Effects of permeability on CO₂ trapping mechanisms and buoyancy‐driven CO₂ migration in saline formations

Abstract

[1] We present the results from a series of numerical simulations to explore systematic k heterogeneity effects on both CO 2 trapping mechanisms and buoyancy-driven CO 2 migration. For this purpose, we generated various permutations of two-dimensional numerical models of subsurface porous media: homogeneous, random, homogenous with a low-permeability (k) lens, and isotropically/anisotropically correlated k fields. For heterogeneous cases, we used a sequential Gaussian simulation technique to generate ten realizations in each model permutation. In each simulation, the amounts of mobile, residually, and aqueously trapped CO 2 were calculated, and the spatial distributions of the CO 2 plumes were quantified using first and second spatial moments. Simulation results from both homogeneous and random k fields suggest that the amount of residually trapped CO 2 increases as the mean effective k increases. These results imply that the overall velocity distribution, which governs the sweeping area of the supercritical-phase CO 2 plume, is a critical factor for controlling residual CO 2 trapping. However, as overall velocity (or effective k field) increases, we predict that the CO 2 plume potentially reaches the caprock more quickly. In addition, results also show that the decrease of variance in ln k increases the amount of residually trapped CO 2 . In simulations of anisotropically correlated k fields, the vertical CO 2 migration distance due to buoyancy shortens as the horizontal correlation length becomes greater. In addition, as the horizontal correlation length becomes greater, residual CO 2 trapping increases and mobile CO 2 decreases because the CO 2 plume spreads farther laterally (i.e., it sweeps a larger area). In summary, results of these analyses suggest that heterogeneous k fields with greater anisotropic correlation ratios potentially maximize residual CO 2 trapping and minimize buoyancy-driven CO 2 migration. Our findings also suggest that when heterogeneous k fields have a certain structure such as a low-k lens or other hydraulic barriers (e.g., faults), the amount of residually trapped CO 2 may increase and depend more on the geometry of geological structures than the magnitude of effective k.

Dynamic modelling of a UK North Sea saline formation for CO 2 sequestration

“…Increasing the permeability of the storage 556 site without changing the porosity results in the plume diameter increasing. This result is supported 557 by the findings of Han et al (2010) who showed that a larger area of the storage site is swept by CO 2 558 when the formation permeability is increased. Similarly Jahangiri & Zhang (2011) found that the 559 overall plume spread in all directions is increased when formation permeability is higher.…”

Sensitivity Study of Simulation Parameters Controlling CO2 Trapping Mechanisms in Saline Formations

“…A detailed description of the GEM simulator is discussed in previous studies of subsurface CO 2 plume behavior (Kumar et al 2005;Bryant et al 2008;Saadatpoor et al 2009;Han et al 2010a). …”

“…Bryant et al (2008) and Saadatpoor et al (2009) showed that the capillary pressure scaled with spatially correlated permeabilities in heterogeneous formations may lead to channeling of vertically migrating CO 2 plumes and impact both the overall configuration of CO 2 plumes and the degree of CO 2 trapping. Ide et al (2007), Flett et al (2007), Green and Ennis-King (2010), and Han et al (2010a) performed more detailed analyses of CO 2 trapping mechanisms including the time-scale of convective mixing in various degrees of subsurface heterogeneity (e.g., permeability variance, correlation length, and amount of shale fraction/thickness). Different from the studies associated with subsurface heterogeneity, Juanes et al (2006) performed numerical simulations at reservoir scales to understand how the changes in engineering parameters such as injection rate, injection ratio of water and CO 2 (alternating water injection), and bottom-hole pressure may influence residual CO 2 trapping.…”

The effect of designing parameter of WAG injection on enhancement of CO2 trapping in heterogeneous formations: A numerical study

“…Permeability heterogeneity is a critical factor influencing CO 2 migration and trapping in saline aquifers. The effect of permeability heterogeneity on the transport and trapping behaviors of CO 2 has been studied since about a decade ago . For a reservoir formed in a depositional environment, permeability heterogeneity can stem from both the spatial distribution of depositional faces and the variations in grain size distributions within each face .…”