient dropped from 23.5 to 7°C/km. Below the zone the gradient increased again to 17.4°C/km (Fig. 3). The temperature anomaly is interpreted to be caused by water influx through conjugated open natural fractures which are identified in the logs by lower density, sonic anisotropy, changes in the brittleness index polarity, increase of the fracture volumetric density, increasing fracture aperture and the presence of Stoneley chevron up-going and down-going reflections (Ciuperca et al. 2021). Another explanation can be that parts of this interval was acting as a loss zone during the drilling operation, meaning that the colder drilling fluid has propagated into and cooled parts of this formation interval. In comparison with the average temperature gradient, 22°C/km in the DGE-1, the upper part in FFC-1, above 2610 m, has a 2°C/km higher gradient, but in its lower part, below 2880 m, the gradient is 5°C/km less. The bottomhole temperature in DGE-1 was 85.1°C at around 3700 m depth and the bottomhole temperature in FFC-1 was just 1°C less but it was measured at 3100 m depth. The extrapolated temperature in FFC-1 at 3700 m is 94.3°C, using the lower temperature gradient, 17°C/km. Rosberg and Erlström (2019) reported that the temperature gradient in the sedimentary succession in DGE-1 was much lower than expected and that is an explanation of the lower bottomhole temperature in DGE-1. In FFC-1, the temperature gradient in the sedimentary succession was like gradients, between 28–32°C/km, observed in other wells located in the sedimentary succession in southwest Skåne (Erlström et al. 2018).
The lower gradient in FFC-1, 17.4°C/km, is comparable with gradient measured in other deep wells in the Fennoscandian basement., such as the temperature gradient in the 6957 m deep borehole Gravberg-1, varies between 14 and 18°C/km (Juhlin et al. 1998) and the gradient, 14 and 17°C/km in the Outokumpu R-2500 research borehole in Finland (Kukkonen et al. 2011). Similar values are also presented in Sundberg et al. (2009) from measurements down to around 1400 m depth in Laxemar, southeast of Sweden and gradients between 15 and 20°C/km are obtained in the 1820 m deep borehole, Bh32012, in Lake Vättern, Sweden. The gradient in the deep wells OTN1-3 in Espoo Finland is also 17°C/km(Kukkonen and Pentti 2021).
The higher gradient in FFC-1, 23.5°C/km, is higher than the gradient measured in other deep wells in the Fennoscandian basement, as well as in the gradient of 20°C/km that was observed for the 2500 m deep COSC-1 borehole in the Swedish Caledonides in west central Sweden (Lorenz et al. 2015). The higher gradient in FFC-1 and the one measured in DGE-1 are more like the gradients between, 21 and 28°C/km measured in the KTB borehole in the upper central European crust in Germany (Emmermann and Lauterjung 1997) and gradient of 24°C/km measured in the Precambrian Canadian Shield (Majorowicz et al. 2014).
Heat flow
In the upper part of the open hole section, above 2610 m, most of the calculated heat flow is between 60 and 70 mW/m2, with an average of 66 mW/m2, see Fig. 3. In the lower part of the open hole section most of the calculated heat flow varies between 40 and 60 mW/m2, with an average around 51 mW/m2. The zone in between seems to be thermally disturbed, which has been mentioned previously, and the calculated heat flow values for this section will most probably differ from values obtained during undisturbed thermal conditions. Therefore, are those values not considered in the further evaluation of the calculated heat flow values. The heat flow values in the upper part of FFC-1 are close to the values reported for DGE-1 (Rosberg and Erlström 2019). The heat flow values are also close to the values reported for other deep boreholes in the Danish Basin and the heat flow model of the Eugeno transverse for the southwest margin of the Fennoscandian Shield (EUGENO-s working group 1988; Balling 1995). Balling (1995) also reports that the values for the central parts of the shield is less, around 40–50 mW/m2, which corresponds more to the values calculated for the lower part for the open hole section in FFC-1. Similar values are also presented in Aldahan et al. (1991) for the Gravberg-1 borehole in central Sweden, the Bh32012 drilling in Lake Vättern (Sundberg et al. 2016), the Outokumpu R-2500 research borehole in Finland (Kukkonnen et al. 2011) and for OTN-1 well in Espoo (Kukkonen and Pentti 2021).
Heat production
The calculated heat production (A) for the crystalline section in FFC-1 using the concentrations of the radiogenic isotopes of uranium (U), thorium (Th) and potassium (K) from the spectral gamma ray log in Fig. 7 and the photo density log is presented in Fig. 3. The average heat production is around 3.0 µW/m3. However, the heat production is considerably lower in intervals dominated by metabasite/amphibolite, around 1.5 µW/m3. In addition, the upper part down to around 2340 m depth also shows lower values around 2.4 µW/m3, which are influenced by the low concentration of uranium content measured over this section. It can also be seen in Fig. 7 that the concentrations of potassium and thorium differs significantly between the metabasite-amphibolite and gneiss-granite rock types, but the uranium content is quite similar. The same pattern can be from the spectral gamma ray log from DGE-1, see Fig. 7. The heat production in DGE-1 is higher, values reaching up to 8 µW/m3, in the open hole section down to around 3040 m depth. Below this depth the average heat productivity is around 3.5 µW/m3 and the values are in the same order as the ones calculated in FFC-1. This is given by the relatively higher content of uranium in the red-brown gneiss interval between 2160 and 3040 m in DGE-1 that contribute to a higher heat productivity in comparison to FFC-1. The average value for the entire open section in DGE-1 is 5.4 µW/m3. Heat production values within the same range as the ones calculated for FFC-1 can also be found in other wells located within the Fennoscandian Shield, such as Gravberg-1 borehole in central Sweden, Laxemar, southeast of Sweden and the Outokumpu R-2500 research borehole in Finland (Aldahan et al. 1991; Sundberg et al. 2009; Kukkonen et al. 2011).
Summary of thermal data from FFC-1 in relation to other deep boreholes
The previous presented thermal data from FFC-1 is summarized in Table 4, as well as compared with data from other deep wells drilled in the upper crystalline crust within the Fennoscandian Shield A comparison is also made with the Hunt well, which represents a similar geological setting on another Precambrian shield margin, i.e. Canadian Shield.
Table 4
Comparison of thermal data from the FFC-1 well with other deep boreholes drilled in the crystalline upper crust. The table is partly based on information in Rosberg and Erlström (2019).
|
FFC-1
|
DGE-11)
|
Gravberg-1 2)
|
Bh32012 in Lake Vättern3)
|
Outokumpu
R-2500 4)
|
Hunt well, Western Canada 5)
|
Depth, m
|
3133
|
3702
|
6957
|
1820
|
2516
|
3518
|
Temperature, °C
|
84
|
85
|
116
|
37
|
40
|
95
|
Q, mW/m2
|
51–66*
|
58*
|
50
|
35–47
|
40–45
|
72
|
Gradient, ˚C/km
|
17–24
|
22–24
|
14–18
|
15–20
|
14–17
|
24
|
K, W/(m K)
|
2.4–3.6
|
2.5–3.6
|
3.4
|
1.8–2.8
|
2.5–3.3
|
2.2–3.6
|
A, µW/m3
|
3.0*
|
5.8*
|
2.0–5.0
|
0.1*
|
1.6–5.4
|
0.9
|
Dominating rock types
|
Gneiss, amphibolite
|
Gneiss, amphibolite
|
Granite
|
Diorite
|
Metasediments Pegmatitic granite, ultramafic rocks
|
Gneiss, granite
|
1) Rosberg and Erlström (2019), 2) Aldahan et al. 1991, 3) Sundberg et al. 2016, 4) Kukkonen et al. 2011, 5) Majorovicz et al. (2014), *Weighted average |
The thermal data acquired in FFC-1 is a valuable contribution, since there are limited number of deep wells located in the Fennoscandian Shield, as well as in the upper crystalline crust. In addition, the thermal data is acquired in the only deep crystalline basement well in the Danish Basin. However, new temperature measurements are required to in depth explain the different temperature gradients obtained in the well and the calculated heat flow. It seems like the temperature survey was conducted under thermal conditions different from the pre-drilling conditions. However, the acquired values still give an indication of the general thermal regime in the well. Unfortunately, new temperature measurements will be both expensive and difficult to conduct. This is because it is a deep well and special logging equipment is required for entering the well due to there is an off set in the borehole. In other words, there is only a few companies that can make the additional logging, which makes it even more expensive.