Convective dissolution of carbon dioxide in saline aquifers

Abstract

[1] Geological carbon dioxide (CO 2 ) storage is a means of reducing anthropogenic emissions. Dissolution of CO 2 into the brine, resulting in stable stratification, increases storage security. The dissolution rate is determined by convection in the brine driven by the increase of brine density with CO 2 saturation. We present a new analogue fluid system that reproduces the convective behaviour of CO 2 -enriched brine. Laboratory experiments and high-resolution numerical simulations show that the convective flux scales with the Rayleigh number to the 4/5 power, in contrast with a classical linear relationship. A scaling argument for the convective flux incorporating lateral diffusion from downwelling plumes explains this nonlinear relationship for the convective flux, provides a physical picture of high Rayleigh number convection in a porous medium, and predicts the CO 2 dissolution rates in CO 2 accumulations. These estimates of the dissolution rate show that convective dissolution can play an important role in enhancing storage security. [2] The storage of carbon dioxide (CO 2 ) in geological formations has been proposed as a technological means to reduce anthropogenic emissions of this greenhouse gas [Orr, 2009;Benson and Cook, 2006]. The positive buoyancy of supercritical CO 2 relative to the ambient brine filling the pore spaces may lead to leakage along imperfections in the geological seal, which is of considerable concern for the security of long-term storage [Gasda et al., 2004;Pruess, 2005;Neufeld et al., 2009]. One of the primary mechanisms for stable long-term geological storage of CO 2 is the dissolution of injected CO 2 within ambient brine. Under typical conditions injected CO 2 dissolves into the ambient brine thereby increasing the density of the brine [Teng et al., 1997]. This layer of dense, saturated brine forms by the processes of diffusion, dispersion and mechanical mixing during injection and, once it has reached sufficient thickness, becomes rapidly unstable to convective overturning [Ennis-King et al., 2005;Riaz et al., 2006]. The process of convective dissolution of CO 2 has recently been imaged at ambient conditions in a Hele-Shaw cell [Kneafsey and Pruess, 2009], and enhanced mass transfer has been measured at reservoir conditions [Yang and Gu, 2006;Farajzadeh et al., 2007]. Convection is therefore expected in most sequestration sites, and controls the dissolution rate and hence the long-term risk of leakage. Geochemical observations in natural CO 2 reservoirs require large amounts of CO 2 dissolution into the ambient brine and provide field evidence for sustained convective transport of dissolved CO 2 [Gilfillan et al., 2008[Gilfillan et al., , 2009. Convective dissolution of CO 2 is therefore expected in most natural and anthropogenic CO 2 reservoirs, and controls the mobility of carbon in the subsurface. It is therefore an important mechanism in the deep carbon cycle [Sherwood and Ballentine, 2009], and controls the long-term risk of leakage of CO 2 from geological storage.[3] Despi...

Convective Instability and Mass Transport of Diffusion Layers in a Hele-Shaw Geometry

“…Near c w = 0.3, both ν and D are relatively constant reducing any effects of their variability. We note that our experimental set up and fluid system is somewhat similar to those used in [12] and [13]. However, the experiments in [12] appeared to show finite cell-size effects, and our Rayleigh numbers (defined below) are a factor of 10 lower than in [13].…”

“…However, the experiments in [12] appeared to show finite cell-size effects, and our Rayleigh numbers (defined below) are a factor of 10 lower than in [13]. Also,we vary the Rayleigh number (defined below) using K and H as opposed to K and ∆ρ [13].…”

An empirical theory for gravitationally unstable flow in porous media

“…Even if the same approach can indeed be used for CO 2 -oil displacements, this paper entirely focuses on CO 2 -water displacement. The increase of the mass-transfer rate is equivalent to a dissolution of a larger amount of CO 2 in a shorter period of time and faster propagation of CO 2 in porous media (aquifers and hydrocarbon reservoirs) [33,34,38]. The large volume of dissolved CO 2 remains permanently in the liquid (at least as long as the pressure remains unchanged) and poses no threat of leakage, which is favorable for geological storage of CO 2 [23].…”

A step function density profile model for the convective stability of CO $$_2$$ 2 geological sequestration

“…In summary, the configuration for the base state considered in this paper leads to stability characteristics and flow features that are qualitatively similar to those observed in experiments. Direct comparison cannot be made given that the experiments are carried out in finite enclosures and at higher Rayleigh numbers (see [13]- [14]). …”

“…The general scenario that has emerged from experiments is that a quasi-stable regime is initially attained with the formation of a boundary layer that subsequently grows and becomes unstable to convective overturning when some critical thickness is reached. At this point, the boundary layer has acquired enough potential energy to overcome the stabilizing effect of buoyancy diffusion [11]- [14]. From the mathematical standpoint, the motionless state is governed by the diffusion equation with a Dirichlet and Neumann boundary conditions for the concentration at the top and bottom walls, respectively, and subject to some initial value.…”