1D and 2D Reactive Transport Models of CO2 Injection in Depleted Gas Reservoirs: A Geochemical Simulation Workflow

Abstract

Geological CO2 storage is a key technology to achieve net-zero target by 2050. Among the potential target formations for Carbon Capture and Storage (CCS), we here investigate CO2 injection into a depleted gas reservoir by taking into account geochemical modeling for reactive transport and storage. The adopted workflow allows to evaluate the impact of CO2-fluid-rock interactions in the reservoir system. Geochemical and reactive transport models (RTMs) were performed at different scales of investigation to predict the sequestration potential and the long-term behavior of the CO2 plume, while addressing the injectivity and/or containment issues related to CO2 injection. Experimental data propaedeutic to RTMs consisted of core samples tests, mineralogical and petrographical analyses, petrophysical log- derived data, and the reconstructed formation water composition at reservoir conditions. By means of specialized commercial software, 1D and 2D reactive transport simulations aimed at defining the performance of the storage site (injectivity, porosity and permeability variations) to evaluate the impact of CO2-fluid-rock interactions in the reservoir system. 1D vertical RTMs were implemented to reproduce the mineralogical, lithological and petrophysical heterogeneities of the reservoir system. The RTMs were initialized to replicate the reservoir saturated by CO2 and simulated for over 8000 years to check the reaction between rock and CO2. Results show that the geochemical activity of the system is influenced more by petrophysical properties, than by the mineralogy of the rock. The porosity tends to increase on small timescales as a result of carbonates dissolution, whereas over long timescales, geochemical activity is more guided by diffusivity, producing the precipitation of secondary carbonates, which decreases the effective porosity. Within the caprock, geochemical reactions are hindered due to the almost complete absence of CO2, despite diffusion spanning over 8000 years of simulation. Based on the geochemical insights obtained through 1D RTMs, 2D radial RTMs provided an overview on the performance of the storage site over a simulated 1000 years timescales since injection. Similar to 1D simulations, 2D radial RTMs indicate that petrophysical properties influence mineral reactivity on small timescales. Furthermore, on larger timescales, diffusive processes enhance phase dissolution/precipitation phenomena. In the reservoir no relevant variations in terms of loss of injectivity are expected; moreover, after 1000 years, the diffusion of CO2 in the caprock is negligible, and there is no observed significant mineral reactivity at the caprock/reservoir interface. By combining 1D and 2D RTMs, we were able to account for detailed geochemical effects of CO2 injection on multiple scale and integrate them inside reservoir models extended beyond the operational lifetime of the storage site.