Feldspars in CCS reservoirs: overlooked or unimportant?

Feldspars are a common framework grain in CCS sandstone reservoirs. 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 change reservoir properties and mineralise injected CO₂. The general consensus is that these features are unlikely to impact fluid migration during the injection lifespan of any CCS project.

In this contribution, the magnitude of any “feldspar effect” is re-evaluated. Firstly, using petrography, SEM analysis and the determination of 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 understood that different feldspar types 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-basinal scale. Secondly, in a series of hydrostatic chemical dissolution experiments conducted at CCS reservoir pressure and reservoir temperature and higher temperatures, enhanced fracturing and dissolution of some feldspar types was observed along with some precipitation of secondary minerals, whereas other feldspar types were apparently unaffected.

The magnitude of the feldspar effect is illustrated using pilot studies conducted on sandy turbidite fairways comprising the Eocene Forties Sandstone and Lower Cretaceous Captain Sandstone members in the Central North Sea. The latter is the target reservoir unit for CO₂ injection as part of the Acorn CCS Project.

The outcome of our re-evaluation is that the impact of feldspars in CCS reservoirs is likely 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 capacity.