The nature and extent of reactions between CO2 - rich water and fractured continental flood basalts

Abstract

Continental flood basalts have been considered as an unconventional reservoir for geological carbon storage where vesiculated basalt intervals serve as reservoirs and massive basalt zones as a caprock. However, the presence of fractures in the massive layer may lead to CO2 leakage. The objective of this study is to understand the reactions of CO2-saturated fluid on fractured basalt and the respective changes in the fracture and adjacent connected pore network geometry at an early stage of the evolving geochemical system.

Batch reaction experiments, high-resolution sample characterisation, and modeling are used to better evaluate basalts as CO2 storage reservoirs. The nature and extent of initial reactions between CO2-rich water and fractures including the connected pore network were determined.

Reactions of CO2-saturated water with powdered massive (MB) and with vesicular basalts (VB) was studied at three different P/T conditions to understand the mobilisation of ions from crystalline basalt at far-from-equilibrium conditions. Reaction path modeling was undertaken for experiments involving MB to estimate the amount of dissolved and precipitated minerals. Mobilisation of ions was unexpectedly low at high experimental pressure and temperature conditions suggesting concurrent mineral precipitation, which was confirmed by the model. Secondary minerals formed a coating on the surface of the primary minerals, which controlled the primary mineral dissolution rate. The nature of secondary minerals was studied in a separate experiment involving basalt wafers at pressure and temperature conditions representing a depth of approximately 800 m. Mostly clay minerals and zeolites were observed at the surface of the wafers and in suspension, which agreed with the change in water composition. Certain clay minerals and zeolites were highly supersaturated based on the outlet fluid composition. Siderite (FeCO3) was close to equilibrium. The volume of dissolved and precipitated minerals was quantified in a third experiment using an artificially fractured basalt core at pressure and temperature conditions representing a depth of approximately 800 m. Net mineral precipitation occurred and the pore network structure changed, while only a very minor reduction in the net connected pore network volume was observed. These observations provide evidence for concurrent dissolution and precipitation processes leading to complex changes in the pore network geometry

In conclusion, precipitation of secondary silicate minerals dominants at an early stage of CO2 -saturated water and basalt reaction and the presence of fractures in basalt enhances the geochemical reactions.