Pore-Scale Study of Transverse Mixing Induced CaCO₃Precipitation and Permeability Reduction in a Model Subsurface Sedimentary System

Abstract

A microfluidic pore structure etched into a silicon wafer was used as a two-dimensional model subsurface sedimentary system (i.e., micromodel) to study mineral precipitation and permeability reduction relevant to groundwater remediation and geological carbon sequestration. Solutions containing CaCl(2) and Na(2)CO(3) at four different saturation states (Ω = [Ca(2+)][CO(3)(2-)]/K(spCaCO(3))) were introduced through two separate inlets, and they mixed by diffusion transverse to the main flow direction along the center of the micromodel resulting in CaCO(3) precipitation. Precipitation rates increased and the total amount of precipitates decreased with increasing saturation state, and only vaterite and calcite crystals were formed (no aragonite). The relative amount of vaterite increased from 80% at the lowest saturation state (Ω(v) = 2.8 for vaterite) to 95% at the highest saturation state (Ω(v) = 4.5). Fluorescent tracer tests conducted before and after CaCO(3) precipitation indicate that pore spaces were occluded by CaCO(3) precipitates along the transverse mixing zone, thus substantially reducing porosity and permeability, and potentially limiting transformation from vaterite to the more stable calcite. The results suggest that mineral precipitation along plume margins can decrease both reactant mixing during groundwater remediation, and injection and storage efficiency during CO(2) sequestration.

Dissolution-Driven Permeability Reduction of a Fractured Carbonate Caprock

“…Similar self-sealing behavior has been observed in wellbore cements (Huerta et al, 2011). Conditions that favor carbonate mineral precipitation within advection-controlled leakage pathways may require mixing of higher-pH interstitial waters with calcium-and carbonate-enriched reservoir brines (Zhang et al, 2010;Nogues et al, 2012). Core flow studies conducted at ambient conditions and low CO 2 partial pressure have demonstrated complex alterations along limestone fracture pathways (Gouze et al, 2003;Noiriel et al, 2007).…”

Hydrodynamic Fingering Instability Induced by a Precipitation Reaction

“…The reverse case of precipitation is not expected to destabilize an interface as the related decrease in permeability and, hence, in mobility behind the front is expected to block the flow rather than destabilize it. There is, however, increased interest to understand the effect of precipitation reactions during flow displacements in porous media in the context of CO 2 sequestration techniques [11][12][13][14]. Mineralization by which CO 2 injected in a porous medium could undergo precipitation reactions (to yield carbonates, for instance [13][14][15][16]) is indeed promising in view of a permanent safe storage of CO 2 in geological strata.…”

“…The present study paves the way to future detailed analysis of the influence of precipitation reactions on the stability of displacement processes in porous media. It explains, for instance, why local precipitation of carbonates [13,14], among others, might destabilize the flow during CO 2 injection processes at the heart of sequestration techniques. …”

An Incompressible, Depth-Averaged Lattice Boltzmann Method for Liquid Flow in Microfluidic Devices with Variable Aperture

“…First, we depth-average Equations (1) and (2). Second, we assume the horizontal velocity components u 3D (x, y, z) and v 3D (x, y, z) have parabolic velocity profiles across the aperture h(x, y), a familiar result from the literature in lubrication theory [15,16], making it possible to evaluate derivatives and integrals with respect to the vertical (z) coordinate.…”

“…Also referred to as micromodels, these devices serve as model porous media in laboratory experiments studying pore-scale reactive transport processes [1,2] (see Figure 1). Micromodel experiments of solute mixing and reaction [3], calcium carbonate (CaCO 3 ) precipitation [4], and biomass growth [5,6] have been numerically modeled by solving the Stokes equations for the flow field followed by the advection-diffusion-reaction equations for concentration fields.…”