Experimental and microstructural analysis of feldspar solubility in CCS reservoirs

Feldspars are a common framework grain in sandstone reservoirs targeted for carbon capture and storage (CCS). They are mechanically weak under reservoir conditions and are very likely to react with CO₂ injected into saline aquifers or depleted hydrocarbon reservoirs.  Reactions could dissolve feldspar and precipitate new minerals to an extent that fundamentally changes reservoir properties and potentially mineralises injected CO₂. The current general consensus is that these features are unlikely to impact fluid migration during the injection lifespan of any CCS project. However, the response of feldspars to saturation with aggressive CO₂-enriched fluids under stressed reservoir conditions is poorly understood.

In this contribution, the magnitude of any “feldspar effect” is re-evaluated using sandstone samples obtained from the Lower Cretaceous Captain Sandstone in the Central North Sea, which is the target reservoir for CO₂ injection in the Acorn Project (UK). 

Firstly, using petrography, SEM analysis and Pb isotopic compositions of detrital feldspars, sediment provenance and subsequent diagenesis are shown to be significant drivers on feldspar composition and texture prior to injection. This is important because it is already understood that different feldspars react with CO₂-rich fluids at different rates: thus any feldspar effect could significantly vary within a reservoir with mixed provenance and burial history on a sub-basin scale. Secondly, we conducted a suite of novel reaction experiments conducted using a triaxial ‘Nimonic’ deformation rig to investigate chemical dissolution in sandstone core plugs saturated with both CO₂-enriched fluids and water under subsurface conditions. Experiments were run at CCS reservoir pressures (70MPa confining pressure, 50MPa pore fluid pressure) and a range of temperatures (80°C – 550°C) to accelerate reaction rates and promote geological reactions in a short timescale. Microstructural and elemental analysis of post-mortem experimental samples showed enhanced fracturing and dissolution of certain feldspars along with precipitation of secondary minerals, whereas other feldspars were apparently unaffected. Experiments performed above 400°C showed replacement and dissolution of K-feldspar grains with Ca-rich plagioclase and K-bearing clays.

The outcome of our re-evaluation is that the impact of feldspars in CCS reservoirs has likely been overlooked, but until further experimental work is carried out to constrain how quickly feldspar interactions will impact fluid flow within the reservoir, uncertainties will remain with regard to their impact on CO₂ injectivity and storage capacity.