the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Sep 2025
Feldspar alteration by disequilibrium CO2-H2O fluids in reservoir sandstones: Implications for CCS
Abstract. Understanding how the minerals in reservoir rocks respond to CO2 injection is vital for the success and safety of Carbon Capture and Storage (CCS) projects. Feldspars are the most common mineral in the Earth’s crust and act as primary framework grains in sandstones. Compared to quartz, feldspars are mechanically weak and chemically reactive. Dissolved feldspars can re-precipitate as clays, which in CCS reservoirs could impact fluid-flow. While caprock mineral stability is well studied, reservoir mineral reactivity, particularly of feldspars, remains understudied. To address this knowledge gap, we present microstructural and geochemical data from batch experiments that reacted CO2-enriched fluids with feldspar-bearing sandstone sampled from the Captain Sandstone Member, the primary reservoir for the Acorn CCS Project (UK).
Experiments were conducted in a hydrostatic pressure vessel at 70 MPa confining pressure, 50 MPa pore pressure, and temperatures ranging from 80 °C to 550 °C, using CO2-enriched water to simulate reservoir conditions. Pre- and post-reaction samples were analysed using XRD, SEM-EDS, and XCT to assess microstructural and mineralogical changes. Results show that CO2:feldspar interactions differ significantly from control experiments involving water alone. At reservoir-relevant temperatures (80 °C), incongruent dissolution of K-feldspar weakened grains which led to microfracturing. At 250 °C, CO2 fluids caused total dissolution of calcite grains and cement and selective leaching of calcium from oligoclase, enriching the pore fluid with Ca²⁺. Above 400 °C, coupled dissolution–precipitation processes were observed, including congruent K-feldspar dissolution, secondary porosity development, and localised precipitation of Ca-aluminosilicates and K-bearing phases around dissolving K-feldspars. These transformations could alter reservoir flow pathways and induce mechanical risks, i.e. destabilising nearby faults or initiating reservoir collapse. Given feldspars’ prevalence in crustal rocks and CCS sandstone reservoirs, their reactive behaviour under in-situ conditions and in the presence of aggressive fluids demands greater attention.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-4419', Giacomo Medici, 02 Oct 2025
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RC1: 'Comment on egusphere-2025-4419', Laura Airaghi, 26 Nov 2025
This is a very interesting work, with broad and applied implications and that reads very well. To strengthen and calrify the manuscript I suggest the authors to adress the following issues :
- While microstructures show preferential K-feldspar dissolution expecially at high temperature (i.e. Fig. 4), estimation for volume amont of K-feldspar grains in starting material up to higher temperature for experiments with CO2-bearing fluid do not seems to show any variation within the uncertainty (Fig. 11B). Do the authors have an estimate of the aboundance of secondary minerals as clay replacing K-feldspar ? Instead the vol % of plagioclase seem to significantly decreases (more the one of K-feldpar) in CO2-bearing experiences (Fig. 11C). I suggest to revise the discussion taking this observations into account.
- As also stated by the authors, low temperature conditions and heterogeneous physical properties of the starting material (porosity, permeability etc.) favour chemical equilibrium at the grain-scale. It will be useful that the authors therefore further clarify how images presented in Fig. 4, 6 and 7 are rapresentative of what seen in all grains of the sample (for the same phase), or, if it is not the case, how such microstructures depend on a particular physico-chemical environnement (microstructural site relative to cracks/faults, domains with different initial grainsize, domains with different proportions of the different mineral phases etc.)
- The vol % of calcite grains and ciment has been estimated at 1 % by X-ray diffraction. 1 vol % is at the limit of what can be detected with image processing by tresholding. This opens two questions : first, how these estimates depend on the chosen area. The volume of calcite being so low and local, the choice of the analyzed area can strongly affects its volume estimate (as one cas see in Fig. S4 where calcite can make up a significant part of the surface). Secondly, how a decrease from 0.3-1 vol % to 0-0.2 vol % can be efficiently detected with the image tresholding on EDS images given locally the small grainsize? More generally, can the authors clarify the uncertainty that are associated to estimates based on EDS image tresholding (due to zone sampling biaisis and processing procedure) and compare them with estimated volume decrease for calcite and other phases ?
- I find very interesting that the main reactional features are acquired at 80°C and then their effects (in terms of changes in mineral proportions) remained rather unchanged for higher temperatures. This may appear controintuitive because one may expect that the higher the temperature, the fastest and most complete the reaction, included the mineral dissolution. Can the authors comment on this and link them with the decreasing of porosity that instead goes the other way from 250-300°C ?
- To make the manuscript easier to follow, I would suggest to summarize the occurrence of mineral phases in each sample in a table and to write explicitly reactions going on in each sample.
- I did not well understand what the authors mean by « weakening » mechanism of feldspar. I would rather define cracking as a deformation mechanism or a mechanism of strain accommodation in feldspar that may lead to weakening of the bulk aggregate because of grainsize reduction and transformation of the strong feldspar phase into a fine-grained weaker secondary phase (as illite). Furthermore, feldspar is generally considered as the strong phases relative to quartz, expecially at such low temperature conditions where it cracks instead of deforming plastically , even if, I completely agree, it is more reactive than quartz.
Citation: https://doi.org/10.5194/egusphere-2025-4419-RC1 -
RC2: 'Comment on egusphere-2025-4419', Alexis Cartwright-Taylor, 12 Dec 2025
This interesting paper addresses an important knowledge gap with a novel experimental methodology and is well-written. It presents key findings about the degree of K-feldspar alteration under reservoir pressures and temperatures relevant for CCS. Increasing temperatures were used as a proxy for increased reaction rates and significant K-feldspar dissolution and fracturing were observed, among other dissolution/precipitation behaviour.
I recommend the following minor changes and clarifications:
- Table 1 - why did Pc conditions vary from 70-80MPa? Could this variability have influenced reaction kinetics?
- Line 259 – reference here to more detail in the Sup. Mat. but there is no further information there. Suggest including specific details of the imagej processing and analysis workflow and parameters that were used.
- Lines 261-262 – mention here of additional 3D micro-CT from pre- and post-experiment samples to assess grain deformation and pore networks but there is no information about how these images were processed and no micro-CT data is presented in the results. Please clarify.
- Line 279 – check figure number in the reference – Fig. 3 is the experimental schematic, but text is discussing the BSE images.
- Fig 6. caption – specify which images relate to which temperatures (not clear from caption or main text).
- Line 422 – mentions only H2O pore area decrease at 400 ºC but CO2 showed similar pore area decrease at 400º
- Lines 430-435 and Fig 11b – the figure does not appear to support the text here. The grey boxes show very little variation in K-feldspar % across the temperature range, while the blue H2O boxes show much more variation (decreasing and then increasing and then decreasing again) and wider interquartile ranges than the grey boxes. Fig 11 seems inconsistent with Fig 4 which shows K-feldspar dissolution – why is this? Are there competing precipitation reactions that might balance out the average % changes? Would be helpful to summarise the expected reactions for each phase/fluid in the main text somewhere.
- Lines 438-439 and Fig 11c – the figure does not appear to support the text. The blue H2O boxes show some small variability around 6% but without knowing the error on the measurements and how large of a change is significant it would be difficult to infer that the plagioclase area had increased with temperature.
- Fig 11 – what are the errors of the mean for these measurements?
- Fig 11 – was an experiment done at freezing point (0ºC) as stated in the figure? Or should this be shown as room temp?
- Fig 11d – these values are very small % changes – what is the resolution on the segmentation? Is it possible to distinguish such small changes?
- Line 577 – mention of enhanced K-feldspar dissolution in CO2 enriched fluids but this is not supported by Fig 11b, although supported by Fig. 4. Please clarify (see also comment #7).
Citation: https://doi.org/10.5194/egusphere-2025-4419-RC2
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General comments
Very good research. Please, follow my specific comments to bring the impact of your research out.
Specific comments
Lines 51-54. “This requires both a specific knowledge of the pre-injection micro-scale reservoir properties, e.g., pore volume, pore connectivity…safety”. Insert recent literature on the pore connectivity/effective porosity of sandstone aquifers, both the researches discuss CO2 storage:
- Agbotui, P.Y., Firouzbehi, F., Medici, G. 2025. Review of effective porosity in sandstone aquifers: insights for representation of contaminant transport. Sustainability, 17(14), 6469.
- Lai, J., Wang, G., Wang, Z., Chen, J., Pang, X., Wang, S., ... & Fan, X. (2018). A review on pore structure characterization in tight sandstones. Earth-Science Reviews, 177, 436-457.
Line 92. Clearly state the specific objectives of your research by using numbers (e.g., i, ii, and iii).
Lines 112-113. Fluvial, aeolian or turbiditic sandstone? Please, specify the palaeo-environment.
Line 142. Provide update for this reference.
Line 144. Specify the total number of samples analyzed.
Line 145. Convert depth in meters. You need to use international units.
Line 289. “seams’ what do you mean? Some authors use the term granulation seams for deformation bands of tectonic origin in sandstones.
Lines 335-336. Ok such features are related to dissolution. Make sure clarity throughout the manuscript.
Figures and tables
Figures 1, 2 and 4-10. Spatial scale very difficult to read.
Figure 2. Specify that the images are produced by a SEM. The same for the other figures analyzed using SEM.
Figure 11. Make the letters and the numbers larger.