A Major Carbon Mitigation Strategy For Reducing Emissions Of CO2
Geological CO2 sequestration (GCS) in carbonate formations has been considered as a major carbon mitigation strategy for reducing emissions of CO2 to the atmosphere (Metz et al., 2005; Reichle et al., 1999). One of the major concerns is the CO2 leakage through highly conductive pathways, such as faults, wells, and local high-permeable zones, which may cause environmental risks of GCS (Harvey et al., 2012; Jun et al., 2013; Lions et al., 2014). During a GCS process, the dissolved CO2 in brine can form a carbonate acid that can dissolve carbonate rocks by various geochemical reactions (Assayag et al., 2009; Gaus et al., 2002; Metz et al., 2005). Such CO2-rock-brine interactions cause mineral dissolutions that may enhance the existing natural fracture system and eventually forms highly conductive pathways for possible CO2 leakage.
One of the CO2 leakage scenarios is the local leakage that might occur in a sudden or short-term period as the CO2 would escape through the conductive pathways near the wellbore during the CO2 injection (Metz et al., 2005). Kharaka et al. (2009) observed field data at the Frio field site-South Liberty oil field in Texas and reported the CO2 leakage was related to the dissolution of carbonate minerals. Gaus (2010) and Lemieux (2011) provided the literature review on the potential leakage pathways during the GCS processes and concluded that geochemical reactions between CO2-saturated water and carbonated rocks were one of the important causes of the leakage. Extensive experimental investigations on CO2-rock-brine interactions have further revealed that percolation of CO2 saturated brine into core sample can lead to significant pore space alteration.
Andreani et al. (2008) observed the increases in fracture apertures and the core sample permeability by core flooding experiment with injected CO2-saturated brine. Luquot and Gouze (2009) presented the experiments by CO2-saturated brine injected into carbonate rocks and observed the significant changes in porosity and permeability induced by mineral dissolution. Cao et al. (2013) injected CO2-saturated brine through a core sample and observed the significant increases in both porosity and permeability. Steel et al. (2018) presented the hydrothermal experiments to investigate the effect of CO2-rock-brine interactions on the evolution of rock properties under the reservoir conditions.
Both field observation and experimental findings indicated that the mineral dissolution can change the rock properties significantly and may lead to the CO2 leakage that causes the failure of the GCS implementation. As an alternative method to field and laboratory experiments, numerical models have been applied to quantify the coupling effects of chemical reactions and fluid flow and better understand the factors that control the mineral reactions since the reactions are dependent on the mineral composition of the rocks, injection scenarios, and geological structures. Extensive mathematical models have been proposed to solve the reactive transport problems (Fan et al., 2012; Li et al., 2011, 2010, 2008; Qiao et al., 2015; Steefel, 2009; Steefel and Lasaga, 1994; Yuan et al., 2017, 2016).
Based on the different length scales, Navier-Stokes or Stokes equation has been applied as a momentum equation at the pore scale, and Darcy’s equation has been applied at the continuum scale and field scale. In a fractured carbonate reservoir, Yuan et al. (2016) presented a 2-D mathematical model that couples the Stokes-Brinkman equation and reactive-transport equations to describe calcite dissolution in a single mineral system at the continuum scale during the waterflooding process. Compared to Darcy’s equation, the Stokes-Brinkman equation is a unified approach for modeling fluid flow in both porous media and free flow regions with a single equation in the entire domain, which is an ideal candidate for modeling of porosity alteration and fracture enhancement due to mineral dissolution.
And then Yuan et al. (2017) employed the 2-D coupled model to describe mineral dissolutions in a multiple mineral system during the GCS process. In this paper, as an extension of previous work (Yuan et al., 2017), we developed a 3-D numerical model that couples the Stokes-Brinkman equation and the reactive-transport equations for modeling the mineral dissolution during CO2 injection in a multi-mineral system. A sequential method (Liu et al., 1997; Liu and Ortoleva, 1996; Panga et al., 2005; Qiao et al., 2015; Steefel and Lasaga, 1994) was applied for solving the coupled mathematical equations numerically. The Stokes-Brinkman equation is solved by the staggered grid finite difference method, which was proposed by Harlow and Welch (1965) and was widely applied for solving the Navier-Stokes equation (Bell et al., 1989; Ge and Fotis, 2007).
The reactive-transport equations are solved by the control volume finite difference method. In the proposed numerical procedure, (1) Stokes-Brinkman equation is first solved for pressure and velocity; (2) reactive-transport equations are then solved for the concentrations of primary aqueous species with calculated velocity distribution from step (1); and (3) the rock property models are used to characterize the porosity and permeability alterations based on the concentrations of the primary species. The numerical model has been validated using a CO2-saturated brine flooding experiment from the existing publication (Andreani et al., 2008; Yuan et al., 2017).
