Dieter Pudlo

Modelling CO2-induced fluid–rock interactions in the Altensalzwedel gas reservoir. Part I: from experimental data to a reference geochemical model

Modelling fluid–rock interactions induced by CO2 is a key issue when evaluating the technical feasibility and long-term safety assessment of CO2 storage projects in deep formations. The German R&D programme CLEAN (CO2 Large-Scale Enhanced Gas Recovery in the Altmark Natural Gas Field) investigated the almost depleted onshore gas reservoir located in the Rotliegend sandstone at over 3,000-m depth. The high salinity of the formation fluids and the elevated temperature in the reservoir exceed the validity limits of commonly available thermodynamic databases needed for predictive geochemical modelling. In particular, it is shown that the activity model of Pitzer has to be applied, even if necessary input data for this model are incomplete or inconsistent for complex systems and for the considered temperatures. Simulations based on Debye-Hückel activity model lead to severe, systematic discrepancies already in the simple proposed reference case where experimental data could be used for comparison. A simplified geochemical model, consistent with the average measured composition of formation fluids and the prevailing mineralogical assemblage of the host rock, identifies the mineral phases most likely to be considered at equilibrium with the formation fluid. The simulated reactions due to CO2 injection, under the hypothesis of local thermodynamical equilibrium, result in a moderate reactivity of the system, with the dissolution of anhydrite cementation and haematite being the most relevant expected mineral reactions. This is compensated, at equilibrium, by the precipitation of new carbonates, like calcite and siderite, for an overall very small loss of porous space. The simulated rather small effect of mineral alteration is also due to the scarce amount of water available for reactions in the reservoir. The results of the model are qualitatively in line with observations from batch experiments and from natural analogues.

Plume related alkaline magmatism in central Africa—the Meidob Hills (W Sudan)

Abstract We investigated in a combined petrological, geochemical and isotope study the Miocene to Holocene volcanic activity in the Meidob Hills Volcanic Field, the north-eastern part of the Darfur Dome Volcanic Province in western Sudan. The Darfur Dome is related to a domal uplift, associated with a negative Bouguer anomaly, and crustal shearing along the Central African Fault Zone. Magmas of the Meidob Hills evolved from basanites to phonolites by fractionation of olivine, clinopyroxene, magnetite, and in a late stage by plagioclase–anorthoclase and apatite. Significant contamination by crustal rocks is observed in basaltic to benmoreitic–trachytic (the latter mostly pyroclastic) rocks. Isotope data demonstrate the presence of a HIMU-like component, which is most pronounced in 7 Ma old basanites ( 87 Sr / 86 Sr =0.70306, 206 Pb / 204 Pb =20.075, 207 Pb / 204 Pb =15.690, 208 Pb / 204 Pb =39.785 ). The basanites are interpreted as a mixture between hypothetical sources of depleted mantle and HIMU with small amounts of some other sources. 3 He / 4 He -isotope ratios in olivine from mafic rocks are much lower than the primitive He mantle source (PHEM) and range between 6.6 and 9.2 Ra, corrected for atmospheric contamination, measured values between 5.4 and 7.5 Ra. Trace element ratios of Ba, Nb, La and Zr show a wide range and indicate source heterogeneities, possibly a subcontinental lithospheric source. The Darfur Dome provides an example for an isolated intracontinental mantle plume with a low He—high HIMU-type composition. Its igneous activity is not related to graben structures, and we see no connection to the Cameroon Line, the Tibesti and the volcanism of the Red Sea and the Afar plume.

The impact of diagenetic fluid–rock reactions on Rotliegend sandstone composition and petrophysical properties (Altmark area, central Germany)

In the framework of the German R&D joint project CLEAN (CO2 large-scale enhanced gas recovery in the Altmark natural gas field), Rotliegend reservoir sandstones of the Altensalzwedel block in the Altmark area (Saxony-Anhalt, central Germany) have been studied to characterise litho- and diagenetic facies, mineral content, geochemical composition, and petrophysical properties. These sands have been deposited in a playa environment dominated by aeolian dunes, dry to wet sand flats and fluvial channel fills. The sediments exhibit distinct mineralogical, geochemical, and petrophysical features related to litho- and diagenetic facies types. In sandstones of the damp to wet sandflats, their pristine red colours are preserved and porosity and permeability are only low. Rocks of the aeolian environment and most of the channel fill deposits are preferentially bleached and exhibit moderate to high porosity and permeability. Although geochemical element whole rock content in these rocks is very similar, element correlations are different. Variations in porosity and permeability are mainly due to calcite and anhydrite dissolution and differences in clay coatings with Fe-bearing illitic-chloritic composition exposed to the pore space. Moreover, mineral dissolution patterns as well as compositions (of clays and carbonate) and morphotypes of authigenic minerals (chlorite, illite) are different in red and bleached rocks. Comparison of the geochemical composition and mineralogical features of diagenetically altered sandstones and samples exposed to CO2-bearing fluids in laboratory batch experiments exhibit similar character. Experiments prove an increase in wettability and water binding capacity during CO2 impact.

The alkaline Meidob volcanic field (Late Cenozoic, northwest Sudan)

The Meidob volcanic field (MVF) forms part of the Darfur Volcanic Province and developed from 7 Ma to 5 ka as indicated by K/Ar, thermoluminescence and 14C ages. It is situated in an uplifted high of the Pan-African basement, which consists of greenstones, high-grade gneisses and granites, and which is covered by Cretaceous sandstone. The MVF basaltic lavas, which originated from more than 300 scoria cones, formed a lava plateau of 50×100 km and up to 400 m thickness in the time between 7 and < 0.3 Ma. Young phonolitic mesa flows, together with rare trachyticbenmoreitic lava flows, trachytic pumice fallout deposits, ignimbrites and maars, form the central part of the field. The total amount of volcanic rocks is between 1400 and 1800 km3, with 98 vol.% being basaltic rocks, which results in an integrated magma output rate of ∼ 0.0002 km3 a−1. A combination of age data of the lavas with erosional features yields uplift rates for the Darfur Dome of ∼30 m Ma-1 in the MVF area. Magma was generated by 3–5% melting of predominantly asthenospheric mantle with a HIMU contribution. Fractionation of olivine, pyroxene, An-poor plagioclaseanorthoclase, magnetite and apatite leads to a differentiation from basanite to phonolite. Assimilation of crustal rocks near the top of the phonolitic upper crustal magma chambers - facilitated by volatile enrichment - produced magmas which gave way to benmoreitic and trachytic lavas, as well as to trachytic ignimbrites and pumice fallout deposits. Ultramafic cumulate xenoliths indicate the existence of major magma reservoirs at the crust-mantle boundary during MVF activity. Magma ascent occurred in a tensional regime, which changed its orientation at around 1 Ma. Early during MVF development, west-east and subordinately northeastsouthwest trending lineaments were active whereas volcanic activity younger than 1 Ma took place along northwest-southeast and northeast-southwest trending systems. The Central African Fault Zone, a transcontinental, lithospheric shear zone, played an important role for the rise of magmas in the Darfur Dome.

The Darfur Dome, western Sudan: the product of a subcontinental mantle plume

Field investigations, K-Ar age determinations and chemical data were used to describe the development of an intraplate volcanic province, the Darfur Dome, Sudan. Magmatism started 36 Ma ago at a small subvolcanic complex (Jebel Kussa) in the center of the dome and was active in the same area between 26 and 23 Ma. Two major volcanic fields (Marra Mountains and Tagabo Hills) developed between 16 and 10 Ma. Volcanism started again at 6.8 Ma with a third volcanic field (Meidob Hills) and at 4.3 Ma in the Marra Mountains and with the reactivation of the center. Activity then continued until the late Quaternary. Having started in the center of the Darfur Dome, volcanism moved in 36 Ma 200 km towards the NNE and 100 km SSW No essential difference in the alkaline magma types (basanitic to phonolitic-trachytic, with different amounts of assimilation of crustal material) in the different fields, was observed. Magmatism is thought to have been produced by a rising mantle plume and volcanism was triggered by stress resolution along the Central African Fault Zone.

Dike rock generation and magma interactions in the Bir Safsaf igneous complex, south-west Egypt: Implications for the Pan-African evolution in north-east Africa

The geological setting, ages, petrography and geochemistry of late Pan-African (≈ 580 Ma) calc-alkaline and tholeiitic dike rocks in the Bir Safsaf igneous complex of south-west Egypt are discussed. These basaltic to rhyolitic dikes intruded contemporaneously and shortly after the intrusion of granitoids. The major and trace element data, Sr and Nd isotope relations, in combination with textural observations, confirm complex interactions between most of the intermediate calcalkaline dike melts and plutonic melts, with different degrees of mixing, assimilation, replenishment and tapping of magma chambers. Trachytic and rhyolitic dikes are strongly differentiated melts from the granitic pluton. The tholeiitic dikes evolved dominantly by fractional crystallization processes. It is inferred that open system and closed system processes operated in calc-alkaline magma chambers, and that the calc-alkaline melts came from a garnet-and amphibole-bearing mantle, modified by a subduction component. Tholeiitic rocks were formed later by fractional crystallization and assimilation processes. Magma ascent of both dike types took place in an extensional environment and the presumed subduction zone has to be seen in connection with the Atmur-Delgo suture zone.

The chemical dissolution and physical migration of minerals induced during CO2 laboratory experiments: their relevance for reservoir quality

The characterization of the quality and storage capacity of geological underground reservoirs is one of the most important and challenging tasks for the realization of carbon capture and storage (CCS) projects. One approach for such an evaluation is the upscaling of data sets achieved by laboratory CO2 batch experiments to field scale. (Sub)-microscopic, petrophysical, tomographic, and chemical analytical methods were applied to reservoir sandstone samples from the Altmark gas field before and after static autoclave batch experiments at reservoir-specific conditions to study the relevance of injected CO2 on reservoir quality. These investigations confirmed that the chemical dissolution of pore-filling mineral phases (carbonate, anhydrite), associated with an increased exposure of clay mineral surfaces and the physical detachment and mobilization of such clay fines (illite, chlorite) are most appropriate to modify the quality of storage sites. Thereby the complex interplay of both processes will affect the porosity and permeability in opposite ways—mineral dissolution will enhance the rock porosity (and permeability), but fine migration can deteriorate the permeability. These reactions are realized down to ~µm scale and will affect the fluid–rock reactivity of the reservoirs, their injectivity and recovery rates during CO2 storage operations.

Cenozoic intra-plate magmatism in the Darfur volcanic province: mantle source, phonolite-trachyte genesis and relation to other volcanic provinces in NE Africa

Chemical and Sr, Nd and Pb isotopic compositions of Late Cenozoic to Quaternary small-volume phonolite, trachyte and related mafic rocks from the Darfur volcanic province/NW-Sudan have been investigated. Isotope signatures indicate variable but minor crustal contributions. Some phonolitic and trachytic rocks show the same isotopic composition as their primitive mantle-derived parents, and no crustal contributions are visible in the trace element patterns of these samples. The magmatic evolution of the evolved rocks is dominated by crystal fractionation. The Si-undersaturated strongly alkaline phonolite and the Si-saturated mildly alkaline trachyte can be modelled by fractionation of basanite and basalt, respectively. The suite of basanite–basalt–phonolite–trachyte with characteristic isotope signatures from the Darfur volcanic province fits the compositional features of other Cenozoic intra-plate magmatism scattered in North and Central Africa (e.g., Tibesti, Maghreb, Cameroon line), which evolved on a lithosphere that was reworked or formed during the Neoproterozoic.

The H2STORE Project: Hydrogen Underground Storage – A Feasible Way in Storing Electrical Power in Geological Media?

The large scale storage of energy is a great challenge arising from the planned transition from nuclear and CO2-emitting power generation to renewable energy production, by e.g. wind, solar, and biomass in Germany. The most promising option for storing large volumes of excess energy produced by such renewable sources is the usage of underground porous rock formations as energy reservoirs. Some new technologies are able to convert large amounts of electrical energy into a chemical form, for example into hydrogen by means of water electrolysis. Porous formations can potentially provide very high hydrogen storage capacities. Several methods have to be studied including high hydrogen diffusivity, the potential reactions of injected hydrogen, formation fluids, rock composition, and the storage complex.

Mineralogical and geochemical analysis of Fe-phases in drill-cores from the Triassic Stuttgart Formation at Ketzin CO2 storage site before CO2 arrival

Abstract Reactive iron (Fe) oxides and sheet silicate-bound Fe in reservoir rocks may affect the subsurface storage of CO2 through several processes by changing the capacity to buffer the acidification by CO2 and the permeability of the reservoir rock: (1) the reduction of three-valent Fe in anoxic environments can lead to an increase in pH, (2) under sulphidic conditions, Fe may drive sulphur cycling and lead to the formation of pyrite, and (3) the leaching of Fe from sheet silicates may affect silicate diagenesis. In order to evaluate the importance of Fe-reduction on the CO2 reservoir, we analysed the Fe geochemistry in drill-cores from the Triassic Stuttgart Formation (Schilfsandstein) recovered from the monitoring well at the CO2 test injection site near Ketzin, Germany. The reservoir rock is a porous, poorly to moderately cohesive fluvial sandstone containing up to 2–4 wt% reactive Fe. Based on a sequential extraction, most Fe falls into the dithionite-extractable Fe-fraction and Fe bound to sheet silicates, whereby some Fe in the dithionite-extractable Fe-fraction may have been leached from illite and smectite. Illite and smectite were detected in core samples by X-ray diffraction and confirmed as the main Fe-containing mineral phases by X-ray absorption spectroscopy. Chlorite is also present, but likely does not contribute much to the high amount of Fe in the silicate-bound fraction. The organic carbon content of the reservoir rock is extremely low (<0.3 wt%), thus likely limiting microbial Fe-reduction or sulphate reduction despite relatively high concentrations of reactive Fe-mineral phases in the reservoir rock and sulphate in the reservoir fluid. Both processes could, however, be fuelled by organic matter that is mobilized by the flow of supercritical CO2 or introduced with the drilling fluid. Over long time periods, a potential way of liberating additional reactive Fe could occur through weathering of silicates due to acidification by CO2.