Berthierine formation in reservoir rocks from the Siri oilfield (Danish North Sea) as result of fluid-rock interactions: Part III. Determining mineral stability and CO2-sequestering capacity of glauconitic sandstones

- Applying hydrogeochemical modeling.
- Determining the effects of pH-EH conditions on glauconite dissolution.
- Analyzing CO2-sequestering capacity of glauconitic sandstones.
- Demonstrating that berthierine formation limits carbonate formation.
- Identifying the effects of the overall mineral assemblage on CO2-sequestration.

Berthierine was proven as one of the most important products of glauconite alteration in the Siri oilfield (Danish North Sea). However, there is an ongoing debate regarding the main product of glauconite dissolution: siderite, berthierine, or berthierine as a precursor of siderite and/or magnesium-bearing carbonate. In order to investigate the consequences of glauconite dissolution in view of thermodynamic admissibility and the resulting CO2-sequestering capacity, a hydrogeochemical model, which is based on thermodynamics of chemical equilibrium, was developed. Calculating various modeling scenarios helps to conclude on the pH-EH conditions of glauconite dissolution as well as of berthierine formation and dissolution in generic, aqueous systems under elevated temperature-pressure conditions.Our modeling results highlight that carbonate formation cannot be triggered exclusively by CO2 addition into glauconitic sandstones. The injection of pure CO2 into glauconitic sandstones leads to acidic and anoxic oxidizing conditions under which glauconite remains stable. To intensify glauconite alteration by CO2 injection, glauconitic sandstones have to be in contact with degradable organic matter, or, alternatively, reducing agents have to be co-injected with CO2. Sufficient electron transfer to ferric iron bound in glauconite is the ultimate control for intense glauconite alteration and for subsequent berthierine precipitation.Once formed, berthierine remains stable over a broad pH range and is not transformed to any carbonate under reducing conditions. Thus, CO2 injection into glauconitic sandstones under reducing conditions mainly leads to formation of berthierine instead of iron- and magnesium-carbonates. However, hydrogeochemical conditions in the subsurface can affect CO2 sequestration via glauconite dissolution and the resulting carbonate formation, including the pH-EH conditions, the chemical composition of glauconite, and the overall mineralogical composition of glauconitic sandstones.