Geophysical Methods for Characterizing and Monitoring the in-situ CO2 Mineral Storage Site in Helguvik, Iceland - Field Experiments and Modelling Results

The DemoUpStorage pilot project is the first field-scale, in-situ CO₂ mineral storage project that utilizes saline water instead of fresh water for injection. Our goal is to monitor the precipitation of secondary carbonates in the pore space of the reservoir using geophysical and geochemical methods. Here, we focus on the geophysical part.

First, we present the geophysical site characterization using electrical resistivity tomography (ERT) and crosshole seismic measurements. The seismic and geoelectric measurements are in good agreement and highlight decameter-thick, basaltic layers that are persistent throughout the reservoir. We use the geophysical data to build a simplified porosity and permeability model of the reservoir to discuss potential flow paths of the injected fluid, highlighting potential locations of carbonate precipitation. The estimated porosity ranges between 6% and 26%, whereas permeability is found to range between 2*10^-15 m² and 2*10^-13 m².

Then, we use a rock physics modelling approach and synthetic seismic modelling to investigate the sensitivity of time-lapse crosshole seismics to monitor secondary carbonate precipitation. We find that precipitation of about 17 kg calcite per cubic meter of rock volume causes a 1% p-wave velocity increase that is feasible to monitor with crosshole seismics, given a dense enough survey geometry, high signal-to-noise ratio and a high sampling rate.