Belshaw, Grace
(2022)
Rock-fluid interactions in geothermal reservoir systems and the implications for carbon sequestration.
PhD thesis, University of Nottingham.
Abstract
This thesis aims to examine whether andesitic rock samples are likely to be good targets for permanent carbon sequestration via mineral trapping, as an alternative to basaltic-type reservoirs which have been proven to be successful. The CarbFix project in Iceland and the Wallula Basalt Pilot Scale project both reported results from field scale studies that suggested carbonation reactions had occurred within just two years in basaltic-type reservoirs, which is a significant improvement on the thousands of years needed for other geological carbon storage methods to be considered permanent. Carbon dioxide was successfully sequestered in these systems, after mineral dissolution reactions with the injected acidic fluids, which released divalent cations able to combine with the dissolved CO2 and precipitate carbonate minerals. However, there is a significant lack of research examining whether alternative volcanic rocks such as andesite may also be suitable. If alternative volcanic rocks can also be utilised as targets for permanent carbon sequestration in short timescales, this would expand the accessibility of the CarbFix project to more global locations and help mitigate the impacts of rising CO2 concentrations in the atmosphere.
Rock samples from the region of Rantau Dedap on the Island of Sumatra, Indonesia, were first characterised in detail to examine their mineralogical and chemical compositions, with results indicating they were of andesitic composition. Batch reactor experiments were conducted under elevated temperatures of 100oC and pH values close to 3 using crushed andesite type rock samples, to simulate carbon sequestration conditions close to the site of injection. The release of silicon into the experimental fluids was used to calculate a bulk rock dissolution rate of the order 10-11 mol/m2/s for the andesite type rocks, which is roughly 1-2 orders of magnitude slower than reported basaltic dissolution rates. Mineral dissolution in basaltic systems is considered to be the rate limiting step in the permanent carbon sequestration via mineral trapping process, and so these results indicate that permanent sequestration of CO2 in andesitic systems may take slightly longer than the basaltic systems which saw sequestration in just two years. However, significant divalent cation release was observed as a result of mineral dissolution, which suggests andesitic samples may still have a good carbon sequestration potential.
To expand on these findings, batch experiments were conducted using resin embedded andesite rocks, at temperatures of 100oC and increased concentrations of CO2 and calcium, to simulate conditions further from the site of injection. Some mineral dissolution was observed as well as divalent cation release. Evidence from electron microscope and energy dispersive x-ray techniques indicated two of the samples which had a feldspar composition with a higher anorthite content had an increased amount of calcite minerals present at the end of the 3-day experiments. These results give a positive indication that andesite samples are likely to support relatively rapid carbon sequestration via mineral trapping under conditions relevant to CO2 injection. The two samples also had a higher content of alteration minerals present compared to the third sample which indicates that the presence of calcic-plagioclase and/or alteration minerals increase the potential for permanent carbon sequestration in andesitic-type reservoir systems.
Batch experiments were conducted to examine the impact of using fluids with increase NaCl concentrations up to 2.1 M, instead of freshwater, on the dissolution of feldspar minerals. If saline type fluids can be used as well as freshwater to dissolve CO2 for subsurface injection in volcanic rock systems, the feasibility of using this technique for carbon sequestration will be made more accessible in new locations worldwide. Silicon and aluminium release into the fluids in all experiments of a similar magnitude with varying NaCl concentrations indicates that using saline fluids is unlikely to have a negative impact on the carbon sequestration potential of a volcanic rock reservoir. Comparison between two feldspar mineral compositions indicates that a sample with a higher anorthite content may be more effective at buffering the acidity of acidic saline fluids, which is essential for initiating carbonation reactions, and so targeting such mineralogy’s is likely to be most effective.
The water adsorption isotherms of samples before and after use in rock-fluid experiments were collected for andesitic rock samples, to determine whether a change in mineralogy as a result of acidic fluid dissolution has an impact on the water adsorption capacity of the rocks. The results indicate the andesitic samples have a water adsorption capacity of roughly 5.0 – 9.2 mg/g, which is comparable to previous studies using rock samples from other geothermal reservoir systems. The collected isotherms indicate that the water adsorption capacity of the samples increases after reaction with representative geothermal fluids, particularly if acidic fluids are used. The authors interpret this is a result of increased reactive surface areas after reaction and gives some indication that the water adsorption capacity of a reservoir is unlikely to be negatively impacted as a result of changing mineralogy during injection of acidic fluids.
Overall, the results from this whole project indicate that andesitic rock samples should be further examined as potential targets for permanent carbon sequestration and that utilising seawater instead of freshwater for CO2 injection is likely to be a viable alternative and make the technique more globally accessible. However, significant research is still needed in these areas, to examine more long-term impacts of such suggestions and in particular, the impact of inducing such precipitation reaction will have on the overall permeability of a volcanic rock reservoir system. The impact of alterations minerals on the carbon sequestration potential in terms of possible cation release on dissolution, and permeability reducing properties on precipitation should be examined particularly closely.
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