Cross borehole electrical resistivity tomography (ERT) was used to image the resistivity distribution before and during two infiltration experiments. In both cases water was introduced into the vadose zone, and the change in resistivity associated with the plume of wetted soil was imaged as a function of time. The primary purpose of this work was to study the capabilities and limitations of ERT to image underground structure and ground water movement in the vadose zone. A secondary goal was to learn specifics of unsaturated flow in a complex geologic setting. Tomographs of electrical resistivity taken before infiltration image coarser, well‐drained soils (sands and gravels) as more resistive zones, whereas finer grained soils (silts and clays), which hold more water by capillarity, are imaged as more conductive. Images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology. In the ERT images we see the effects of capillary barriers and infer differences between capillary‐driven flow through fine sediments and gravity‐driven flow through very permeable sediments. Images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs. ERT can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements.

The ultimate fate of CO 2 injected into saline aquifers for environmental isolation is governed by three interdependent yet conceptually distinct processes: CO 2 migration as a buoyant immiscible fluid phase, direct chemical interaction of this rising plume with ambient saline waters, and its indirect chemical interaction with aquifer and cap-rock minerals through the aqueous wetting phase. Each process is directly linked to a corresponding trapping mechanism: immiscible plume migration to hydrodynamic trapping, plume-water interaction to solubility trapping, and plume-mineral interaction to mineral trapping. In this study, reactive transport modelling of CO 2 storage in a shale-capped sandstone aquifer at Sleipner has elucidated and established key parametric dependencies of these fundamental processes, the associated trapping mechanisms, and sequestration partitioning among them during consecutive 10-year prograde (active-injection) and retrograde (post-injection) regimes. Intra-aquifer permeability structure controls the path of immiscible CO 2 migration, thereby establishing the spatial framework of plume-aquifer interaction and the potential effectiveness of solubility and mineral trapping. Inter-bedded thin shales-which occur at Sleipner-retard vertical and promote lateral plume migration, thereby significantly expanding this framework and enhancing this potential. Actual efficacy of these trapping mechanisms is determined by compositional characteristics of the aquifer and cap rock: the degree of solubility trapping decreases with increasing formation-water salinity, while that of mineral trapping is proportional to the bulk concentration of carbonate-forming elements-principally Fe, Mg, Ca, Na, and Al. In the near-field environment of Sleipner-like settings, 80-85% by mass of injected CO 2 remains and migrates as an immiscible fluid phase, 15-20% dissolves into formation waters, and less than 1% precipitates as carbonate minerals. This partitioning defines the relative effectiveness of hydrodynamic, solubility, and mineral trapping on a mass basis. Seemingly inconsequential, mineral trapping has enormous strategic significance: it maintains injectivity, delineates the storage volume, and improves cap-rock integrity. We have identified four distinct mechanisms: dawsonite [NaAlCO 3 (OH) 2 ] cementation occurs throughout the intra-aquifer plume, while calcite-group carbonates [principally, (Fe,Mg,Ca)CO 3 ] precipitate via disparate processes along lateral and upper plume margins, and by yet another process within inter-bedded and cap-rock shales. The coupled mineral dissolution/precipitation reaction associated with each mechanism reduces local porosity and permeability. For Sleipner-like 2 settings, the magnitude of such reduction for dawsonite cementation is near negligible; hence, this process effectively maintains initial CO 2 injectivity. Of similarly small magnitude is the reduction associated with formation of carbonate rind along upper and lateral plume boundaries; these processes effectively delinea...

No abstract

The concept of "capillary," or "matric," potentials is commonly used in soil physics to describe water movement in unsaturated soils. The rigorous definition of these and other potentials is presented from fundamental thermodynamic principles at the microscopic level and extended to the macroscopic level by averaging over a representative elementary volume. Of special interest is the treatment of adsorptive surface forces and their associated potentials. Porous medium potentials are extended to a domain containing multiple fluid phases and multiple components. A macroscopic motion equation for a fluid phase (Darcy's law) is derived, incorporating the effect of potentials and surface forces. It relates advective fluxes to gradients of macroscopic chemical potentials and temperature. It reduces to the usual form of Darcy's law only when the aqueous phase is sutt•ciently dilute and temperatures are uniform. Kelvin's law, which relates relative humidity to matric potential, is extended to the case of multiple multicomponent fluid phases in a porous medium domain. The concept of "irreducible" (or "residual") wetting fluid saturation and its relationship to capillary pressure, surface forces, and the Gibbs chemical potential, are discussed. Common methods for determining the matric potential are reexamined in light of this work. the gaseous phase approaches zero as water saturation approaches its irreducible value?2. Under drying conditions, water saturations may go below the irreducible value. What, then, is the meaning of "irreducible water saturation"? When drying takes place to saturations below the so-called irreducible saturation, the liquid phase pressure and the matric potentials are referred to in many references as being "undefined." Yet this pressure is needed in the mathematical model of a drying process.3.Paper number 95WR02715. 0043-1397/96/95WR-02715 $05.00 6. What is the role of potentials and surface forces on diffusive transport? What is the thermodynamic basis for methods determining the matric potential?We felt that these questions cannot be answered by using currently accepted concepts. This motivated us to examine deeper some of the underlying fundamental concepts and definitions.One of the first concepts that we encountered was the definition of "potential." The total potential at a point in the soil was defined by a committee of the International Soil Science Society [Aslyng, 1963, p. 7] as the amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally to the soil water at a considered point, an infinitesimal quantity of water from a reference pool. The latter is at the elevation, the temperature, and the external gas pressure of the considered point, and contains a solution identical in composition to the soil water at the considered point.The fact that this definition refers to "soil water at a considered point" indicates that that the defined potential is at the macroscopic level and not at the microscopic one, as the latter would be a f...

No abstract