Evaluation of the Lund deep geothermal exploration project in the Romeleåsen Fault Zone, South Sweden: a case study

Abstract

The bedrock of Skåne, the southernmost province of Sweden, has been targeted for geothermal feasibility studies since the late 1970s. An exploration project concerning the geothermal potential in the Romeleåsen Fault Zone was launched outside the town of Lund in 2001. Besides geophysical imaging of the thrust fault zone, the investigations included drilling and investigations of a 3701.8-m-deep exploration well, DGE-1, with the aim to find > 100 °C warm and hydraulically conductive fractured crystalline bedrock associated to the fault zone. The well penetrates a heavily thrusted and predominantly strongly inclined sedimentary succession in hanging rock blocks along the main fault before entering the fractured crystalline basement at 1946 m, primarily composed of gneiss, granite, and metabasite. This paper represents the first comprehensive description and evaluation of the geological, physical, and hydrological properties of the bedrock at these depths in the Romeleåsen Fault Zone coupled to a geothermal assessment. In addition, the applicability of the four drilling methods used in the crystalline basement section is discussed. The outcome of the DGE-1 well shows significant fracturing in the crystalline bedrock at target depth. The investigations show an average thermal gradient of 22 °C/km, an average heat flow of 58 mW/m 2 , and an average heat production of 5.8 µW/m 3 . The values are relatively high in comparison to thermal conditions noted in other deep wells in the Fennoscandian Shield. However, a bottomhole temperature of around 85 °C and insufficient fluid production rate made a commercial geothermal system unviable. Despite this, the experiences from drilling and investigations of the crystalline bedrock at several kilometers depth constitute important proxies for assessing the geothermal potential in similar geological settings and for engineered geothermal systems in the crystalline bedrock of south Sweden. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Deep basin temperature and heat-flow field in Denmark – New insights from borehole analysis and 3D geothermal modelling

“…Significantly lower values of ca. 60 mW•m -2 observed for the STZ/SKP are in good agreement with those reported in the adjacent tectonic units of the Baltic Shield (50-60 mW•m -2 from: Slagstad et al, 2009;Balling, 2013;Veikkolainen et al 2017;Rosberg and Erlström;.…”

Evaluation of Deep Geothermal Exploration Drillings in the Crystalline Basement of the Fennoscandian Shield Border Zone in South Sweden.

“…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.…”

“…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 in ux is only reported from FFC-1.…”

“…This increasing interest yields the necessity to build knowledge on technical issues related to drilling, rock mass composition, geo-mechanical properties and, thermal and hydraulic conditions of the crystalline bedrock at great depths. In line with this 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 besides the drilling performance, rock mass composition and physical properties, fracturing and thermal properties, which all play a signi cant role in assessing the bedrock prerequisites for EGS in south Sweden.…”

“…In [2002][2003] Lund Energy AB (today Kraftringen AB) and the Department of Engineering Geology at the Lund University performed a geothermal exploration drilling, DGE-1, with the aim of nding hot water in fractured crystalline bedrock associated to the Romeleåsen Fault Zone on the margins of the Fennoscandian Shield in Skåne, south Sweden (Rosberg and Erlström 2019). However, the project ended after encountering less fractured and conductive rock mass than expected.…”