@article{KuehnAltmannsbergerHens2016,
  author    = {K{\"u}hn, Michael and Altmannsberger, Charlotte and Hens, Carmen},
  title     = {Waiweras WarmwasserreservoirWelche Aussagekraft haben Modelle?},
  series = {Grundwasser : Zeitschrift der Fachsektion Hydrogeologie in der Deutschen Gesellschaft f{\~A}¼r Geowissenschaften (FH-DGG)},
  volume    = {21},
  journal   = {Grundwasser : Zeitschrift der Fachsektion Hydrogeologie in der Deutschen Gesellschaft f{\~A}¼r Geowissenschaften (FH-DGG)},
  publisher = {Springer},
  address   = {Heidelberg},
  issn      = {1430-483X},
  doi       = {10.1007/s00767-016-0323-2},
  pages     = {107 -- 117},
  year      = {2016},
  abstract  = {The warm water geothermal reservoir below the village of Waiwera in New Zealand has been known by the native Maori for centuries. Development by the European immigrants began in 1863. Until the year 1969, the warm water flowing from all drilled wells was artesian. Due to overproduction, water up to 50 A degrees C now needs to be pumped to surface. Further, between 1975 and 1976, all warm water seeps on the beach of Waiwera ran dry. Within the context of sustainable water management, hydrogeological models must be developed as part of a management plan. Approaches of varying complexity have been set-up and applied since the 1980s. However, none of the models directly provide all results required for optimal water management. Answers are given simply to parts of the questions, nonetheless improving resource management of the geothermal reservoir.},
  language  = {de}
}
@article{HennigKuehn2021,
  author    = {Hennig, Theresa and K{\"u}hn, Michael},
  title     = {Surrogate model for multi-component diffusion of Uranium through Opalinus Clay on the host rock scale},
  series = {Applied Sciences : open access journal},
  volume    = {11},
  journal   = {Applied Sciences : open access journal},
  number    = {2},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2076-3417},
  doi       = {10.3390/app11020786},
  pages     = {21},
  year      = {2021},
  abstract  = {Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick's law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32-. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.},
  language  = {en}
}
@article{StedingKempkaZirkleretal.2021,
  author    = {Steding, Svenja and Kempka, Thomas and Zirkler, Axel and K{\"u}hn, Michael},
  title     = {Spatial and temporal evolution of leaching zones within potash seams reproduced by reactive transport simulations},
  series = {Water / Molecular Diversity Preservation International (MDPI)},
  volume    = {13},
  journal   = {Water / Molecular Diversity Preservation International (MDPI)},
  number    = {2},
  publisher = {Molecular Diversity Preservation International},
  address   = {Basel},
  issn      = {2073-4441},
  doi       = {10.3390/w13020168},
  pages     = {21},
  year      = {2021},
  abstract  = {Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of Peclet (Pe) and Damkohler (Da). For Pe > 2 and Da > 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da > 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential.},
  language  = {en}
}
@article{HennigStockmannKuehn2020,
  author    = {Hennig, Theresa and Stockmann, Madlen and K{\"u}hn, Michael},
  title     = {Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay},
  series = {Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry},
  volume    = {123},
  journal   = {Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0883-2927},
  doi       = {10.1016/j.apgeochem.2020.104777},
  pages     = {9},
  year      = {2020},
  abstract  = {Diffusive transport and sorption processes of uranium in the Swiss Opalinus Clay were investigated as a function of partial pressure of carbon dioxide pCO(2), varying mineralogy in the facies and associated changes in porewater composition. Simulations were conducted in one-dimensional diffusion models on the 100 m-scale for a time of one million years using a bottom-up approach based on mechanistic surface complexation models as well as cation exchange to quantify sorption. Speciation calculations have shown, uranium is mainly present as U(VI) and must therefore be considered as mobile for in-situ conditions. Uranium migrated up to 26 m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16 m was the maximum. The main species was the anionic complex CaUO2(CO3)(3)(2-) . Hence, anion exclusion was taken into account and further reduced the migration distances by 30 \%. The concentrations of calcium and carbonates reflected by the set pCO(2) determine speciation and activity of uranium and consequently the sorption behaviour. Our simulation results allow for the first time to prioritize on the far-field scale the governing parameters for diffusion and sorption of uranium and hence outline the sensitivity of the system. Sorption processes are controlled in descending priority by the carbonate and calcium concentrations, pH, pe and the clay mineral content. Therefore, the variation in porewater composition resulting from the heterogeneity of the facies in the Opalinus Clay formation needs to be considered in the assessment of uranium migration in the far field of a potential repository.},
  language  = {en}
}
@article{WetzelKempkaKuehn2018,
  author    = {Wetzel, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Quantifying rock weakening due to decreasing calcite mineral content by numerical simulations},
  series = {Materials},
  volume    = {11},
  journal   = {Materials},
  number    = {4},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {1996-1944},
  doi       = {10.3390/ma11040542},
  pages     = {19},
  year      = {2018},
  abstract  = {The quantification of changes in geomechanical properties due to chemical reactions is of paramount importance for geological subsurface utilisation, since mineral dissolution generally reduces rock stiffness. In the present study, the effective elastic moduli of two digital rock samples, the Fontainebleau and Bentheim sandstones, are numerically determined based on micro-CT images. Reduction in rock stiffness due to the dissolution of 10\% calcite cement by volume out of the pore network is quantified for three synthetic spatial calcite distributions (coating, partial filling and random) using representative sub-cubes derived from the digital rock samples. Due to the reduced calcite content, bulk and shear moduli decrease by 34\% and 38\% in maximum, respectively. Total porosity is clearly the dominant parameter, while spatial calcite distribution has a minor impact, except for a randomly chosen cement distribution within the pore network. Moreover, applying an initial stiffness reduced by 47\% for the calcite cement results only in a slightly weaker mechanical behaviour. Using the quantitative approach introduced here substantially improves the accuracy of predictions in elastic rock properties compared to general analytical methods, and further enables quantification of uncertainties related to spatial variations in porosity and mineral distribution.},
  language  = {en}
}
@misc{WetzelKempkaKuehn2018,
  author    = {Wetzel, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Quantifying rock weakening due to decreasing calcite mineral content by numerical simulations},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1092},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-47308},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-473089},
  pages     = {21},
  year      = {2018},
  abstract  = {The quantification of changes in geomechanical properties due to chemical reactions is of paramount importance for geological subsurface utilisation, since mineral dissolution generally reduces rock stiffness. In the present study, the effective elastic moduli of two digital rock samples, the Fontainebleau and Bentheim sandstones, are numerically determined based on micro-CT images. Reduction in rock stiffness due to the dissolution of 10\% calcite cement by volume out of the pore network is quantified for three synthetic spatial calcite distributions (coating, partial filling and random) using representative sub-cubes derived from the digital rock samples. Due to the reduced calcite content, bulk and shear moduli decrease by 34\% and 38\% in maximum, respectively. Total porosity is clearly the dominant parameter, while spatial calcite distribution has a minor impact, except for a randomly chosen cement distribution within the pore network. Moreover, applying an initial stiffness reduced by 47\% for the calcite cement results only in a slightly weaker mechanical behaviour. Using the quantitative approach introduced here substantially improves the accuracy of predictions in elastic rock properties compared to general analytical methods, and further enables quantification of uncertainties related to spatial variations in porosity and mineral distribution.},
  language  = {en}
}
@misc{WetzelKempkaKuehn2017,
  author    = {Wetzel, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Predicting macroscopic elastic rock properties requires detailed information on microstructure},
  series = {Energy procedia},
  volume    = {125},
  journal   = {Energy procedia},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1876-6102},
  doi       = {10.1016/j.egypro.2017.08.195},
  pages     = {561 -- 570},
  year      = {2017},
  abstract  = {Predicting variations in macroscopic mechanical rock behaviour due to microstructural changes, driven by mineral precipitation and dissolution is necessary to couple chemo-mechanical processes in geological subsurface simulations. We apply 3D numerical homogenization models to estimate Young's moduli for five synthetic microstructures, and successfully validate our results for comparable geometries with the analytical Mori-Tanaka approach. Further, we demonstrate that considering specific rock microstructures is of paramount importance, since calculated elastic properties may deviate by up to 230 \% for the same mineral composition. Moreover, agreement between simulated and experimentally determined Young's moduli is significantly improved, when detailed spatial information are employed.},
  language  = {en}
}
@misc{MaLiKuehnetal.2018,
  author    = {Ma, Jianli and Li, Qi and K{\"u}hn, Michael and Nakaten, Natalie Christine},
  title     = {Power-to-gas based subsurface energy storage},
  series = {Renewable and Sustainable Energy Reviews},
  volume    = {97},
  journal   = {Renewable and Sustainable Energy Reviews},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {1364-0321},
  doi       = {10.1016/j.rser.2018.08.056},
  pages     = {478 -- 496},
  year      = {2018},
  abstract  = {The Renewable energy power generation capacity has been rapidly increasing in China recently. Meanwhile, the contradiction between power supply and demand is becoming increasingly more prominent due to the intermittence of renewable energies. On the other hand, on the mitigation of carbon dioxide (CO2) emissions in China needs immediate attention. Power-to-Gas (PtG), a chemical energy storage technology, can convert surplus electricity into combustible gases. Subsurface energy storage can meet the requirements of long term storage with its large capacity. This paper provides a discussion of the entire PtG energy storage technology process and the current research progress. Based on the comparative study of different geological storage schemes for synthetic methane, their respective research progress and limitations are noted. In addition, a full investigation of the distribution and implementation of global PtG and CO2 capture and storage (CCS) demonstration projects is performed. Subsequently, the opportunities and challenges of the development of this technology in China are discussed based on techno-economic and ecological effects analysis. While PtG is expected to be a revolutionary technology that will replace traditional power systems, the main issues of site selection, energy efficiency and the economy still need to be adequately addressed. Additionally, based on the comprehensive discussion of the results of the analysis, power-to-gas and subsurface energy storage implementation strategies, as well as outlook in China are presented.},
  language  = {en}
}
@article{HennigKuehn2021,
  author    = {Hennig, Theresa and K{\"u}hn, Michael},
  title     = {Potential uranium migration within the geochemical gradient of the opalinus clay system at the Mont Terri},
  series = {Minerals},
  volume    = {11},
  journal   = {Minerals},
  number    = {10},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2075-163X},
  doi       = {10.3390/min11101087},
  pages     = {22},
  year      = {2021},
  abstract  = {Transport properties of potential host rocks for nuclear waste disposal are typically determined in laboratory or in-situ experiments under geochemically controlled and constant conditions. Such a homogeneous assumption is no longer applicable on the host rock scale as can be seen from the pore water profiles of the potential host rock Opalinus Clay at Mont Terri (Switzerland). The embedding aquifers are the hydro-geological boundaries, that established gradients in the 210 m thick low permeable section through diffusive exchange over millions of years. Present-day pore water profiles were confirmed by a data-driven as well as by a conceptual scenario. Based on the modelled profiles, the influence of the geochemical gradient on uranium migration was quantified by comparing the distances after one million years with results of common homogeneous models. Considering the heterogeneous system, uranium migrated up to 24 m farther through the formation depending on the source term position within the gradient and on the partial pressure of carbon dioxide pCO2 of the system. Migration lengths were almost equal for single- and multicomponent diffusion. Differences can predominantly be attributed to changes in the sorption capacity, whereby pCO2 governs how strong uranium migration is affected by the geochemical gradient. Thus, the governing parameters for uranium migration in the Opalinus Clay can be ordered in descending priority: pCO2, geochemical gradients, mineralogical heterogeneity.</p>},
  language  = {en}
}
@article{DeLuciaKuehnLindemannetal.2021,
  author    = {De Lucia, Marco and K{\"u}hn, Michael and Lindemann, Alexander and L{\"u}bke, Max and Schnor, Bettina},
  title     = {POET (v0.1): speedup of many-core parallel reactive transport simulations with fast DHT lookups},
  series = {Geoscientific model development : an interactive open access journal of the European Geosciences Union},
  volume    = {14},
  journal   = {Geoscientific model development : an interactive open access journal of the European Geosciences Union},
  number    = {12},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {1991-959X},
  doi       = {10.5194/gmd-14-7391-2021},
  pages     = {7391 -- 7409},
  year      = {2021},
  abstract  = {Coupled reactive transport simulations are extremely demanding in terms of required computational power, which hampers their application and leads to coarsened and oversimplified domains. The chemical sub-process represents the major bottleneck: its acceleration is an urgent challenge which gathers increasing interdisciplinary interest along with pressing requirements for subsurface utilization such as spent nuclear fuel storage, geothermal energy and CO2 storage. In this context we developed POET (POtsdam rEactive Transport), a research parallel reactive transport simulator integrating algorithmic improvements which decisively speed up coupled simulations. In particular, POET is designed with a master/worker architecture, which ensures computational efficiency in both multicore and cluster compute environments. POET does not rely on contiguous grid partitions for the parallelization of chemistry but forms work packages composed of grid cells distant from each other. Such scattering prevents particularly expensive geochemical simulations, usually concentrated in the vicinity of a reactive front, from generating load imbalance between the available CPUs (central processing units), as is often the case with classical partitions. Furthermore, POET leverages an original implementation of the distributed hash table (DHT) mechanism to cache the results of geochemical simulations for further reuse in subsequent time steps during the coupled simulation. The caching is hence particularly advantageous for initially chemically homogeneous simulations and for smooth reaction fronts. We tune the rounding employed in the DHT on a 2D benchmark to validate the caching approach, and we evaluate the performance gain of POET's master/worker architecture and the DHT speedup on a 3D benchmark comprising around 650 000 grid elements. The runtime for 200 coupling iterations, corresponding to 960 simulation days, reduced from about 24 h on 11 workers to 29 min on 719 workers. Activating the DHT reduces the runtime further to 2 h and 8 min respectively. Only with these kinds of reduced hardware requirements and computational costs is it possible to realistically perform the longterm complex reactive transport simulations, as well as perform the uncertainty analyses required by pressing societal challenges connected with subsurface utilization.},
  language  = {en}
}
@article{TranterDeLuciaKuehn2021,
  author    = {Tranter, Morgan Alan and De Lucia, Marco and K{\"u}hn, Michael},
  title     = {Numerical investigation of barite scaling kinetics in fractures},
  series = {Geothermics : an international journal of geothermal research and its applications},
  volume    = {91},
  journal   = {Geothermics : an international journal of geothermal research and its applications},
  publisher = {Elsevier},
  address   = {Amsterdam [u.a.]},
  issn      = {0375-6505},
  pages     = {14},
  year      = {2021},
  abstract  = {Barite stands out as one of the most ubiquitous scaling agents in deep geothermal systems, responsible for irreversible efficiency loss. Due to complex parameter interplay, it is imperative to utilise numerical simulations to investigate temporal and spatial precipitation effects. A one-dimensional reactive transport model is set up with heterogeneous nucleation and crystal growth kinetics. In line with geothermal systems in the North German Basin, the following parameters are considered in a sensitivity analysis: temperature (25 to 150 degrees C), pore pressure (10 to 50 MPa), fracture aperture (10(-4) to 10(-2) m), flow velocity (10(-3) to 10(0) m s(-1)), molar volume (50.3 to 55.6 cm(3) mol(-1)), contact angle for heterogeneous nucleation (0 degrees to 180 degrees), interfacial tension (0.07 to 0.134 J m(-2)), salinity (0.1 to 1.5 mol kgw(-1) NaCl), pH (5 to 7), and supersaturation ratio (1 to 30). Nucleation and consequently crystal growth can only begin if the threshold supersaturation is exceeded, therefore contact angle and interfacial tension are the most sensitive in terms of precipitation kinetics. If nucleation has occurred, crystal growth becomes the dominant process, which is mainly controlled by fracture aperture. Results show that fracture sealing takes place within months (median 33 days) and the affected range can be on the order of tens of metres (median 10 m). The presented models suggest that barite scaling must be recognised as a serious threat if the supersaturation threshold is exceeded, in which case, large fracture apertures could help to minimise kinetic rates. The models further are of use for adjusting the fluid injection temperature.},
  language  = {en}
}
@misc{KuehnSchoene2017,
  author    = {K{\"u}hn, Michael and Sch{\"o}ne, Tim},
  title     = {Multivariate regression model from water level and production rate time series for the geothermal reservoir Waiwera (New Zealand)},
  series = {Energy procedia},
  volume    = {125},
  journal   = {Energy procedia},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1876-6102},
  doi       = {10.1016/j.egypro.2017.08.196},
  pages     = {571 -- 579},
  year      = {2017},
  abstract  = {Water management tools are necessary to guarantee the preservation of natural resources while ensuring optimum utilization. Linear regression models are a simple and quick solution for creating prognostic capabilities. Multivariate models show higher precision than univariate models. In the case of Waiwera, implementation of individual production rates is more accurate than applying just the total production rate. A maximum of approximately 1,075 m3/day can be pumped to ensure a water level of at least 0.5 m a.s.l. in the monitoring well. The model should be renewed annually to implement new data and current water level trends to keep the quality.},
  language  = {en}
}
@misc{KuehnLiNakatenetal.2017,
  author    = {K{\"u}hn, Michael and Li, Qi and Nakaten, Natalie Christine and Kempka, Thomas},
  title     = {Integrated subsurface gas storage of CO2 and CH4 offers capacity and state-of-the-art technology for energy storage in China},
  series = {Energy procedia},
  volume    = {125},
  journal   = {Energy procedia},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1876-6102},
  doi       = {10.1016/j.egypro.2017.08.039},
  pages     = {14 -- 18},
  year      = {2017},
  abstract  = {Integration and development of the energy supply in China and worldwide is a challenge for the years to come. The innovative idea presented here is based on an extension of the "power-to-gas-to-power" technology by establishing a closed carbon cycle. It is an implementation of a low-carbon energy system based on carbon dioxide capture and storage (CCS) to store and reuse wind and solar energy. The Chenjiacun storage project in China compares well with the German case study for the towns Potsdam and Brandenburg/Havel in the Federal State of Brandenburg based on the Ketzin pilot site for CCS.},
  language  = {en}
}
@article{MaLiKempkaetal.2019,
  author    = {Ma, Jianli and Li, Qi and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Hydromechanical response and impact of gas mixing behavior in subsurface CH4 storage with CO2-based cushion gas},
  series = {Energy \& fuels},
  volume    = {33},
  journal   = {Energy \& fuels},
  number    = {7},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0887-0624},
  doi       = {10.1021/acs.energyfuels.9b00518},
  pages     = {6527 -- 6541},
  year      = {2019},
  abstract  = {Power-to-gas (PtG) stores chemical energy by converting excess electrical energy from renewable sources into an energy-dense gas. Due to its higher available capacity compared to surface-based storage technologies, subsurface storage in geological systems is the most promising approach for efficient and economic realization of the PtG system's storage component. For this purpose, methane (CH4) produced by methanation by means of hydrogen (H2) and carbon dioxide (CO2) is stored in a geological reservoir until required for further use. In this context, CO2 is used as the cushion gas to maintain reservoir pressure and limiting working gas, i.e., (CH4) losses during withdrawal periods. Consequently, mixing of both gases in the reservoir is inevitable. Therefore, it is necessary to minimize the gas mixing region to optimize the efficiency of the PtG system's storage component. In the present study, the physical properties of CH4, CO2 and their mixtures are reviewed. Then, a multicomponent flow model is implemented and validated against published data. Next, a hydromechanically coupled model is established, considering fluid flow through porous media and effective stresses to investigate the mixing behavior of both gases and the mechanical reservoir stability. The simulation results show that, with increasing reservoir thickness and dip angle, the mixing region is reduced during gas injection if CO2 is employed as the cushion gas. In addition, the degree of mixing is lower at higher temperatures. Feasible injection rates and injection schedules can be derived from the integrated reservoir stability analysis. The methodology developed in the present study allows the determination of optimum strategies for storage reservoir selection and gas injection scheduling by minimizing the gas mixing region.},
  language  = {en}
}
@article{WetzelKempkaKuehn2020,
  author    = {Wetzel, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Hydraulic and mechanical impacts of pore space alterations within a sandstone quantified by a flow velocity-dependent precipitation approach},
  series = {Materials},
  volume    = {13},
  journal   = {Materials},
  number    = {14},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {1996-1944},
  doi       = {10.3390/ma13143100},
  pages     = {20},
  year      = {2020},
  abstract  = {Geochemical processes change the microstructure of rocks and thereby affect their physical behaviour at the macro scale. A micro-computer tomography (micro-CT) scan of a typical reservoir sandstone is used to numerically examine the impact of three spatial alteration patterns on pore morphology, permeability and elastic moduli by correlating precipitation with the local flow velocity magnitude. The results demonstrate that the location of mineral growth strongly affects the permeability decrease with variations by up to four orders in magnitude. Precipitation in regions of high flow velocities is characterised by a predominant clogging of pore throats and a drastic permeability reduction, which can be roughly described by the power law relation with an exponent of 20. A continuous alteration of the pore structure by uniform mineral growth reduces the permeability comparable to the power law with an exponent of four or the Kozeny-Carman relation. Preferential precipitation in regions of low flow velocities predominantly affects smaller throats and pores with a minor impact on the flow regime, where the permeability decrease is considerably below that calculated by the power law with an exponent of two. Despite their complete distinctive impact on hydraulics, the spatial precipitation patterns only slightly affect the increase in elastic rock properties with differences by up to 6.3\% between the investigated scenarios. Hence, an adequate characterisation of the spatial precipitation pattern is crucial to quantify changes in hydraulic rock properties, whereas the present study shows that its impact on elastic rock parameters is limited. The calculated relations between porosity and permeability, as well as elastic moduli can be applied for upscaling micro-scale findings to reservoir-scale models to improve their predictive capabilities, what is of paramount importance for a sustainable utilisation of the geological subsurface.},
  language  = {en}
}
@article{StedingKempkaKuehn2021,
  author    = {Steding, Svenja and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {How insoluble inclusions and intersecting layers affect the leaching process within potash seams},
  series = {Applied Sciences : open access journal},
  volume    = {11},
  journal   = {Applied Sciences : open access journal},
  number    = {19},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2076-3417},
  doi       = {10.3390/app11199314},
  pages     = {21},
  year      = {2021},
  abstract  = {Potash seams are a valuable resource containing several economically interesting, but also highly soluble minerals. In the presence of water, uncontrolled leaching can occur, endangering subsurface mining operations. In the present study, the influence of insoluble inclusions and intersecting layers on leaching zone evolution was examined by means of a reactive transport model. For that purpose, a scenario analysis was carried out, considering different rock distributions within a carnallite-bearing potash seam. The results show that reaction-dominated systems are not affected by heterogeneities at all, whereas transport-dominated systems exhibit a faster advance in homogeneous rock compositions. In return, the ratio of permeated rock in vertical direction is higher in heterogeneous systems. Literature data indicate that most natural potash systems are transport-dominated. Accordingly, insoluble inclusions and intersecting layers can usually be seen as beneficial with regard to reducing hazard potential as long as the mechanical stability of leaching zones is maintained. Thereby, the distribution of insoluble areas is of minor impact unless an inclined, intersecting layer occurs that accelerates leaching zone growth in one direction. Moreover, it is found that the saturation dependency of dissolution rates increases the growth rate in the long term, and therefore must be considered in risk assessments.},
  language  = {en}
}
@article{TillnerLangerKempkaetal.2016,
  author    = {Tillner, Elena and Langer, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Fault damage zone volume and initial salinity distribution determine intensity of shallow aquifer salinisation in subsurface storage},
  series = {Hydrology and earth system sciences : HESS},
  volume    = {20},
  journal   = {Hydrology and earth system sciences : HESS},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {1027-5606},
  doi       = {10.5194/hess-20-1049-2016},
  pages     = {1049 -- 1067},
  year      = {2016},
  abstract  = {Injection of fluids into deep saline aquifers causes a pore pressure increase in the storage formation, and thus displacement of resident brine. Via hydraulically conductive faults, brine may migrate upwards into shallower aquifers and lead to unwanted salinisation of potable groundwater resources. In the present study, we investigated different scenarios for a potential storage site in the Northeast German Basin using a three-dimensional (3-D) regional-scale model that includes four major fault zones. The focus was on assessing the impact of fault length and the effect of a secondary reservoir above the storage formation, as well as model boundary conditions and initial salinity distribution on the potential salinisation of shallow groundwater resources. We employed numerical simulations of brine injection as a representative fluid. Our simulation results demonstrate that the lateral model boundary settings and the effective fault damage zone volume have the greatest influence on pressure build-up and development within the reservoir, and thus intensity and duration of fluid flow through the faults. Higher vertical pressure gradients for short fault segments or a small effective fault damage zone volume result in the highest salinisation potential due to a larger vertical fault height affected by fluid displacement. Consequently, it has a strong impact on the degree of shallow aquifer salinisation, whether a gradient in salinity exists or the saltwater-freshwater interface lies below the fluid displacement depth in the faults. A small effective fault damage zone volume or low fault permeability further extend the duration of fluid flow, which can persist for several tens to hundreds of years, if the reservoir is laterally confined. Laterally open reservoir boundaries, large effective fault damage zone volumes and intermediate reservoirs significantly reduce vertical brine migration and the potential of freshwater salinisation because the origin depth of displaced brine is located only a few decametres below the shallow aquifer in maximum. The present study demonstrates that the existence of hydraulically conductive faults is not necessarily an exclusion criterion for potential injection sites, because salinisation of shallower aquifers strongly depends on initial salinity distribution, location of hydraulically conductive faults and their effective damage zone volumes as well as geological boundary conditions.},
  language  = {en}
}
@misc{TillnerLangerKempkaetal.2016,
  author    = {Tillner, Elena and Langer, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Fault damage zone volume and initial salinity distribution determine intensity of shallow aquifer salinisation in subsurface storage},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {548},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-41185},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-411854},
  pages     = {19},
  year      = {2016},
  abstract  = {Injection of fluids into deep saline aquifers causes a pore pressure increase in the storage formation, and thus displacement of resident brine. Via hydraulically conductive faults, brine may migrate upwards into shallower aquifers and lead to unwanted salinisation of potable groundwater resources. In the present study, we investigated different scenarios for a potential storage site in the Northeast German Basin using a three-dimensional (3-D) regional-scale model that includes four major fault zones. The focus was on assessing the impact of fault length and the effect of a secondary reservoir above the storage formation, as well as model boundary conditions and initial salinity distribution on the potential salinisation of shallow groundwater resources. We employed numerical simulations of brine injection as a representative fluid. Our simulation results demonstrate that the lateral model boundary settings and the effective fault damage zone volume have the greatest influence on pressure build-up and development within the reservoir, and thus intensity and duration of fluid flow through the faults. Higher vertical pressure gradients for short fault segments or a small effective fault damage zone volume result in the highest salinisation potential due to a larger vertical fault height affected by fluid displacement. Consequently, it has a strong impact on the degree of shallow aquifer salinisation, whether a gradient in salinity exists or the saltwater-freshwater interface lies below the fluid displacement depth in the faults. A small effective fault damage zone volume or low fault permeability further extend the duration of fluid flow, which can persist for several tens to hundreds of years, if the reservoir is laterally confined. Laterally open reservoir boundaries, large effective fault damage zone volumes and intermediate reservoirs significantly reduce vertical brine migration and the potential of freshwater salinisation because the origin depth of displaced brine is located only a few decametres below the shallow aquifer in maximum. The present study demonstrates that the existence of hydraulically conductive faults is not necessarily an exclusion criterion for potential injection sites, because salinisation of shallower aquifers strongly depends on initial salinity distribution, location of hydraulically conductive faults and their effective damage zone volumes as well as geological boundary conditions.},
  language  = {en}
}
@misc{MartensJuhlinBruckmanetal.2018,
  author    = {Martens, Sonja and Juhlin, Christopher and Bruckman, Viktor J. and Mitchell, Kristen and Griffiths, Luke and K{\"u}hn, Michael},
  title     = {Editorial: energy, resources and the environment},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  number    = {834},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-42788},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-427880},
  pages     = {6},
  year      = {2018},
  abstract  = {Every year, the European Geosciences Union (EGU) brings together experts from all over the world at its General Assembly, covering all disciplines of the Earth, planetary and space sciences. The EGU Division on Energy, Resources and the Environment (ERE) is concerned with one of the humankind's most challenging goals - providing affordable, reliable and sustainable energy and other georesources. A collection of contributions from the ERE Division at the EGU General Assembly 2018 is assembled within the present special issue in Advances in Geosciences.},
  language  = {en}
}
@misc{KuehnKempkadeLuciaetal.2017,
  author    = {K{\"u}hn, Michael and Kempka, Thomas and de Lucia, Marco and Scheck-Wenderoth, Magdalena},
  title     = {Dissolved CO2 storage in geological formations with low pressure, low risk and large capacities},
  series = {Energy procedia},
  volume    = {114},
  journal   = {Energy procedia},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1876-6102},
  doi       = {10.1016/j.egypro.2017.03.1607},
  pages     = {4722 -- 4727},
  year      = {2017},
  abstract  = {Geological CO2 storage is a mitigation technology to reduce CO2 emissions from fossil fuel combustion. However, major concerns are the pressure increase and saltwater displacement in the mainly targeted deep groundwater aquifers due to injection of supercritical CO2. The suggested solution is storage of CO2 exclusively in the dissolved state. In our exemplary regional case study of the North East German Basin based on a highly resolved temperature and pressure distribution model and a newly developed reactive transport coupling, we have quantified that 4.7 Gt of CO2 can be stored in solution compared to 1.5 Gt in the supercritical state.},
  language  = {en}
}
@article{WetzelKempkaKuehn2021,
  author    = {Wetzel, Maria and Kempka, Thomas and K{\"u}hn, Michael},
  title     = {Diagenetic trends of synthetic reservoir sandstone properties assessed by digital rock physics},
  series = {Minerals},
  volume    = {11},
  journal   = {Minerals},
  number    = {2},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2075-163X},
  doi       = {10.3390/min11020151},
  pages     = {21},
  year      = {2021},
  abstract  = {Quantifying interactions and dependencies among geometric, hydraulic and mechanical properties of reservoir sandstones is of particular importance for the exploration and utilisation of the geological subsurface and can be assessed by synthetic sandstones comprising the microstructural complexity of natural rocks. In the present study, three highly resolved samples of the Fontainebleau, Berea and Bentheim sandstones are generated by means of a process-based approach, which combines the gravity-driven deposition of irregularly shaped grains and their diagenetic cementation by three different schemes. The resulting evolution in porosity, permeability and rock stiffness is examined and compared to the respective micro-computer tomographic (micro-CT) scans. The grain contact-preferential scheme implies a progressive clogging of small throats and consequently produces considerably less connected and stiffer samples than the two other schemes. By contrast, uniform quartz overgrowth continuously alters the pore space and leads to the lowest elastic properties. The proposed stress-dependent cementation scheme combines both approaches of contact-cement and quartz overgrowth, resulting in granulometric, hydraulic and elastic properties equivalent to those of the respective micro-CT scans, where bulk moduli slightly deviate by 0.8\%, 4.9\% and 2.5\% for the Fontainebleau, Berea and Bentheim sandstone, respectively. The synthetic samples can be further altered to examine the impact of mineral dissolution or precipitation as well as fracturing on various petrophysical correlations, which is of particular relevance for numerous aspects of a sustainable subsurface utilisation.},
  language  = {en}
}
@article{DeLuciaKuehn2021,
  author    = {De Lucia, Marco and K{\"u}hn, Michael},
  title     = {DecTree v1.0-chemistry speedup in reactive transport simulations},
  series = {Geoscientific model development : an interactive open access journal of the European Geosciences Union},
  volume    = {14},
  journal   = {Geoscientific model development : an interactive open access journal of the European Geosciences Union},
  number    = {7},
  publisher = {Copernicus},
  address   = {G{\"o}ttingen},
  issn      = {1991-959X},
  doi       = {10.5194/gmd-14-4713-2021},
  pages     = {4713 -- 4730},
  year      = {2021},
  abstract  = {The computational costs associated with coupled reactive transport simulations are mostly due to the chemical subsystem: replacing it with a pre-trained statistical surrogate is a promising strategy to achieve decisive speedups at the price of small accuracy losses and thus to extend the scale of problems which can be handled. We introduce a hierarchical coupling scheme in which "full-physics" equation-based geochemical simulations are partially replaced by surrogates. Errors in mass balance resulting from multivariate surrogate predictions effectively assess the accuracy of multivariate regressions at runtime: inaccurate surrogate predictions are rejected and the more expensive equation-based simulations are run instead. Gradient boosting regressors such as XGBoost, not requiring data standardization and being able to handle Tweedie distributions, proved to be a suitable emulator. Finally, we devise a surrogate approach based on geochemical knowledge, which overcomes the issue of robustness when encountering previously unseen data and which can serve as a basis for further development of hybrid physics-AI modelling.},
  language  = {en}
}
@book{Kuehn2013,
  author    = {K{\"u}hn, Michael},
  title     = {Beitr{\"a}ge der Georessource Untergrund zur Energiewende : Antrittsvorlesung 2013-06-12},
  publisher = {Univ.-Bibl.},
  address   = {Potsdam},
  year      = {2013},
  abstract  = {Die Ziele der Energiewende sind ehrgeizig. Der Vortrag zeigt, welche Nutzungsoptionen der Untergrund bietet - z.B. geothermische Energiegewinnung oder geologische Speicherung. F{\"u}r eine gesellschaftsweite, sachliche Diskussion werden konkrete Zahlen nicht nur zu den Chancen, sondern auch zu den Risiken ben{\"o}tigt.},
  language  = {de}
}
@article{TranterDeLuciaKuehn2021,
  author    = {Tranter, Morgan Alan and De Lucia, Marco and K{\"u}hn, Michael},
  title     = {Barite scaling potential modelled for fractured-porous geothermal reservoirs},
  series = {Minerals},
  volume    = {11},
  journal   = {Minerals},
  number    = {11},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2075-163X},
  doi       = {10.3390/min11111198},
  pages     = {22},
  year      = {2021},
  abstract  = {Barite scalings are a common cause of permanent formation damage to deep geothermal reservoirs. Well injectivity can be impaired because the ooling of saline fluids reduces the solubility of barite, and the continuous re-injection of supersaturated fluids forces barite to precipitate in the host rock. Stimulated reservoirs in the Upper Rhine Graben often have multiple relevant flow paths in the porous matrix and fracture zones, sometimes spanning multiple stratigraphical units to achieve the economically necessary injectivity. While the influence of barite scaling on injectivity has been investigated for purely porous media, the role of fractures within reservoirs consisting of both fractured and porous sections is still not well understood. Here, we present hydro-chemical simulations of a dual-layer geothermal reservoir to study the long-term impact of barite scale formation on well injectivity. Our results show that, compared to purely porous reservoirs, fractured porous reservoirs have a significantly reduced scaling risk by up to 50\%, depending on the flow rate ratio of fractures. Injectivity loss is doubled, however, if the amount of active fractures is increased by one order of magnitude, while the mean fracture aperture is decreased, provided the fractured aquifer dictates the injection rate. We conclude that fractured, and especially hydraulically stimulated, reservoirs are generally less affected by barite scaling and that large, but few, fractures are favourable. We present a scaling score for fractured-porous reservoirs, which is composed of easily derivable quantities such as the radial equilibrium length and precipitation potential. This score is suggested for use approximating the scaling potential and its impact on injectivity of a fractured-porous reservoir for geothermal exploitation.},
  language  = {en}
}
@article{TranterDeLuciaWolfgrammetal.2020,
  author    = {Tranter, Morgan Alan and De Lucia, Marco and Wolfgramm, Markus and K{\"u}hn, Michael},
  title     = {Barite scale formation and injectivity loss models for geothermal systems},
  series = {Water},
  volume    = {12},
  journal   = {Water},
  number    = {11},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2073-4441},
  doi       = {10.3390/w12113078},
  pages     = {24},
  year      = {2020},
  abstract  = {Barite scales in geothermal installations are a highly unwanted effect of circulating deep saline fluids. They build up in the reservoir if supersaturated fluids are re-injected, leading to irreversible loss of injectivity. A model is presented for calculating the total expected barite precipitation. To determine the related injectivity decline over time, the spatial precipitation distribution in the subsurface near the injection well is assessed by modelling barite growth kinetics in a radially diverging Darcy flow domain. Flow and reservoir properties as well as fluid chemistry are chosen to represent reservoirs subject to geothermal exploration located in the North German Basin (NGB) and the Upper Rhine Graben (URG) in Germany. Fluids encountered at similar depths are hotter in the URG, while they are more saline in the NGB. The associated scaling amount normalised to flow rate is similar for both regions. The predicted injectivity decline after 10 years, on the other hand, is far greater for the NGB (64\%) compared to the URG (24\%), due to the temperature- and salinity-dependent precipitation rate. The systems in the NGB are at higher risk. Finally, a lightweight score is developed for approximating the injectivity loss using the Damkohler number, flow rate and total barite scaling potential. This formula can be easily applied to geothermal installations without running complex reactive transport simulations.},
  language  = {en}
}
@article{WilkeSchettlerViethHillebrandetal.2018,
  author    = {Wilke, Franziska Daniela Helena and Schettler, Georg and Vieth-Hillebrand, Andrea and K{\"u}hn, Michael and Rothe, Heike},
  title     = {Activity concentrations of U-238 and Ra-226 in two European black shales and their experimentally-derived leachates},
  series = {Journal of Environmental Radioactivity},
  volume    = {190},
  journal   = {Journal of Environmental Radioactivity},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0265-931X},
  doi       = {10.1016/j.jenvrad.2018.05.005},
  pages     = {122 -- 129},
  year      = {2018},
  abstract  = {We performed leaching tests at elevated temperatures and pressures with an Alum black shale from Bomholm, Denmark and a Posidonia black shale from Lower Saxony, Germany. The Alum shale is a carbonate free black shale with pyrite and barite, containing 74.4 mu g/g U. The Posidonia shales is a calcareous shale with pyrite but without detectable amounts of barite containing 3.6 mu g/g U. Pyrite oxidized during the tests forming sulfuric acid which lowered the pH on values between 2 and 3 of the extraction fluid from the Alum shale favoring a release of U from the Alum shale to the fluid during the short-term and in the beginning of the long-term experiments. The activity concentration of U-238 is as high as 23.9 mBq/ml in the fluid for those experiments. The release of U and Th into the fluid is almost independent of pressure. The amount of uranium in the European shales is similar to that of the Marcellus Shale in the United States but the daughter product of U-238, the Ra-226 activity concentrations in the experimentally derived leachates from the European shales are quite low in comparison to that found in industrially derived flowback fluids from the Marcellus shale. This difference could mainly be due to missing Cl in the reaction fluid used in our experiments and a lower fluid to solid ratio in the industrial plays than in the experiments due to subsequent fracking and minute cracks from which Ra can easily be released.},
  language  = {en}
}