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.
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.