Geological Survey of Denmark and Greenland Bulletin 41, 2018, 13-16
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Potential for brine storage near the gas storage facility at Lille Torup, northern Jylland, Denmark Morten Leth Hjuler, Morten Sparre Andersen, Carsten Møller Nielsen, Anders Mathiesen, Lars Kristensen, Nina Skaarup and Lars Henrik Nielsen This study is based on a feasibility study for the Danish En- Geological background erginet.dk to identify potential formations for brine storage The Lille Torup area is located centrally in the Danish Ba- near the gas storage facility at Lille Torup, northern Jylland, sin, where the Upper Permian–Mesozoic succession is 5–5.5 Denmark (Fig. 1; Hjuler et al. 2017). km thick. The basin was formed in the Late Carbonifer- Located on top of a salt structure, the gas storage facility com- ous–Early Permian with basal Rotliegendes coarse-grained prises seven caverns, which have been washed out by circulating clastic sediments and thick Zechstein salts overlain by Trias- water in the salt dome. One cavern contains c. 520.000 m3 of sic sandstone, mudstone, carbonate rocks and salt (Nielsen intrusive brine that must be disposed of in order to increase the 2003). These are followed by Lower Jurassic mudstone, Mid- storage volume for gas. One option is to inject the brine into the dle Jurassic sandstone, Upper Jurassic–Lower Cretaceous subsurface if a target with appropriate storage properties can be mudstone and siltstone with few sandstone layers. The Mes- identified, but it is a prerequisite that the stored brine does not ozoic succession terminates with c. 1200 m thick carbonate compromise freshwater reservoirs. Due to cost considerations, deposits. the brine storage should be situated within a radius of 50 km of The salt structure at Lille Torup consists of mobilised the gas storage facility and at a depth not exceeding 2000 m. Zechstein salt penetrating the Mesozoic succession. Its top Based on the national geothermal research conducted point is c. 250 m below the present-day surface. during the last decade, a number of sandy formations are The Haldager Sand Formation in the northern part of the considered potential storage reservoirs (Fig. 2; e.g. Mathiesen basin is 2–150 m thick, but may exceed 200 m in rim syn- et al. 2009; Vosgerau et al. 2016). Around Lille Torup, these clines of salt structures, where sandstone commonly domi- include the Bunter Sandstone/Skagerrak, Gassum, Haldager nates the lithology. The Frederikshavn Formation is primar- Sand and Frederikshavn formations where the two former ily present in the northern part of the basin and frequently formations are discarded due to present-day burial depths ex- includes sandstone layers. Its thickness decreases southwards ceeding 2000 m. In addition, the Chalk Group is considered from 150–300 m to a few metres. The more than 1000 m a potential storage formation due to its importance as a hy- thick chalk- and limestone-dominated Chalk Group consti- drocarbon reservoir in the North Sea, however, due to risk of tutes the topmost pre-Quaternary formation in large parts of leakage to the younger sediments and risk of environmental the Danish Basin. issues, the chalk was discarded as potential storage zone. Aalborg N o r w e g i a n - D a n i s h B a s i n 1 Mors-1 R i n g k ø b i n g - F y n H i g h Erslev-2 Hyllebjerg-1 N o r t h G e r m a n B a s i n Erslev-1 Aars-1 Farsø-1 50 km 2 3 Rødding-1 Hobro 4 Skive-1 Skive-2 Hobro-1 Gassum-1 5 Skive Kvols-1 2D seismic line Viborg Lille Torup gas Randers storage facility Holstebro Selected storage area Fig. 1. The study area and selected storage areas Fault within a radius of 50 km of the Lille Torup gas Aarhus Salt diapir storage facility. 15 km Silkeborg © 2018 GEUS. Geological Survey of Denmark and Greenland Bulletin 41, 13–16. Open access: www.geus.dk/bulletin 13 System Lithostratigraphic unit Potential formations for brine storage Quaternary Post Chalk Group Five potential storage areas were defined based on reservoir Cretaceous Chalk Group Assumed seal quality assessments obtained by integration of well-log data Lower Cretaceous unit Assumed seal and WebGIS data improved with locally refined seismic in- Jurassic Frederikshavn Fm Reservoir terpretations. Injectivity assessments were performed using Børglum Fm ECLIPSE 100 reservoir simulation software and Petrel soft- Flyvbjerg Fm ware. See Hjuler et al. (2017) for details. The Haldager Sand Formation (Figs 3A–C) is presently Haldager Sand Fm Reservoir buried more than 2000 m in large parts of the study area, Fjerritslev Fm Assumed seal but more shallow occurrences exist. The generally 50–150 Triassic Gassum Fm Reservoir m thick formation is dominated by sandstone known to be Vinding Fm quartz-rich, which points to good reservoir properties. In ar- Oddesund Fm eas of relatively shallow burial (<2000 m), the Haldager Sand Tønder Fm Formation may constitute a storage formation. Falster Fm The Frederikshavn Formation (Figs 3D–F) is buried Ørslev Fm less than 2000 m and generally more than 100 m thick; it Bunter Sandstone Fm Reservoir is thickest east of Lille Torup. Several potential storage res- Permian Zechstein Group ervoirs with sufficient lateral extent can be identified. The Frederikshavn Formation constitutes a storage option. Fig. 2. Lithostratigraphic chart showing potential reservoirs (yellow) and The Chalk Group (Figs 3G, H) is buried at 100–700 m assumed seals (brown). depth and is more than 1 km thick. On top of the Lille Torup salt structure, the salt movements may have fractured the c. 250 m of chalk, and increased permeability and thus reser- voir quality. However, the overlaying Quaternary deposits Methods are not expected to possess sealing qualities and brine storage The subsurface within a radius of 50 km of the Lille Torup in the chalk could lead to environmental issues. The Chalk storage facility was screened for potential sandstone reser- Group is therefore discarded as a potential storage formation. voirs suitable for storage. The local database comprises 11 vertical deep wells and an open grid of regional 2D seismic profiles of variable quality and resolution (Fig. 1). Some wells Potential seals were excluded from the database due to location on top of The Chalk Group outside the top of the Lille Torup salt salt structures (Erslev-1–2 and Skive-1–2), uncertain data structure is expected to effectively seal off pore water from quality (Aars-1) or separation from the Lille Torup storage the sandy formations beneath it due to its low permeability facility by fjord water (Mors-1). and great thickness. In addition, the clayey Lower Creta- The Danish geothermal WebGIS application (Vosgerau ceous unit is assumed to be of sufficiently low permeability et al. 2016) provided maps of formation depth, formation to prevent pressure and pore-water propagation from below. thickness and potential reservoir sandstone thickness as well as reservoir parameters of relevant wells. The potential reser- voir sandstone thickness map was developed for assessment Reservoir parameters of the formations of the geothermal potential and is used in this study as an The Haldager Sand Formation mainly comprises sandstone indicator for injection capacity. layers with porosities in the 18–22% range and permeabili- Reservoir properties derived from well logs include the ties in the 140–360 mD range (Table 1). Disregarding burial depths of formation top and base, formation thickness, gross depth, the Haldager Sand Formation is assumed to provide sand thickness (i.e. cumulated thickness of all sandstone lay- suitable storage properties. ers), potential reservoir sandstone thickness (i.e. cumulated The sandstone layers of the Frederikshavn Formation thickness of sandstone layers with a shale content <30% have porosities in the 17–30% range, permeabilities in the and a porosity >15%), as well as averaged values of porosity, 110–1500 mD range and the thickness of potential reservoir permeability and transmissivity of the potential reservoir sandstone in the 6–66 m range (Table 1). In the Kvols-1 well, sandstone. For uncertainty considerations, see http://dybge- however, the formation seems to have little or no storage po- otermi.geus.dk/. tential, assumedly because clay minerals reduce both pore 14 Depth to formation Formation thickness Potential reservoir sand thickness A B C M-1 Hy-1 M-1 Hy-1 M-1 Hy-1 F-1 A-1 F-1 A-1 F-1 A-1 Ho-1 Ho-1 Ho-1 K-1 G-1 K-1 G-1 K-1 G-1 Haldager Sand Fm D E F M-1 Hy-1 M-1 Hy-1 M-1 Hy-1 F-1 A-1 F-1 A-1 F-1 A-1 S-1 S-1 S-1 R-1 S-2 Ho-1 R-1 S-2 Ho-1 R-1 S-2 Ho-1 K-1 G-1 K-1 G-1 K-1 G-1 Frederikshavn Fm Frederikshavn Thickness 50 km G H ≥ 15 m M-1 Hy-1 M-1 Hy-1 F-1 A-1 F-1 A-1 S-1 S-1 R-1 S-2 Ho-1 R-1 S-2 Ho-1 K-1 G-1 K-1 G-1 Chalk Group Lille Torup gas Wells: storage facility E-1 Erslev-1 E-2: Erslev-2 Salt diapir Above sea level Not present Not present at 800–3000 m F-1: Farsø-1 G-1: Gassum-1 m below mean sea level Thickness below mean sea level Fault 0–100 800–900 0–50 m Ho-1: Hobro-1 100–200 900–1000 50–100 400–450 Hy-1: Hyldebjerg-1 200–300 1000–1100 100–150 450–500 K-1: Kvols-1 300–400 1100–1200 150–200 500–550 M-1: Mors-1 Study area 400–500 1200–1300 200–250 550–600 R-1: Rødding-1 500–600 1300–1400 250–300 600–650 S-1: Skive-1 600–700 1400–1500 300–350 650–700 S-2: Skive-2 700–800 1500–1600 350–400 > 700 A-1: Aars-1 Fig.