Density-Driven Flow Simulation in Anisotropic Porous Media: Application to CO2 Geological Sequestration

Abstract

Carbon dioxide (CO 2 ) sequestration in saline aquifers is considered as one of the most viable and promising ways to reduce CO 2 concentration in the atmosphere. CO 2 is injected into deep saline formations at supercritical state where its density is smaller than the hosting brine. This motivates an upward motion and eventually CO 2 is trapped beneath the cap rock. The trapped CO 2 slowly dissolves into the brine causing the density of the mixture to become larger than the host brine. This causes gravitational instabilities that is propagated and magnified with time. In this kind of density-driven flows, the CO 2 -rich brines migrate downward while the brines with low CO 2 concentration move upward. With respect to the properties of the subsurface aquifers, there are instances where saline formations can possess anisotropy with respect to their hydraulic properties. Such anisotropy can have significant effect on the onset and propagation of flow instabilities. Anisotropy is predicted to be more influential in dictating the direction of the convective flow. To account for permeability anisotropy, the method of multipoint flux approximation (MPFA) in the framework of finite differences schemes is used. The MPFA method requires more point stencil than the traditional two-point flux approximation (TPFA). For example, calculation of one flux component requires 6-point stencil and 18-point stencil in 2-D and 3-D cases, respectively. As consequence, the matrix of coefficient for obtaining the pressure fields will be quite complex. Therefore, we combine the MPFA method with the experimenting pressure field technique in which the problem is reduced to solving multitude of local problems and the global matrix of coefficients is constructed automatically, which significantly reduces the complexity. We present several numerical scenarios of density-driven flow simulation in homogeneous, layered, and heterogeneous anisotropic porous media. The numerical results emphasize the significant effects of anisotropy in driving the migration of dissolved CO 2 along the principal direction of anisotropy even if the porous medium is highly heterogeneous. Furthermore, the impacts of the increase of density difference between the brine and the CO 2 -saturated brine with respect to the onset time of convection, the CO 2 flux, and the CO 2 total dissolved mass are also discussed in this paper. Motivation and backgroundNowadays, global warming issue has become the world-widely discussed topic due to the change of climate on the Earth. One of the causes of the global warming is the greenhouse gasses emission, i.e., the release of CO 2 into the atmosphere due to the use of fossil fuels in the industry activities such as power plant, refineries, petrochemical industries, oil and gas processing, etc. The amount of CO 2 concentration in the atmosphere has steadily increased particularly in the last century, from 280 ppm (prior to the industrial revolution) to 390 ppm (at the current time) with annual increase of 1 to 2 ppm [Firoozabadi ...

Multiphase flow simulation with gravity effect in anisotropic porous media using multipoint flux approximation

“…In this approach, a set of experimenting predefined fields are used to operate on the local system of discretized equations to automatically obtain the global matrix of coefficients [17]. This greatly simplifies the coding particularly when solving complex stencils like those encountered in MPFA [14][15][16][17][18][19].…”

“…This approach is so-called multipoint flux mixed finite element (MPFMFE). In the last decade, the implementation of MPFA into single-phase or multiphase flow models for 2-D and 3-D problems has been deeply discussed [12][13][14][15][16].…”

“…This is, tremendously, beneficial particularly when there are long expressions of discretized algebraic equations. This technique has been implemented successfully in many engineering applications involving anisotropy of media properties [14][15][16][17][18][19].…”

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