Energy

ProcediaEnergy Procedia 00 (2010) 000–000

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Mineral changes in CO2 experiments – Examples from Danish onshore saline aquifers.

Weibel, R.a*, Kjøller, C. a, Bateman, K.b, Nielsen, L. H. a, Frykman, P. a, Springer, N. a, Laier, T. a

aGEUS, Geological Survey of Denmark and Greenland, Oester Voldgade 10, 1350 Copenhagen K, Denmark.bBGS, British Geological Survey, Kingsley Dunham Centre,Keyworth, Nottingham, UK.

Elsevier use only: Received date here; revised date here; accepted date here

Abstract

The onshore parts of the Norwegian-Danish Basin and the northern rim of the North German Basin contain extensive saline aquifers of potential interest for CO2 storage. The Triassic Bunter Sandstone and Skagerrak formations comprise arkoses, subarkoses and lithic arkoses, with feldspar being dominated by K-feldspar. The authigenic phases are dominated by carbonates(mainly calcite and dolomite), anhydrite, clay minerals. The Upper Triassic – Lower Jurassic shallow marine and paralic sandstones (GassumFormation) comprises mainly subarkoses and sublitharenites. The authigenic phases are typically carbonates(typically siderite and ferroan dolomite), clay minerals and pyrite. The Middle Jurassic fluvial, estuarine and shallow marine sandstones (Haldager Sand Formation) is characterised by quartz arenites and subarkoses. The authigenic phases comprise clay minerals, pyrite, carbonate, quartz and feldspar. Experiments in the laboratory were designed and performed in order to investigate the response of these mineralogical different reservoir sandstones to CO2 storage.

In the batch experiments, the sandstone plugs were exposed to CO2 saturated synthetic fluid at present day reservoir temperatures and pressures (70°C at 20 MPa) for a period of 7–14 months. Parallel experiments with N2 gas show minor adjustments of the chemistry of the pore fluids until equilibrium is reached prior to first sampling (~30 days). After 7 months of experiment the fluids show increased values of HCO3

¯, Ca2 + and Mg2+ in the Skagerrak and Bunter Sandstone formations. Whereas the Gassum formation (and to a lesser extend the Haldager Sand Formation) show increased values of HCO3

¯, total Fe and Mn+ in the fluid. These changes are probably related to the dissolution of the typical carbonates in the different formations.

© 2010 Elsevier Ltd. All rights reserved

Keyworths: CO2; experiment; mineral changes; carbonate dissolution; Bunter Sandstone Fm; Skagerrak Fm; Gassum Fm; Haldager Sand Fm.

1. Introduction

What happens when reservoir rocks are exposed to CO2? Are all reservoir sandstones equally suitable for CO2storage, or are specific mineralogical compositions preferable? In an attempt to address these fundamental questions laboratory batch experiment were conducted on different reservoir rocks at present day reservoir conditions. Plugs of the Bunter Sandstone, Skagerrak, Gassum and Haldager Sand formations are exposed to CO2 saturated synthetic fl uids under reservoir conditions (70°C and 20 MPa) in the laboratory experiments. By doing so we aim to come

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closer to what actually happens when well-known reservoir rocks are exposed to CO2 and to what degree the primary mineralogy influence the reactions.

2. Geological setting

Important Danish reservoir rocks for CO2 storage are the Triassic and Jurassic sandstones formations, which have the required burial depth of 800–3000 m, reasonable thicknesses and sand net-gross content (Chadwick et al. 2008; Mathiesen et al. 2009). The aeolian and emphermal fl uvial deposits of the Lower Triassic Bunter Sandstone Formation occurs in the northern rim of the North German Basin (Fig. 1; Bertelsen 1980) with thicknesses up to 300 m and burial depths varying from 1140–1800 m (Mathiesen et al. 2009). The proximal fluvial deposits of the Triassic, of the more or less contemporaneously, Skagerrak Formation (Fig. 1) is found only in the Norwegian-Danish Basin (Fig. 1). The Skagerrak Formation has present day burial depths from 600–5000 m and total thicknesses up to 1700 meters; and with a net-gross relation estimated to 0.3 (Mathiesen et al. 2009). The Upper Triassic – Lower Jurassic shallow marine and paralic sandstones (Gassum Formation) are present in most of Denmark (Fig. 1; Nielsen 2003) with thicknesses up to 400 m and net-gross relations of approximately 0.4 (Mathiesen et al. 2009) and present day burial depths varying from 550–3350 m. The Middle Jurassic fluvial, estuarine and shallow marine sandstones (Haldager Sand Formation) occur in restricted areas in the northern part of Denmark Jurassic (Fig. 1; Nielsen 2003) with thicknesses up to 200 m and net-gross relations of approximately 0.7 (Mathiesen et al. 2009) and present day burial depth of 400–2500 m.

Figure 1. A. Map of Denmark showing the regional potential for CO2 storage in the Triassic-Jurassic formations. Modified after Anthonsen and Nielsen (2008). B. Stratigraphic scheme simplified after Michelsen and Clausen (2002) and Nielsen (2003).

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3. Methodology

Petrography was evaluated from transmitted light and reflect ed light microscopy of polished thin sections. Modal compositions of the sandstones were obtained by point counting minimum 300 points in each thin section, which were stained for K-feldspar. Supplementary studies of crystal morphologies, dissolution features and paragenetic relationships were performed on gold coated rock chips mounted on stubs and on carbon coated thin sections using a Phillips XL 40 scanning electron microscope (SEM). The scanning electron microscope was equipped withsecondary electron detector (SE), back-scatter detector (BSE); and with a Thermo Nanotrace 30 mm2 detector surface window and a Pioneer Voyager 2.7 10 mm2 window Si(Li) detector energy dispersive X-ray analysis (EDX) system. The electron beam was generated by a tungsten filament operating at 17 kV and 50-70 μA.

The batch experiments were conducted at the British Geological Survey according to the procedures described by Bateman (2009). Sandstones plugs were taken from the Bunter Sandstone Formation in the Tønder-4 well, the Skagerrak Formation in the Vedsted-1 well, the Gassum Formation in the Vedsted-1 and Stenlille-18 wells and the Haldager Sand Formation in the Vedsted-1 well (Fig. 1). The plugs were placed in synthetic pore fluids (Laier 2008) in pressure vessels at 70°C and 20 MPa. CO2 saturated synthetic pore fluids was injected at the beginning of the experiments. The duration of the experiments is in all 14 months. The results presented here are based on rock samples exposed to CO2 in 7 months, whereas fluid samples were taken after 1, 2, 3 and 7 months.

The fluid composition was analysed for major and some minor cations (Si, Al, Fe, Sr, Ba, Co, Ni, Cu, Zn, Cr, Mo, Cd, P, Pb, V, Li, B, As, Se and total S) using inductively coupled plasma - optical emission spectroscopy (ICP-OES), and for major and some minor anions (Fˉ, Brˉ, Clˉ, SO4

2-, NO3ˉ, HPO42-) using ion chromatography (IC).

Analysis of HCO3ˉ was performed by titration against sulphuric acid. Measurements of pH were made on cooled, depressurised samples using an Orion® 900A pH meter calibrated using Whatman® NBS traceable buffers at pH 4, 7 and 10.

4. Results

4.1. Mineralogy of the original material

The Bunter Sandstone Formation consists mainly of arkoses and subarkoses (Weibel and Friis 2004). K-feldspar is more common than plagioclase, and plagioclase seems to be more intensively altered and dissolved than K-feldspar. Some samples contain numerous ooids, carbonate clasts or clay intraclasts from overbank strata. The volumetrically important authigenic phases in the Bunter Sandstone Formation are carbonates (calcite and dolomite), anhydrite and clay minerals (illite, chlorite, mixed-layer illite/smectite, mixed-layer smectite/chlorite). Dolomite, calcite and anhydrite may occur as pore-filling cement. Common subordinate authigenic minerals areanalcime, feldspar overgrowths, quartz overgrowths, barite, red coatings of primarily hematite, anatase and other authigenic opaque minerals (Weibel and Friis 2004).

The Skagerrak Formation is dominated by arkoses, lithic arkoses and subarkoses (Weibel 1998). The feldspar group is completely dominated by K-feldspar, though rare plagioclase grains have been identified. Alteration of feldspar grains includes sericitisation, clay mineral or carbonate replacement and dissolution. The rock fragments are mainly igneous and rare metamorphic, but volcanic rock fragments occur in a specific area. Clay intraclasts which probably are rip-up clasts from overbank deposits are highly abundant in few samples. Mica is relatively common. The dominating porosity reducing cements are carbonates (typically dolomite, but also calcite and ferroan dolomite) and clays, which are typically smectite in the shallow wells, but mixed-layer illite/smectite and illite with increased burial depth (Weibel 1999). Of minor volumetrically importance are quartz overgrowths, feldspar overgrowths and anatase. Anhydrite is a rare late authigenic phase.

The Gassum Formation consists mainly of subarkoses and arkoses (Friis 1987). The feldspar abundance varies across the basin and feldspar is relatively more abundant in the northwestern part than in the eastern part. K-feldspar is more common than albite and Ca-rich plagioclase has not been observed. Alteration of feldspar grains includes partly dissolution, replacement by kaolin or carbonate. Mica is present in all samples. The porosity reduction is mainly due to compaction in shallow wells, i.e. burial depths <1500 m (Friis 1987). Changes in the porosity are mainly the result of limited amounts of authigenic phases as siderite, pyrite and calcite besides generation of

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secondary porosity by feldspar dissolution. When the formation is buried >1500m the porosity reduction is caused in particular by quartz diagenesis and locally by ferroan dolomite, and kaolin. Volumetrically minor authigenic phases include illite, chlorite, albite.

The Haldager Sand Formation is characterized by quartz arenites and subordinate subarkoses. Quartz dominates the framework grains. K-feldspar is more common than plagioclase, which is extremely rare in the deepest buried sediments. Rock fragments are of plutonic, metamorphic or sedimentary origin. Organic matter is abundant in some samples and commonly concentrated together with mica. The authigenic phases are similar to those of the Gassum Formation, however their abundance is typically lower in the Haldager Sand Formation compared to the Gassum Formation. Early quartz and kaolinite occur randomly distributed in the fluvio-deltaic sandstones, whereas calcite cement may be important in the marine deposits (Nielsen and Friis 1984). Siderite, pyrite, iron-oxide/hydroxides and anatase are common minor authigenic phases.

4.2. Fluid chemistry

Most elements show an increase in abundance within the first month of both the N2 and CO2 experiment. Unfortunately, the variations of the Si and Al contents are within the uncertainty of the analytical methods and have consequently been left out. Some of the elements and ions showing the largest variation over time between the N2and CO2 experiments are presented in Fig. 2. Though the basic pore fluid content of Ca2+ and Mg2+ varies between the Bunter Sandstone, Gassum and Haldager Sand formations they all show increased values during the C O2experiments relative to the N2 experiments. The HCO3

- content increases in all CO2 experiments after 7 months but not in the N2 experiments. The Mn content increases clearly in the CO2 experiments for the Gassum Formation and possibly also for the Haldager Sand Formation, but remains unaffected for the Bunter Sandstone Formation. The FeTOTAL content increases in the CO2 experiments but decreases in the N2 experiments for both the Gassum and Haldager Sand formations, but remains constant for the Bunter Sandstone Formation.

Figure 2. Fluid composition evolution during experiments for the Bunter Sandstone, Gassum and Haldager Sand formations.

4.3. Mineral changes during experiment

Mineral changes after CO2 and N2 experiments has been evaluated by petrographic methods. However, the drawback is that though dissolution phenomenons on crystal face are recognizable, continued dissolution of

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diagenetically altered minerals cannot be evaluated. In a similar way evaluation of dissolution of clay minerals like chlorite and illite is difficult due to their delicate crystal shapes.

Detrital albite is partly dissolved in the Bunter Sandstone Formation prior to the experiment, as well as after the CO2 experiment. Authigenic albite, however, show no signs of alteration after the experiment (Figs 3A, 3B). Detrital K-feldspar appears with K-feldspar overgrowths, some of which being unaffected and others showing possible weak signs of corrosion after the CO2 experiment (Fig. 3C, 3D). Analcime occurs with internal dissolution voids, which possibly are enlarged the CO2 experiment. Calcite rhombs show weak signs of corrosion on crystal faces after theCO2 experiment (Figs 4A, 4B). No clear observations of any changes in the anhydrite cement during the CO2experiment. Detrital albite and K-feldspar may be intensively dissolved in the Skagerrak Formation both prior to and after the C O2 experiment. However, the authigenic albite overgrowths seem unaffected by the experiments. Pore-filling ferroan dolomite show clear dissolution along crystal faces and penetrating along cleavage planes (Figs 7C, 7D). Kaolin shows no sign of corrosion after the CO2 experiment.

Intense dissolution of both detrital albite and K-feldspar is common in the Gassum Formation of the Vedsted-1 and Stenlille-18 wells both prior and after the experiments. However, authigenic albite and authigenic K-feldspar, precipitated on the remnants of the detrital feldspar, show no signs of corrosion after the CO2 experiment. Pore filling ferroan dolomite show intensive dissolution along crystal phases and cleavage planes. Also siderite spherulites occur with dissolution features after the CO2 experiment. Detrital K-feldspar are partly dissolved and partly replaced by kaolin in the Haldager Sand Formation both prior and after the CO2 experiment.

5. Discussion

The most prominent mineral changes during the CO2 batch experiments of all samples are carbonate dissolution. The dissolution of ferroan dolomite in both the Skagerrak and Gassum formations is followed by increased iron, manganese and bicarbonate contents in the fluid samples, though the increase of calcium and magnesium contents is obliterated by relatively high levels in the initial fluids. The corrosion of calcite rhombs in the Bunter Sandstone Formation is accompanied by a relatively higher increase in the calcium content in the pore fluid than observed for the other formations. The fluid composition in the Haldager Sand Formation experiment suggests incipient dissolution of siderite, though this still requires petrographic documentation.

Other laboratory experiments conducted with siliciclastic sandstones exposed to CO2 saturated fluids also result in dissolution of carbonates, dolomite, calcite and siderite (e.g Pearce et al. 1996; Shiraki and Dunn 2000). However, secondary calcite precipitation was also observed in the 8 months batch experiments of the TriassicSherwood Sandstone, UK, reacted with CO2-saturated seawater-resembling fluids (Pearce et al. 1996). In natural CO2 fi elds and fields of enhanced oil recovery (EOR), where the exposure to C O2 has been of longer duration than what is possible in the laboratory, both dissolution and precipitation of carbonates have been reported. Corrosion of dolomite in the natural C O2 fields has occasionally been reported (Pearce et al. 1996; Pauwels et al. 2007). However, precipitation of carbonates, including calcite, ferroan dolomite, ankerite, dawsonite (NaAlCO3(OH)2) and alumohydrocalcite (CaAl2(CO3)2(OH)4

.3H2O) seem to be a more common phenomenon (Baines and Worden 2004; Rochelle et al. 2004).

The differences between the short term laboratory experiments and long term exposure in natural CO2 fi elds can be explained by the reaction rates. Reaction rates and time defines which stage the process has reached. CO2dissolution and dissociation will lead to an initial decrease in pH (Baines and Worden 2004). The rate of carbonate dissolution is so fast compared to silicate minerals dissolution that carbonate minerals will partially dissolve (Worden and Smith 2004, Pauwels et al. 2007). Over time thermodynamic equilibrium will be established and silicate minerals will dissolve whereas carbonate mineral will grow in stead. Minerals likely to liberate ions for carbonate formation include feldspars, zeolite, and Fe, Mg or Na rich clays (Baines and Worden 2004; Wilkinson et al. 2009). As silicate minerals are dissolved, and the liberated ions react with bicarbonate to form carbonate minerals, which causes the pH to rise. Though potentially unstable silicate minerals are present in the Bunter Sandstone, Skagerrak, Gassum and Haldager Sand formations consisting of K-feldspar, albite, clays (mainly illite and kaolin and only minor chlorite), their reactivity is slow, which probably lead to a fast decrease in pH and the following dissolution of carbonates. As the investigations of the Danish Triassic–Jurassic formations are based on 7 months of experiment the rock-fluid-C O2 system has not reached equilibrium.

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Other lab experiments are performed on crushed samples whereas our experiments were performed on plugs, which may have slowed down the reactions in the centre and reduced the amount of changed minerals but on the other hand gives us the opportunity to study how the mineral reaction penetrates into the sandstone. Also in natural CO2 fields the sandstones may still contain potentially reactive minerals as feldspar and clays and consequently have not yet reached thermodynamic equilibrium.

Dissolution of feldspar is a common diagenetic phenomenon in the Triassic–Jurassic sediments, though the process did not necessarily continue during the experiments, as described by some authors (Pearce et al. 1996; Shiraki and Dunn 2000; Baines and Worden 2004) and suggested from geochemical modeling (e.g. Rochelle et al. 2004). The reason being that authigenic albite in the Skagerrak and Gassum formations and authigenic K-feldspar in the Gassum and Bunter Sandstone formations do not show any signs of corrosion. The possibility exists that the corroded and partly dissolved part of the feldspar grains are more easily dissolved during experiments than the authigenic. Dissolution of clay minerals in the Triassic–Jurassic sediments is difficult to evaluate due to their fragile morphology, though the relatively large kaolin crystals seem to be unaffected.

Figure 3. Unaffected authigenic feldspar during CO2 experiment. Unaffected authigenic albite prior to (A) and after CO2

experiment (B), Skagerrak Formation, Vedsted-1, 2064.87m. Unaffected K-feldspar overgrowths prior to (C) and after CO2 experiment (D), Bunter Sandstone Formation, Tønder-4 well, 1663.27m.

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6. Conclusion

The investigated sandstones from the Bunter Sandstone, Skagerrak, Gassum and Haldager Sand formations all proved suitable as reservoir sandstones for CO2 storage. The carbonate cement is dissolved in the experiments after 7 months of exposure to CO2, as it will be dissolved in the beginning of a CO2 storage case. On the other hand the carbonate cement is not so extensive that this would influence the stability of the reservoir.

Beyond the time frame of the experiment the mineral reactions are more speculative. Potentially active minerals as feldspar and clay minerals are present in all investigated formations. Other experiments and modeling (see for example Kjøller et al. (2010)) suggest that these silicates gradually will dissolve and lead to precipitation of various carbonate minerals in the long term. Thus the lower reaction rates of the silicate minerals compared to carbonate minerals means that the carbonate dissolution will occur in the short term, whereas the carbonate precipitation is expected to take place as the silicate alteration intensifies and the reactions towards equilibrium continues.

7. Acknowledgement

The paper is part of the Aqua-DK research project supported by the Danish EUDP-programme funded by the Danish Energy Agency, DONG Energy and Vattenfall. DONG Energy and Vattenfall are thanked for their interest

Figure 4. Carbonate dissolution during CO2 experiment. Calcite rhomb unaffected prior to the experiments (A) and partly dissolved after CO2 experiment (B), Bunter Sandstone Formation, Tønder-4 well, 1663.27m. Porefilling ferroan dolomite cement unaffected (C) prior to and partly dissolved at crystal faces and along cleavage planes after CO2 experiments (D), Skagerrak Formation, Vedsted-1, 2064.87m.

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in the project and their permission to publish this paper, which is also published with the permission of the Geological Survey of Denmark and Greenland.

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