Effects of physical and chemical heterogeneities on transport and reaction processes in porous media.

摘要

We experimentally and numerically investigated the effect of chemical and physical heterogeneity on fluid flow, transport and reaction in porous media. We also proposed a new reactive transport model to simulate the soil formation process from Marcellus shale parent rock, which helped us in determining the key controlling parameters of the mineral dissolution and precipitation processes in natural settings.;In order to determine how physical heterogeneity structure, in particular correlation length, controls flow and solute transport, we used non-reactive solute transport in two-dimensional (2D) sand boxes (21.9 cm by 20.6 cm) and four modeling approaches, including 2D Advection-Dispersion Equation (ADE) with explicit heterogeneity structure, 1D ADE with average properties, and non-local Continuous Time Random Walk (CTRW) and fractional ADE (fADE). The goal of the physical heterogeneity part of the work was to answer two questions: 1) How and to what extent does correlation length control effective permeability and breakthrough curves (BTC)? 2) Which model can best reproduce data under what conditions? Sand boxes were packed with the same 20% (v/v) fine and 80% (v/v) coarse sands in three patterns that differ in correlation length. The Mixed cases contained uniformly distributed fine and coarse grains. The Four-zone and One-zone cases had four and one square fine zones, respectively. A total of 7 experiments were carried out with permeability variance of 0.10 (LC), 0.22 (MC), and 0.43 (HC). Experimental data show that the BTC curves depend strongly on correlation length, especially in the HC cases. The HC One-zone (HCO) case shows distinct breakthrough steps arising from fast advection in the coarse zone, slow advection in the fine zone, and slow diffusion, while the LCO and MCO BTCs do not exhibit such behavior. With explicit representation of heterogeneity structure, 2D ADE reproduces BTCs well in all cases. CTRW reproduces temporal moments with smaller deviation from data than fADE in all cases except HCO, where fADE has the lowest deviation.;Well-mixed batch reactor reaction rate studies result in high dissolution rates, which are usually up to five orders of magnitude greater than field-scale rates. In the natural subsurface, solid materials of different properties are distributed unevenly with various spatial patterns. Numerous factors have been examined to explain the discrepancies between well-mixed laboratory rates and those measured in fields. Parameters such as chemical and physical heterogeneities, velocity, and flow distribution are commonly ignored in the well-mixed batch reactor rate measurements. Some modeling studies have shown that spatial distribution of minerals in porous media affects large-scale mineral dissolution. Experimental studies on the effect of spatial pattern of distribution of chemical heterogeneity on mineral dissolution reaction rate are scarce except for a few studies on magnesite dissolution rates. Large-scale dissolution rates can be affected by both physical and chemical heterogeneities.;In the chemical heterogeneity part of the study we examined the effect of calcite spatial distribution on its dissolution rate under various flow velocities and permeability contrast conditions. Dissolution data of reactive fluid flow (pH=4) through two-dimensional (2D) flow cells (20.0 cm by 20.0 cm) was collected. The flow cells were packed with the same amount of calcite and sand with Mixed and One-zone patterns. The Mixed case contained uniformly distributed calcite and sand grains while the One-zone case had one square calcite zone in the middle of the flow cell. The experiments were carried out at three flow rates (1.435, 7.175, 14.35 m/d) and the dissolution process was simulated using reactive transport modeling. In addition to velocity, effect of parameters such as permeability ratio (calcite permeability/sand permeability), and transverse dispersivity on calcite dissolution were examined numerically. The goal of this part of the study was to answer the following questions: 1) What is the extent of the effect of physical and chemical heterogeneities on mineral dissolution? 2) What are the parameters that control significance of mineral spatial pattern on overall dissolution?;To understand controls of geochemical reaction rates in natural systems, we modeled soil formation from Marcellus shale parent rock using reactive transport modeling with laboratory measured rate laws. Marcellus Shale is a black shale formation that is rich in organic matter and pyrite. The dissolution of Marcellus shale can lead to release of heavy metals and cause significant environment problems, especially with the extensive use of hydraulic fracturing during the production of natural gas. Here, we use soil formation and aqueous geochemistry data as constraints to understand the processes and develop a reactive transport model during Marcellus shale weathering. The simulation was carried out from approximately 10,000 years ago when the formation was first exposed after the last glacier to the present time. (Abstract shortened by UMI.).