Displays

The mining industry globally produces millions of tons of waste rock every year. The weathering of exposed metal(loid)-rich waste rock can produce poor-quality effluent, and mine sites therefore need to establish water-quality management strategies that predict and mitigate environmental impacts. Technical frameworks to support drainage quality predictions and industrial waste-rock management typically combine classic static and kinetic testing procedures, field-scale experiments and sometimes geochemical equilibrium- and reactive-transport models. However, predictions of waste rock weathering and drainage processes remain challenging on relevant spatiotemporal scales, due to site-specificity in waste rock and local weathering conditions, unresolved heterogeneity in large waste-rock systems and the intricate (non-linear) coupling between chemical kinetics and mass- and heat transfer processes.

We synthesized long-term (>10 yr) hydrogeochemical field data from a multiscale experimental research program at the Antamina mine, Peru. At Antamina, various waste-rock materials have been extensively hydraulically, physically and geochemically characterized and weathered at different spatiotemporal scales. This data set provides a unique opportunity to quantitatively assess the mechanisms that affect drainage from different waste-rock types under field conditions. Monitoring of weathering rates in humidity cell tests (~1 kg), column experiments (~170 kg), field barrel kinetic tests (~350 kg), and mesoscale experimental piles (~6,500,000 kg) revealed that normalized mass loadings from different waste-rock types systematically decreased with increasing experimental scale.

We developed a process-based reactive-transport framework to reproduce the recorded waste-rock drainage trends from the various field experiments. For each of the experiments, 1-D reactive-transport models were constructed in MIN3P-HPC, all including the same formulations for, e.g., transient unsaturated flow, advective-diffusive transport of aqueous species, gas diffusion, gas-liquid partitioning and equilibrium or kinetic mineral dissolution and precipitation reactions. The models were exclusively parameterized with measured field hydrostatics (e.g., tracer testing, volumetric water contents; van Genuchten parameters), analyzed physicochemical bulk waste-rock properties (e.g., bulk geochemistry, mineral content, particle size), or adopted literature values (e.g., kinetic rate laws and constants).

At all experimental scales, the recorded drainage quality evolution could be successfully reproduced with the consistent suite of field-parameterized physical transport processes and kinetic rate laws. A comparison of fitted effective rate coefficients reveals that reduced weathering rates at increasing scales mostly originate from decreasing specific mineral surface areas (particle sizes increase with experimental scale) and possibly by surface passivation, although the effects of flow bypassing and channeling are not yet fully investigated. This work demonstrates that with efforts focused on the identification and parameterization of the relevant physicochemical processes, effective yet process-based models can be developed from readily available bulk waste-rock parameters to predict and upscale mine waste rock weathering and drainage quality trends across laboratory-to-practice-relevant scale ranges.

How to cite: Vriens, B., Seigneur, N., Aranda, C., Mayer, U., and Beckie, R.: Reactive-transport modeling of hydrogeochemical weathering processes in mine waste rock across a wide spatiotemporal scale range, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5853, https://doi.org/10.5194/egusphere-egu2020-5853, 2020.

The interface between reservoir/cap rocks and the Portland cement around boreholes is a possible leakage pathway during deep geological injection of CO₂. To study the alteration of cement and rock, laboratory experiments involving flow along this interface were performed. Cylindrical cores of about 5 cm in length and 2.5 cm in diameter and composed of half-cylinders of cement and rock (sandstone, limestone, marl) were used. They were reacted with a synthetic sulfate-rich saline groundwater under (a) atmospheric conditions (10^-3.4 bar CO₂, 25ºC, pH 6.2) and (b) supercritic conditions (130 bar CO₂, 60ºC, pH about 3) in flow-through reactors. Tracer (LiBr) tests were performed prior to the injection of the saline solution in the atmospheric experiments to characterize cement diffusivity. The evolution of solution chemistry at the outlet was monitored over time. Rock and cement were analyzed at the end of the experiments (SEM, XRD, profilometry).

In the atmospheric experiments pH increased up to about 11 (tracer tests) and 8 (groundwater injection, brucite precipitation). Calculated outlet pH was about 4 under supercritic conditions. Major-element concentrations showed little change during the atmospheric experiments, while Ca excess and S deficit were observed under supercritic conditions. Intense brucite precipitation was observed on the cement surface after the atmospheric experiments, while an apparently amorphous red-colored phase precipitated under supercritic conditions. Rock surfaces evidenced calcite dissolution in the supercritic experiments, while alteration was little in the atmospheric experiments. Some gypsum precipitation was also observed. Interface aperture increased during the supercritic experiments.

2D reactive transport modeling (CrunchFlow) was used to interpret the results. Phase reactivities (surface areas), and in some cases diffusion coefficients (rock and cement), were adjusted to fit models to measurements (solution and solid). Under atmospheric conditions, brucite precipitation (and decrease in porosity) results from the mixing by diffusion of the Mg in the input solution and the alkalinity in the cement. Ca from portlandite dissolution and sulfate from the input solution drives the precipitation of gypsum. For the supercritic experiments, model results show intense dissolution of portlandite, ettringite, siliceous hydrogarnet and hydrotalcite, extending for about 3 mm into the cement and causing an increase in porosity. The Ca released precipitates as calcite, with carbonate provided by the CO₂-rich input solution. As the portlandite front moves into the cement, calcite dissolves next to the interface and some of the Ca precipitates as gypsum. Coupled calcite dissolution and gypsum precipitation also occurs, to a lesser extent, in the rock side. The calculations also result in the precipitation of small amounts of ferrihydrite, gibbsite and boehmite, which could correspond to the observed red-colored precipitates. Importantly, the adjusted values of the reactive surface areas for the different experiments point to a larger reactivity of the cement under supercritic conditions.

How to cite: Soler, J. M., Fernández-Rojo, L., Chaparro, M. C., Queralt, I., Galí, S., and Cama, J.: Flow and reaction along the cement-rock interface during CO2 injection. Laboratory experiments and modeling., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5234, https://doi.org/10.5194/egusphere-egu2020-5234, 2020.

Unconventional oil and gas production involves the use of acidic hydraulic fracturing fluids that interact with the rock matrix bordering the fractures. As a result, fracture permeability and mass transfer between the matrix and the fracture can be altered, affecting production performance. The evolution of the altered zones are controlled by the gradients of pH and concentrations of various species perpendicular to the fracture-matrix interface, mineral reactions in the matrix as the reactive fluid diffuse into the matrix, and potential mineral coating on the fracture surface where the matrix fluid and fracture fluid mix. In this study, we use reactive transport model to investigate the evolution of the altered zones bordering the fractures. The simulations are based on batch and fracture flow experiments of shales and syntheized hydraulic fracturing fluids. Through the simulations, we quantify the reaction front of different mineral phases and the change of local porosity, and examine their dependence on mineral composition and fluid chemistry. We also discuss the impacts of the altered zones on matrix diffusivity and fracture permeability.

How to cite: Deng, H., Molins, S., Steefel, C., Bargar, J., Jew, A., Hakala, A., Lopano, C., and Xiong, W.: Modeling and experimental investigation of the evolution of altered zones at the shale-fluid interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21236, https://doi.org/10.5194/egusphere-egu2020-21236, 2020.

The north-eastern suburbs of Paris are most prone to sinkhole development due to the natural dissolution of gypsum rocks in contact with groundwater flow. This dissolution induces a loss of solid material creating underground voids with different shapes and sizes that can lead to large underground collapse or subsidence. Until now, there is still a high uncertainty regarding the dissolution mechanisms of natural gypsum and the hydrodynamic, chemical and mechanical conditions involved in this process.

This work has two broad aims: a) to evaluate the variability of gypsum dissolution rate as function of the surface roughness and heterogeneity; b) to identify the respective role of particle transport and dissolution processes in the formation of cavities in gypsum horizons. In fact, for gypsum with interstitial porosity, the release of grains and their transport by the flow (suffusion phenomenon) could very strongly increase the growth of the cavity compared to taking into account only the dissolution.

A variety of experimental protocols have been developed to quantify the parameters controlling the studied phenomena. Rotating disk and batch experiments are employed to determine the kinetic rate model parameters of different varieties of natural gypsum with different porosity and insoluble contents  following the empirical rate expression derived from mixed kinetic theory. To get results more representative of in-situ conditions, they are adjusted according to the specific roughness and texture of each sample. The impact of erosion and particle transport related to gypsum dissolution is determined by controlled leaching tests on external surfaces. It consists of immersing entirely a block of gypsum in a horizontal canal filled with water circulating at a low velocity (≃ 10^-4 to 10^-3 m/s) so that the grains detached during dissolution are not carried out by the flow but collected in a container placed under the block. These grains are then observed microscopically and analyzed by X-ray diffraction to better determine their mineralogy. For each gypsum block tested, the particlar flux is found low composed mostly of insoluble grains with only few gypsum grains released. The distribution of insoluble at the interface has a large influence on the dissolution. When they are present as a form of thin layers, they create local reliefs, depending on their cohesion, which disturbs the flow and locally enhance the gypsum dissolution. When they are distributed at the boundary of gypsum grains, they serve as a coating which protects them and drastically slows down the dissolution kinetics. 

How to cite: Zaier, I., Billiotte, J., Charmoille, A., and Laouafa, F.: Quantification of the dissolution kinetics of natural gypsum and particle transport processes in the evolution of dissolution cavities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9277, https://doi.org/10.5194/egusphere-egu2020-9277, 2020.

The understanding of dissolution and precipitation of minerals and its impact on the transport of fluids in fractured media is essential for various subsurface applications including shale gas production using hydraulic fracturing (“fracking”), CO₂ sequestration, or geothermal energy extraction. The implementation of such coupled processes into numerical reactive transport codes requires a mechanistic process understanding and model validation with quantitative experiments. In this context, we developed a microfluidic “lab-on-chip” of a reactive fractured porous medium of 800 µm × 900 µm size with 10 µm depth. The fractured medium consisted of compacted celestine grains (grain size 4 – 9 µm). A BaCl₂ solution was injected into the microreactor at a flow rate of 500 nl min^-1, leading to the dissolution of celestine and an epitaxial growth of barite on its surface (Poonoosamy et al., 2016). Our investigations including confocal Raman spectroscopic techniques allowed for monitoring the temporal mineral transformation at the pore scale in 2D and 3D geometries. The fractured porous medium causes a heterogeneous flow field in the microreactor that leads to spatially different mineral transformation rates. In these experiments, the dynamic evolution of surface passivation processes depends on two intertwined processes: i) the dissolution of the primary mineral that is needed for the subsequent precipitation, and ii) the suppression of the dissolution reaction as a result of secondary mineral precipitation. However, the description of evolving reactive surface areas to account for mineral passivation mechanisms in reactive transport models following Daval et al. (2009) showed several limitations, and prompt for an improved description of passivation processes that includes the diffusive properties of secondary phases (Poonoosamy et al., 2020). The results of the ongoing microfluidic experiments in combination with advanced pore-scale modelling will provide new insights regarding application and extension of the description of surface passivation processes to be included in (continuum-scale) reactive transport models.

Daval D., Martinez I., Corvisier J., Findling N., Goffé B. and Guyotac F. (2009) Carbonation of Ca-bearing silicates, the case of wollastonite: Experimental investigations and kinetic modelling. Chem. Geol. 265(1–2), 63-78.

Poonoosamy J., Curti E., Kosakowski G., Van Loon L. R., Grolimund D. and Mäder U. (2016) Barite precipitation following celestite dissolution in a porous medium: a SEM/BSE and micro XRF/XRD study. Geochim. Cosmochim. Acta 182, 131-144.

Poonoosamy J., Klinkenberg M., Deissmann G., Brandt F., Bosbach D., Mäder U. and Kosakowski G. (2020) Effects of solution supersaturation on barite precipitation in porous media and consequences on permeability: experiments and modelling. Geochim. Cosmochim. Acta 270, 43-60.

How to cite: Poonoosamy, J., Roman, S., Soulaine, C., Deng, H., Molins, S., Tournassat, C., Deissmann, G., Burmeister, A., Kohlheyer, D., and Bosbach, D.: A “lab-on-a-chip” experiment for assessing mineral precipitation processes in fractured porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13613, https://doi.org/10.5194/egusphere-egu2020-13613, 2020.

Bio-grouting using ureolytic microorganisms has been developed over the past decade for civil engineering applications including: (i) sealing fractures in rock, (ii) sealing cracks in cement, (iii) reducing the permeability of porous media  and (iv) soil stabilisation and (v) repair of concrete and stone. This study investigates the potential application of microbially induced carbonate precipitation (MICP) within the oil and gas industry. To deploy MICP in a well abandonment context a more in-depth knowledge of the influence and performance under elevated subsurface pressures and temperatures is required.

Batch experiments investigated the ureolytic activity at subsurface temperatures ranging from 20-90°C and fluid pressures from 1-13MPa for up to 2hrs exposure time. Strong evidence of increased ureolytic activity was observed in specimens at temperatures of 60°C and above, but with increasing exposure time ureolytic activity ceased. In comparison increased fluid pressures had little influence on ureolytic activity. Our results imply that the bacterial cell protects the enzyme from denaturation at elevated temperature conditions.

A second set of experiments consisted of multiple injections of the treatment fluids in a fine-grained sandstone sampled from the Brent sandstone formation of the Dunlin oilfield in the North Sea. With a focus on simulating the in-situ environmental conditions, we set-up a high pressure high temperature system consisting of a HPLC pump, water bath, Hassler core-holder with pressure capabilities of up to 2400psi and a temperature rating of 90°C with high sensitivity pressure transducers and a backpressure regulator. The cores were exposed to realistic North Sea subsurface temperatures of 20, 50, 60°C and fluid pressures of 442, 1326, 1621psi according to their corresponding depths: at 1000ft, 3000ft and 3667ft.

The study investigated the influence of the pressure and temperature conditions on (i) permeability reduction, (ii) distribution of CaCO₃ precipitates via X-CT imaging and (iii) mineralogy via FE-EPMA coupled with EDX/WDX spectroscopy.

Permeability reductions in the coarse-grained sandstones of 5 orders of magnitude were achieved in all the subsurface temperature-pressure combinations tests. Micro xCT scans indicate that CaCO₃ precipitation occurred closer to the inlet as the temperature and pressure increased, due in part to the higher ureolytic activity at higher temperatures and the lower solubility of CaCO₃ at higher temperatures. At elevated pressure and temperature conditions the energy barrier to transform from a calcite dominated system could be overcome and formed predominantly aragonite.

This study has demonstrated the potential for deploying MICP at subsurface conditions in oil and gas applications. The biotechnology itself could be used to seal off reservoir formations in mature oil and gas assets, repair fluid migration pathways or act as an environmental wellbore barrier element and therefore could ultimately reduce the number of well barriers required to be installed during plugging and abandonment.

How to cite: Steinacher, F., Pagano, D. A. G., El Mountassir, D. G., Minto, D. J. M., and Lunn, P. R. J.: Experimental investigation of microbially induced carbonate precipitation in sandstone cores under in-situ North Sea temperature and pressure conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19049, https://doi.org/10.5194/egusphere-egu2020-19049, 2020.

Salts in porous rocks are destructive agents that may derive from various sources such as capillary rise of groundwater, rock weathering, or atmospheric deposition; which later precipitate within or on the surface of the media. This results in clogged pore structures, and hence affects the vapor flux between the evaporation front, where subsurface evaporation takes place, and the rock surface. It is known that salt precipitation results in lower evaporation rate; however, there is still need to investigate various aspects of these processes. In the present study, sodium chloride and magnesium sulfate salts were used for evaporation experiments using loose porous media of different structure: similar grain size of natural sand and well-rounded glass beads. The combined effect of grain angularity and concentration of salt solutions were examined in cylindrical glass containers. For each experiment, the mass loss is calculated with periodic weighting, and visual changes are measured and documented. The laboratory experiments were performed in three stages: a) different type of salts under the same conditions, b) same type of salt in different concentrations, c) same type of salt solution in structurally different porous medium. We found that magnesium sulfate caused decrease in evaporation rate by a factor of 5 compare to the same concentration of sodium chloride. Comparing the sodium chloride solution in different concentrations, the solution with higher concentration showed a slower trend growth on evaporation rate. Regarding the difference of pore structure, sodium chloride created a salt crust that was covering circa 90% of the surface with superficial fractures in the case of natural sand whereas in the case of glass beads, it covered only less than 40% of the surface. Nevertheless, the evaporation rate in the described experiment from natural sand showed a faster trend growth than in the case of the glass beads, which agrees with the observation of the evaporation front, which dropped down relatively faster in natural sand than in the glass beads. This indicates that materials with more rounded grains tend to have lower evaporation rate with less visible salt crust on the surface, whereas materials with rougher grain surface tend to have higher evaporation rate with considerably thicker and wider salt crust on the surface. Therefore, the pore structure might be one of the important determinatives of salt weathering patterns in porous materials.

How to cite: Karatas, T.: Effects of Salt Precipitation on Evaporation Rate in Porous Media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16510, https://doi.org/10.5194/egusphere-egu2020-16510, 2020.

Heavy metals from mines affect soil and groundwater, cause aand severely impact on the health of local residents. The soil samples were characterized the for distribution, and by the chemical speciation method , and then estimated the human health risks of the two mine-affected soils after stabilization were estimated. Two extraction techniques (Tessier and Wenzel methods) were applied to fractionate metals, such as arsenic (As) and zinc (Zn), to quantify the chemical status of metals in the soils. The mobility of As and Zn was predicted using ASTM test and sequential extraction (Tessier and Wenzel) methods results. The correlation coefficients of As and Zn mobility prediction using Tessier and Wenzel Fraction 1 were (0.920 and 0.815), respectively. Sum The of fractionsum of fractions (F1+F2+F3) showed the highest correlation coefficients value and F value for mobility prediction of both As and Zn. The hazardous indices (HI) for non-carcinogenic risk and carcinogenic risk (CR) to humans were evaluated according to the pseudo-total concentrations of metal in soils. The CR values of carcinogenic for As were within the ranges from 1.38 × 10^-4 to 1.25 × 10^-3 and 3.71 × 10^-4 to 3.35 × 10^-3 for both Young Dong (YD) and Dea San (DS), respectively. The HI for non-carcinogenic risk was highest for As in the YD (2.77) and DS (7.46) soils, which covered approximately 96 and 84% of HI, respectively. In summary, the contribution of As to risk from heavy metals was dominant.

How to cite: Choi, J. and Ahn, Y.: Heavy metals speciation with prediction model for heavy metal mobility and risk assessment in mine-affected soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1263, https://doi.org/10.5194/egusphere-egu2020-1263, 2020.

     The potential development of shale gas has brought with it several concerns about environmental impacts, these include: induced seismicity, air pollution, and groundwater contamination. During hydraulic fracturing for shale gas, large volumes of oxic and acidic water are injected into the gas-bearing formations. The injected fluids contain a range of additives and will mix and react with the in-situ groundwater and shale rock with the potential to drive water-rock interactions; release metal contaminants; alter the permeability of the bedrock; with each of these affecting the transport and recovery of water, hydrocarbons, and contamination. The purpose of this study is to understand the geochemical processes and inorganic metals release during hydraulic fracturing to assess the potential contribution of fluid-rock interaction for the composition of produced waters and alteration of shale mechanical properties.
     The study has: 
i) Statistically analysed the chemical composition of hydraulic fracturing in USGS dataset to create prior distributions for the prediction of the salinity and chemical composition of flowback fluids in the UK. 
ii) Statistically analysed the composition and controls on geothermal waters in the UK. Deep geothermal waters are an analogue for the in-situ groundwater composition with which injected fracking fluids will react and mix.
iii) Both sources of information have assisted in the design of the high pressure, high temperature experiments that will simulate the fracking fluid processes
iv) Undertaken sequential extraction of target shales to understand the data from existing batch experiments undertaker

     Future work will include isotope proxy and mineralogical texture studies to predict flowback fluid composition and the post-frack condition of the shale.

How to cite: Hsu, H.-Y. T., Worrall, F., and Aplin, A.: The Potential Water Quality Impacts of Shale Gas Exploitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16255, https://doi.org/10.5194/egusphere-egu2020-16255, 2020.

Studies of groundwater hydrodynamics are rare in desert aquifer system due to its remote location, sparse population, low effort input, and limited field data, but necessary and vital for water security and environment protection. Hydro-meteorological conditions, subsurface electrical resistivity structure, and the chemical and isotopic (¹¹B/¹⁰B, ²H/H, and ¹⁸O/¹⁶O) compositions of groundwater and lake water in a desert inter-dune aquifer system were systematically investigated in order to delineate the origins and processes of groundwater flow. It was found that (1) two possible recharge mechanisms are possible to recharge the shallow sand dune aquifer, namely intensive rainfall infiltration and moisture-heat coupled transport; (2) the electrical resistivity tomography data show that salt lake water intrudes the ambient aquifer and interacts with fresh groundwater. The emergences of uptake of boron and increase of δ¹¹B in salt groundwater further stress the intrusion of salt lake water. (3) Dissolution of ¹⁰B in fresh groundwater leads to a shift of the δ¹¹B value from positive to negative along the flow path. Fresh groundwater streamlines originate from one place with high δ¹¹B value in solution to another place with low δ¹¹B value. Further referring to much high δ¹¹B value in local rainwater, it is suggested that the shallow lakeshore groundwater is probably recharged by local rainfall; (4) Combining with the flow field of lakeshore groundwater and, hydrogen and oxygen isotopic characteristics of groundwater and lake water, the desert lake water is considered to be mainly sustained by shallow lacustrine groundwater discharge.

How to cite: Zhang, X., Luo, X., and Jiao, J. J.: Characterizing the groundwater flow pathways and recharge sources of a desert inter-dune aquifer system by geophysical approaches and multiple isotopes (B, H and O), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4403, https://doi.org/10.5194/egusphere-egu2020-4403, 2020.

Due to human interventions such as the construction of tunnels and bridges as well as in geothermal projects, natural groundwater flow paths are disturbed. Within regions of clay-sulfate rock deposits like Southern Germany and Switzerland („Gipskeuper“) as well as Spain („Red Clay“), the mentioned structural interventions may promote the swelling of such rocks. Consequently, damage to buildings and other infrastructure in their vicinity can be triggered.

Until today, the planning of countermeasures that would minimize or prevent the mentioned swelling is difficult, because it is hardly possible to predict the swelling behavior of clay-sulfate rocks connected with geotechnical constructions. One reason is the limited knowledge of geochemical, hydraulic and geomechanical processes taking place during rock swelling.

The swelling process in clay-sulfate rocks is mainly due to the transformation of anhydrite into gypsum, which leads to an increase in rock volume of up to 60 %. The chemical transformation process includes anhydrite dissolution, possibly transport of dissolved sulfate with groundwater flow, and subsequent gypsum precipitation; and has large impact on flow paths within the swelling rock.

To extend the knowledge of hydromechanical and geochemical processes during the swelling of clay-sulfate rocks, a swelling test facility was installed at the chair of Engineering Geology and Environmental Geotechnics (TU Bergakademie Freiberg, Geotechnical Institute). The basis are six independently controllable apparatuses to conduct swelling experiments under oedometric conditions. First experiments with natural rock samples from a tunnel in Southern Germany are currently carried out.

Because groundwater circulation strongly influences swelling processes in clay-sulfate rocks, and vice versa, we developed a modified setup for swelling experiments. To observe and quantify hydraulic, mechanical and chemical processes during rock swelling in the lab, we combine the swelling apparatuses with a flow-through cell (permeameter). The modified setup allows the realization of an active and controlled fluid flow through the rock sample during swelling. As a result, reactive flow processes such as changes in permeability due to swelling can be observed under variable stress and strain conditions. Our contribution presents the experimental setup of the swelling tests with and without fluid flow. Furthermore, first results from the current experimental runs are presented.

How to cite: Röder, K. and Butscher, C.: Investigating hydromechanical and geochemical processes in swelling clay-sulfate rocks - presentation of a new experimental setup, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2929, https://doi.org/10.5194/egusphere-egu2020-2929, 2020.

Dissolution by a reactive flow is a complex phenomenon influenced by a number of different parameters, including flow rate, diffusion rate of the reactant, reaction rate and the pore space characteristics of the host rock. Depending on the values of these parameters, the dissolution patterns will have different morphological features. In particular, there is range of parameters where the dissolution front becomes unstable, which is accompanied by a formation of pronounced dissolution channels, which are called solution pipes in geological literature and wormholes in the petroleum industry, where they are produced to stimulate the flow from oil reservoirs. In the natural settings, these features are formed in rocks with a very high porosity and then with a rather large flow rate. Their shapes are strongly related to their characteristic sizes. At the macroscale (1-10metres) they are usually almost cylindrical with a diameter from a few cm up to a meter, while at microscale they show a highly ramified, fractal-like shape. To investigate this variability and to understand their formation and evolution, we are conducting microfluidic experiments using a self-constructed microfluidic cell. We are using a system consisting of two polycarbonate chips in which it is possible to have a control on flow rate and on the aperture. The lower plate has an indentation that can be filled with gypsum, while on the upper chip there is a reservoir that allows water to be supplied to the system in a controlled way. We are using powder gypsum during these experiments because it has a very simple chemistry, high solubility in water and therefore allows a greater speed of dissolution The two chips are joined together with an ultrathin, double coated tape of variable thickness that allows us to control the aperture of the system, which can thus be regarded as an analog fracture. As the gypsum chip is dissolved, we observe the appearance of fingers of different shapes, depending on the flow rate and the aperture. We report the results of these experiments and relate the observed features with the natural shapes found in the karst systems. We also investigate how the shapes of the pipes change as we vary the flow rate periodically, which reflects annual variations in the flow in the natural karst systems.

Key words: dissolution, solution pipes, microfluidics

How to cite: Magni, S., Cooper, M., and Szymczak, P.: How does the flow affect the evolution of solution pipes ?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-747, https://doi.org/10.5194/egusphere-egu2020-747, 2020.

In this work we have investigated numerically the formation of channelised dissolution patterns, termed “wormholes”, using initial pore geometries generated from tomographic images of limestone cores. We have employed an OpenFOAM-based Darcy-scale numerical solver,  porousFoam, which combines a Darcy/Darcy-Brinkman flow solver and a reactive transport solver in an evolving pore space. Simulated geometries, of both final and intermediate steps, are compared to dissolution experiments on samples the initial pore geometry is generated from, with the same acid concentration and flow rate applied. 

The initial condition of porosity distribution is set from X-Ray Computed Microtomography (XCMT) images via three phase segmentation into macroporosity, microporosity, and grain regions. Porosity values for microporous regions are set using linear interpolation between pore and grain grayscale values [1]. The inlet boundary conditions of flow rate and acid concentration are set as in the dissolution experiment. To test the effect of the permeability-porosity constitutive relationship we have investigated several options including power laws of varying exponent, and the Carman-Kozeny relation. We have also analyzed the impact of using Darcy versus Darcy-Brinkman flow solvers. Despite a qualitatively similar appearance to experimental results, the simulated wormholes are usually significantly thicker than their experimental counterparts, a fact noted by other researchers as well [2]. We comment on possible reasons for this discrepancy and on the limitations of Darcy-scale solvers in general. Additionally, we find that higher exponents in the power law makes the numerical dissolution very sensitive to grayscale threshold values as a small variation in this value changes the path of the wormhole.

[1] Luquot, L., Rodriguez, O., and Gouze, P.: Experimental characterization of porosity structure and transport property changes in limestone undergoing different dissolution regimes, Transport Porous Med., 101, 507–532, 2014.

[2] Yue Hao, Megan Smith, Yelena Sholokhova, Susan Carroll, CO2-induced dissolution of low permeability carbonates. Part II: Numerical modeling of experiments, Advances in Water Resources, 62, 388-408, 2013 

How to cite: Sharma, R. P., Cooper, M. P., Ladd, A. J. C., and Szymczak, P.: Modeling wormhole formation in digital rock samples: the role of segmentation and permeability-porosity relationships , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-996, https://doi.org/10.5194/egusphere-egu2020-996, 2020.

Dissolution of porous media is a complex process involving nonlinear couplings between flow, transport, the evolving geometry of the media, and the process of dissolution itself. In some cases these couplings lead to the formation of intricate patterns, the characteristics of which depend strongly on flow, mineral dissolution rate, and initial pore space geometry. In particular, finger-like channels, termed "wormholes", are spontaneously formed where the majority of flow is focused. Capturing the dynamics of wormhole growth has so far been largely limited to numerical models, with few studies observing their time evolution in real rocks. In this study we capture the dynamics of both wormhole growth and their alteration of flow in the rock by placing the experiment within neutron and X-Ray tomographs and scanning while actively dissolving limestone cores. 

To observe the evolution of wormhole geometry limestone samples are dissolved in a cell translucent to X-Rays and neutrons. For each experiment a high (30-35 micrometer) resolution scan was taken of the initial sample geometry, as well as the geometry after dissolution. During acidization tomography was performed at 60-70 micrometer resolution with acquisition times ranging from three to six minutes. For several experiments dissolution was paused and and a contrasting agent injected to visualize the flow field within the sample. Flow field experiments were performed with neutron tomography by first injecting heavy water, followed by light water as the contrast agent, and with X-Ray tomography by injection a solution of potassium iodide into light water. Results of dissolution experiments show that wormhole growth can be tracked at sufficiently high spatial and temporal resolution to measure changes to the pore space.

These experiments highlight the importance of the near-tip region on the dynamics of wormhole propagation. In particular, focusing of the flow is shown to take place not only within the wormhole but also significantly (>5mm) into the porous region past the wormhole tip. These "virtual channels" link the tip with the neighboring regions of high porosity. Several such virtual channels can exist, indicating potential paths of further growth, and demonstrate the strong coupling of flow and geometry evolution. Additionally, we observe a dramatic dependence of the dissolution patterns on the initial pore structure, in particular the total initial porosity, distribution of pore sizes and connectivity of the pore space. In pore spaces with poor connectivity and low porosity the wormholes tend to be very tortuous and thin. Such wormholes advance through rapid, almost discontinuous jumps, guided by the above-described pre-focusing mechanism. On the other hand, the advancement of a wormhole in a well-connected rock is much more diffuse, controlled by merging between neighboring pore spaces.

How to cite: Cooper, M., Magni, S., Vu, P., Blach, T., Radlinski, A., Dohnalik, M., Tengattini, A., and Szymczak, P.: Observing the evolution of geometry and flow in dissolving rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1010, https://doi.org/10.5194/egusphere-egu2020-1010, 2020.

Precipitation and dissolution are prime processes in carbonate rocks and being able to monitor them is a major deal of reservoir exploitation for geo-resources (water, gas) or geological storage (CO₂, H₂, waste). Geophysics can be used to monitor these processes non-intrusively and at low cost. Among the existing techniques, we used two electrical methods to monitor the reactivity of a synthetic calcareous porous medium: self-potential (SP) and spectral induced polarization (SIP). SP is a passive technique that consists in measuring the electrical field as it is affected by water fluxes and concentration gradients through electrokinetic and electrochemical couplings. SIP is an active method that provides the electrical conductivity and the chargeability of a porous medium in a low frequency range (mHz to kHz). We carried out a two months laboratory experiment to monitor the geoelectrical signals generated by chemical variations in a synthetic medium composed of pure calcite grains. Three different solutions were injected to alternatively dissolve or precipitate calcite in the sample. The sample is equipped with four aligned non-polarizable Ag/AgCl electrodes in order to geoelectrically monitor the fluid percolation and the ionic concentration gradients changes through the medium. Moreover, we conducted chemical analyses of the downstream fluid to monitor its ionic composition. We made a 1D reactive-transport simulation with the software CrunchFlow to get the concentration gradients of all dissolved ions along the column. Following a theoretical framework, we used a physically based analytical model to relate our electrical signals to ionic concentrations of a multicomponent electrolyte. We find that dissolution and precipitation generate measurable geoelectrical signals because of chemical reactions and ions substitutions. These findings open the possibility to better understand geoelectrical signals in natural media and possibly use them to monitor in situ reactivity.

How to cite: Rembert, F., Jougnot, D., Luquot, L., Zuddas, P., and Guérin, R.: Geoelectrical monitoring of dissolution and precipitation reactions in a saturated calcareous porous medium, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5069, https://doi.org/10.5194/egusphere-egu2020-5069, 2020.

One of the major keys to the success of the carbon capture and storage (CCS) is understanding the geochemical effects that CO₂ has on the storage reservoir. The injection of CO₂ into the reservoir disturbs geochemical equilibrium as it induces acid-generation reactions with subsequent CO₂-brine-mineral interactions, including dissolution of certain host minerals and precipitation of secondary minerals. The mineral precipitation, especially precipitation of carbon-bearing minerals in geological formations, is generally a favorable for CO₂ trapping mechanism that ensures long-term geologic CO₂ sequestration. These precipitates, however, may clog the wellbore and its surroundings, followed by loss of injectivity.

The current study is dedicated towards a better understanding of the geochemistry of the geological CO₂ storage based on the Ketzin CO₂ pilot site. The Ketzin CO₂ storage site, the first on-shore geological CO₂ storage site in the European mainland, is demonstrated a safe and reliable CO₂ storage operation after injection of about 67-kilo tons of CO₂ and offers the unique opportunity to work on data sets from all storage life-cycle (Martens et al., 2014). Through both field measurement and modeling studies, this contribution aims to explore the secondary mineral precipitation mechanisms and identify the major influential factors during the CO₂ sequestration. This approach supports the H2020 project SECURe establishing best practice in baseline investigations for subsurface geoenergy operations, underpinned by data of pilot and research-scale sites in Europe and internationally. The secondary minerals solubility was investigated as a function of the reservoir temperature, pressure, and CO₂ concentration, which occurred in the reservoir. Special focus is set to sulfate minerals, as field evidence exists that gypsum precipitates as a result of reservoir exposition to CO₂. Batch modeling was performed using the PHREEQC code version 3 (Parkhurst and Appelo, 2013) with the Pitzer database (pitzer.dat). The coupling interface OGS#IPhreeqc (He et al., 2015) applied reactive transport modeling, and the coupled reactive-transport processes in the reservoir with complex chemistry can be modeled. Our results suggest that the gypsum precipitation was found to increase as CO₂ concentration ascends. However, no significant porosity and permeability alterations are observed since the gypsum precipitation acts as a Ca²⁺ sink and leads to further carbonate dissolution. The results highlight the high reactivity of the near-well zone due to CO₂ injection and emphasize the need to be monitored in the injection well to avoid the potential formation of gypsum, which could lead to well clogging.

He, W., Beyer, C., Fleckenstein, J.H., Jang, E., Kolditz, O., Naumov, D., Kalbacher, T., 2015. A parallelization scheme to simulate reactive transport in the subsurface environment with OGS#IPhreeqc 5.5.7-3.1.2. Geosci. Model Dev. 8, 3333-3348.

Martens, S., Möller, F., Streibel, M., Liebscher, A., 2014. Completion of Five Years of Safe CO2 Injection and Transition to the Post-closure Phase at the Ketzin Pilot Site. Energy Procedia 59, 190-197.

Parkhurst, D.L., Appelo, C.A.J., 2013. Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, Techniques and Methods, Reston, Virginia, USA, p. 519.

How to cite: Jang, E., Wiese, B., Kalbacher, T., Lu, R., and Schmidt-Hattenberger, C.: Evaluation of secondary mineral precipitation by reactive transport modeling at the Ketzin CO2 storage site, Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9504, https://doi.org/10.5194/egusphere-egu2020-9504, 2020.

The dissolution of fractured or porous media by reactive flow is often occurring preferentially, forming highly conductive channels, so-called “wormholes”. Wormhole formation prevails in subsurface karst where it can form extensive speleological systems, and is also significant for a large range of applications, e.g. well acidizing or CO₂ geo-sequestration. The underlying mechanism involves positive feedback between reaction and transport— the flow pathways that focus the reactive flow dissolve preferentially and increase their conductivity, and in turn their flow. An increased pressure ahead of the longer wormholes screens off the shorter ones, which ultimately cease to grow. Over time, the characteristic spacing between active (growing) wormholes increases, while their number declines, which results in a hierarchical scale-invariant distribution of wormhole lengths. Interestingly, a variety of other pattern-forming processes in nature show a similar competitive dynamics and emergent of hierarchical structures, with examples ranging from viscous fingering to crack propagation in brittle solids and side-branches growth in crystallization [1].

The importance of wormholing and its intriguing dynamics motivated intensive research, including on the emergence of reactive-infiltration instabilities [2], as well as on the effects of medium heterogeneity on the wormhole growth. Here, we study wormholing in anisotropic media using a network model of regular geometry— longitudinal channels (aligned along the main flow direction) and transverse ones, of a different average cross-section. Our simulations show that anisotropy substantially affects wormholing, controlling the characteristic spacing between the wormholes and the overall permeability evolution. In the case of wider transverse channels, wormhole interaction via the pressure field is enhanced, resulting in stronger wormhole competition and hence larger spacing. Conversely, in the extreme case of very narrow transverse channels, spacing becomes minimal and neighboring wormholes tend to merge. Simulations further reveal that narrow transverse channels promote the emergence of thinner and more conical wormholes with several side-branches.

Additionally, we discuss the relation between the wormhole development in an anisotropic medium and viscous fingering phenomena in a network of microfluidic channels [3]. Despite many similarities between these systems we also find important differences— while the spacing between viscous fingers increases linearly with anisotropy, the corresponding relation for wormholes turns out to be nonlinear. This nonlinearity could be attributed to the effect of anisotropy on wormhole shape and advancement velocity and is of interest for future investigation. Our findings contribute to the understanding of wormholing in geological systems and demonstrate how the small-scale features can fundamentally affect the resulting large-scale morphologies.

[1] Krug, J., Adv. Phys., 46, 139, 1997

[2] Ortoleva, P. et al, Amer. J. Sci., 287, 1008, 1987

[3] Budek, A. et al, Phys. Fluids, 27, 112109, 2015

How to cite: Roded, R., Szymczak, P., and Holtzman, R.: Wormholing in anisotropic media: Pore orientation effect on large-scale patterns , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13173, https://doi.org/10.5194/egusphere-egu2020-13173, 2020.

Deep and ultra-deep carbonate reservoir is an important area of petroleum exploration. However, the prerequisite for predicting high quality deep ultra-deep carbonate reservoirs lays on the mechanism of carbonate dissolution/precipitation. It is optimal to perform hydrocarbon generation-dissolution simulation experiments to clarify if burial dissolution could improve the physical properties of carbonate reservoirs, while quantitatively and qualitatively describe the co-evolution process of source rock and carbonate reservoirs in deep layers. In this study, a series of experiments were conducted with the limestone from the Ordovician Yingshan Formation in the Tarim Basin, and the low maturity source rock from Yunnan Luquan, with a self-designed hydrocarbon generation-dissolution simulation equipment. The controlling factors accounted for the alteration of carbonate reservoirs and dissolution modification process by hydrocarbon cracking fluid under deep burial environments were investigated by petrographic and geochemical analytical methods. In the meantime, the transformation mechanism of surrounding rocks in carbonate reservoirs during hydrocarbon generation process of source rock was explored. The results showed that: in the burial stage, organic acid, CO₂ and other acidic fluids associated with thermal evolution of deep source rocks could dissolve carbonate reservoirs, expand pore space, and improve porosity. Dissolution would decrease with the increasing burial depth. Whether the fluid could improve reservoir physical properties largely depends on calcium carbonate saturation, fluid velocity, water/rock ratio, original pore structure etc. This study could further contribute to the prediction of high-quality carbonate reservoirs in deep and ultra-deep layers.

How to cite: Ding, Q., He, Z., and Zhu, D.: The transformation effect of source rock-derived acidic fluid on carbonate reservoir from simulation experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21098, https://doi.org/10.5194/egusphere-egu2020-21098, 2020.

CO2 sequestration in deep geological formation is considered an option to reduce CO2 emissions in the atmosphere. After injection, the CO2 will slowly dissolve into the pore water producing low pH fluids with a high capacity for dissolving carbonates. Limestone rock dissolution induces geometrical parameters changes such as porosity, pore size distribution, or tortuosity which may consequently modify transport properties (permeability, diffusion coefficient). Characterizing these changes is essential for modelling flow and CO2 transport during and after the CO2 injection. Indeed, these changes can affect the storage capacity and injectivity of the formation.

Very few published studies evaluate the transport properties changes (porosity, permeability, pore size distribution, diffusion coefficient) due to fluid-rock interactions (dissolution and/or precipitation).

Here we report experimental results from the injection of acidic fluids (some of them equilibrated with gypsum) into limestone core samples of 25.4 mm diameter and around 25 mm length. We studied two different limestone samples: one composed of 73% of calcite and 27% of quartz, and the second one of 100% of dolomite. Experiments were realized at room temperature. Before and after each acidic rock attack, we measure the sample porosity, the diffusion coefficient and the pore size distribution.

We also imaged the 3D pore network by X-ray microtomography to evaluate the same parameters. During percolation experiments, the permeability changes are recorded and chemical samples taken to evaluate calcite dissolution and gypsum precipitation. Several dissolution/precipitation-characterization cycles are performed on each sample in order to study the evolution and relation of the different parameters.

These experiments show different dissolution regimes depending of the fluid acidity and of the

limestone samples in particular the initial local heterogeneity, and pore size distribution.

How to cite: Luquot, L.: Dissolution and precipitation reactions during acidic fluid percolation through different limestone samples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18873, https://doi.org/10.5194/egusphere-egu2020-18873, 2020.

During the dissolution at a calcite cleavage face, etch pits open around defects. Atomic steps moving outwards from these pit centres are currently considered the general driving mechanism of this dissolution process that results in heterogeneous material flux from the surface. This means that the defects that generate the etch pits are crucial for the surface evolution. Recent kinetic Monte Carlo (kMC) simulation results indicate that not only the density but also the spatial distribution of defects is critical for the influence on dissolution.

In kMC simulations used for crystal dissolution, defect positions are input and can be defined in various ways, e.g., at pre-defined coordinates or randomly drawn from a distribution. The user is free in defining the defects, although it can generally be considered reasonable to choose defect densities and distributions as close as possible to what is expected to occur in nature and technical systems.

The actual spatial distribution of screw dislocations in calcite and their influence on rate variability are still not entirely known. To make the calcite kMC simulations comparable with experimental results, we experimentally determined the etch pit distributions, analyzed them and subsequently used them as input for further kMC studies.

While the direct measurement of defects in the crystal structure is extremely difficult, the indirect approach of measuring etch pits that have formed around defect outcrops during the beginning of dissolution is more feasible. For this, cleaved calcite single crystals were etched using ultra-pure water for 3 to 4 hours to obtain a significant amount of etch pits on the surface. The topography of the crystal surfaces was analysed using Vertical Scanning Interferometry (VSI). The resulting topography maps were stitched to gain a larger area for better statistics, and the centres of visible etch pits marked. This generates two-dimensional point patterns that describe the actual defect distribution more accurately than purely randomly generated coordinates without further constraints.

Based on data analysis of the experiments, we will show the resulting point distributions and synthetic patterns with similar underlying statistics. Using these as input for modelling, we then calculate kMC simulations and geometrical models of a system close to the calcite single crystal from our experiment, and compare them also to simulations using different defect positions as input.

How to cite: Rohlfs, R. D., Trindade Pedrosa, E., Kurganskaya, I., Fischer, C., and Luttge, A.: Etch pit distribution on calcite cleavage surfaces – experiments and simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22076, https://doi.org/10.5194/egusphere-egu2020-22076, 2020.

Advanced imaging techniques such as atomic force microscopy (AFM) allow for direct observations of reactions at mineral surfaces at the nanoscale. This enables reaction mechanisms to be clarified. Aqueous solutions passing over a calcite surface often control coupled reactions of dissolution and precipitation, whereby calcite is replaced pseudomorphically by a more stable phase that precipitates at the calcite-fluid interface. Both molar volume changes as well as solubility changes between parent and product phases most commonly result in a concomitant porosity that then allows the solution to penetrate within the calcite. In this way pollutant elements such as phosphate, from over fertilization of agricultural soils, or water contaminated with elements such as, selenium, arsenic, antimony, chromium (from both natural and anthropogenic sources), can be sequestered within more stable, less soluble phases. Calcium carbonate barriers (such as crushed limestone) within water channels may present an effective and simple method to remove contaminant elements from water systems.

How to cite: Putnis, C. V.: Nanoscale imaging at the calcite-water interface: Implications for potential environmental remediation., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20771, https://doi.org/10.5194/egusphere-egu2020-20771, 2020.

In the past 20 years, basaltic aquifers have been studied as a key geologic carbon storage host due to their high reactivity and widespread distribution. However, many basaltic reservoirs contain substantial alteration minerals and their potential as cation sources for carbon mineralization processes still need to be assessed. A common alteration phase in high temperatures (≥ 200 °C) basalts is epidote. To help determine the possible contribution of this mineral to CO2 sequestration through the release of its constituting cations, the dissolution rates of epidote from the Green Monster Mine (Alaska) were experimentally measured. Far-from equilibrium experiments were conducted over the pH range 2-11 using both batch reactors at 25 °C, and mixed-flow reactors at 100 and 200 °C. Furthermore, mixed-flow reactor experiments at pH ~9 on epidote in presence of CO2 were carried out at 200 °C to study its carbonation potential and to quantify the yields of this reaction compared to basaltic glass. The determination of the extent of this process was monitored by inorganic carbon analyses on both solid and fluid fraction using non-dispersive infra-red (NDIR) CO2 gas analyses. Preliminary results suggest that epidote and potentially other alteration Ca-silicate phases can provide Ca2+ as efficiently as fresh basalts at 25 and 100 °C to promote the precipitation of calcium carbonate. Further experimental and modelling work is ongoing to confirm these findings at different thermal conditions and as a function of injected fluid chemistry.

How to cite: Marieni, C., Saldi, G., Benezeth, P., and Oelkers, E.: Epidote dissolution and its role within carbon storage, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22669, https://doi.org/10.5194/egusphere-egu2020-22669, 2020.

Clay mineral precipitation in geothermal systems can be detrimental for geothermal energy harvesting and subsurface CO₂ storage efforts. One of the clays that precipitate in such systems is sepiolite [1]. Sepiolite precipitation of can lead to a loss in host rock permeability and a decrease in the aqueous Mg concentrations, thereby hindering Mg-carbonate formation and thus limiting CO₂ mineral storage. Conversely, sepiolite dissolution can provide Mg and thus enhance CO₂­ mineral storage. Water rock interactions in such systems occur often close to equilibrium and temporal and regional changes in the saturation state of sepiolite can affect both the dissolution and precipitation rate of this mineral. Hence, to improve quantitative models on CO₂ mineral storage and hydrothermal energy harvesting scenarios and to gain an increased understanding in clay mineral dissolution/precipitation mechanisms in general we have measured sepiolite dissolution and precipitation rates as a function of its saturation state.

A series of mixed flow precipitation and dissolution experiments were performed at 60 °C with varying flow rates and saturation indices ranging from -8 to 18. All experiments were performed in the presence of pure, crystalline sepiolite seeds. The reactors were placed in a shaker water bath to ensure that they were well mixed and at constant temperature. Sepiolite precipitation rates were calculated from the difference in Mg and Si concentration between the inlet and outlet solution.  The solid phase was recovered from a mixed flow experiment that ran for over 3 months. The resulting solid contained approximately 40 w.t% newly precipitated material. The precipitation of crystalline sepiolite was confirmed from the stoichiometric Si/Mg depletion in solution, from Energy-dispersive X-ray spectroscopy and from X-ray diffraction spectra of the solid phase. Precipitation/dissolution rates varied between 10^-16.30 and 10^-18.77 mol/cm²/s depending on the affinity of the precipitation reaction. The results show that the precipitation and dissolution rates of sepiolite depend linearly on the affinity of the precipitation/dissolution reaction, with dissolution and precipitation rates increasing at higher reaction affinities. The linearity of sepiolite precipitation and dissolution further suggests that dissolution is mechanistically the reverse of precipitation and that both processes are consistent with transition state theory. The relatively high precipitation rates at increased Mg/Si concentrations imply that, under basic conditions, sepiolite precipitation could be detrimental to the permeability of the host rock and the availability of Mg during CO₂ sequestration.

[1] Oelkers, E. H., et al. "Using stable Mg isotope signatures to assess the fate of magnesium during the in-situ mineralisation of CO₂ and H₂S at the CarbFix site in SW-Iceland." GCA 245 (2019): 542-555.

How to cite: Mulders, J. and Oelkers, E.: An experimental study of sepiolite precipitation and dissolution rates and mechanisms at 60 C, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11534, https://doi.org/10.5194/egusphere-egu2020-11534, 2020.

There are more than one thousand meters thick of Paleogene volcanic stratigraphy in Liaohe Depression, Bohai Bay Basin, East China. We can identify and divide these Paleogene volcanic stratigraphy into 14 stages (Huang et al., 2014; Feng et al., 2015).

We take samples of buried basalts from depth 1418m to 3951m, then use optical microscope, scanning electron microscopy (SEM), electron probe microanalyzer (EPMA) techniques to analyze mineral transformation during the burial process of basalt. Our goal is to establish a mineral transformation model of buried basalts in this area.

We summarized that pyroxene have no alter during burial; feldspar alteration sequence: plagioclase -- phillipsite/chabazite -- analite; olivine alteration sequence: olivine -- smectite -- mixed-layer minerals (chlorite and smectite) -- chlorite; calcium is precipitated while mineral transformation, which can form calcite and fill the pores.

We concluded that (1) from depth 2400m to 3700m (underground temperature 85~130℃), the effective micropores of phenocryst are mainly intra-crystalline pores of phillipsite; (2) below 3700m (underground temperature above 130℃) the effective pores are mainly calcite dissolved pores. This model may be suitable for portraying the mineral transformation and pore structural evolution during the burial process of alkaline series basalts in the alkaline environment (without the influence of organic acids).

References:

HUANG Y.L., SHAN J.F., BIAN W.H., GU G.Z., FENG Y.H., ZHANG B., WANG P.J., 2014, Facies classification and reservoir significance of the Cenozoic intermediate and mafic igneous rocks in Liaohe Depression, East China [J]. Petroleum Exploration and Development, 41(6), 734-744, https://doi.org/10.1016/S1876-3804(14)60087-2.

FENG Y.H., YU X.J., HUANG Y.L., LIU B.H., GU G.Z., LI H.Y., WANG P.J., 2015, Eruption cycles and stages of Cenozoic volcanic rocks and their significance to hydrocarbon accumulations in Liaohe Basin [J]. Journal of China University of Petroleum (Edition of Natural Science), 39(5), 50-57. (in Chinese with English abstract)

How to cite: Liu, H., Huang, Y., Feng, Y., and Wang, P.: Mineral transformation and pore structural evolution during the burial process of basalt: a case from Liaohe Depression, Bohai Bay Basin, East China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21010, https://doi.org/10.5194/egusphere-egu2020-21010, 2020.