One-Dimensional Reactive Transport Modeling of CO2 Storage Systems – Change in Cap Rock Porosity Triggered by Pressure and Temperature Dependent CO2-Water-Rock Interactions

Abstract:

In carbon capture and storage (CCS) systems supercritical CO₂ is injected into a reservoir and dissolves in the reservoir brine. Subsequently, CO_2(aq) diffuses into the cap rock to regions of lower total pressure and temperature and triggers CO₂-water-rock interactions that are coupled with mass transport and result in precipitation and/or dissolution of minerals along the CO₂ migration path. Such hydrogeochemical interactions change porosities and are responsible for the improvement or deterioration of the long term integrity of the system.

This study presents a semi-generic hydrogeochemical model based on chemical equilibrium thermodynamics, data from several CO₂ storage systems, and plausible assumptions regarding non-available data. One-dimensional reactive transport modeling is performed by using the U.S.G.S. PHREEQC code (3.1.4-8929; phreeqc.dat database) to identify and quantify the loss or gain of total porosity affected by hydrogeochemical reactions driven by diffusive mass transport exposed to pressure and temperature gradients. A fine spatial and temporal discretization, the use of non-reactive tracers, and a broad variety of modeling scenarios enable the calculation of the relevant timescale for simulations of long-term storage of CO₂ and the consideration of the pressure dependent mass action law constants along the CO₂ migration path.

Modeling results show that the relevant timescale for simulations of long-term storage of CO₂ is in the range of 10⁶ years, and that pressure/temperature conditions, heterogeneities (veins and fractures) and the mineralogical composition of the cap rock have the strongest influence on the increase in cap rock porosity (maximum increase from initial 5 % to 7.5 %). Critical parameter combinations – total pressure effects are crucial – could put long-term integrity at risks. Nevertheless, a wide range of conditions and parameter combinations for safe CO₂ storage is identified by other modeling scenarios.