Precipitation and dissolution of reactive solutes in fractures

Abstract

Abstract. The precipitation and dissolution of reactive solutes, transported under the action of fully developed laminar flow in saturated fractures, is analyzed assuming an irreversible first-order kinetic surface reaction for one component. Equations describing solute transport, precipitation and dissolution, and the evolution of fracture aperture were approximated and solved using combined analytical and numerical techniques; dimensionless transport parameters incorporated into the solutions were estimated from data available in the literature. Fractures with initially flat, linearly constricted, and sinusoidal apertures were investigated. The initial fracture geometry and the solute saturation content of the inflowing fluid have a profound effect on the reaction processes. The results show that the evolution of the solute transport and fracture geometry can be adequately described by the Damk6hler and P6clet numbers. Two extreme transport regimes were identified: relatively uniform evolution of fracture apertures and nonuniform evolution of fracture apertures restricted to the inlet region of fractures. In the case of precipitation with half-life times of the order of seconds to years and with fluid residence times of the order of minutes to days, the time for a fracture to close completely is of the order of days to millions of years. This is consistent with the order of magnitude of hydrogeological timescales. In the model the process of dissolution is the inverse of precipitation, although the combined solute transport and reaction processes are irreversible. These results and the applied dimensionless analysis can be used as a basis for the development of more complex models of reactive solute transport, precipitation, and dissolution in saturated fractured media. IntroductionThe transport of contaminants through fractured geological media has received considerable attention in recent years. While many studies have analyzed the transport of conservative solutes in discrete fracture networks, relatively little attention has been devoted to the analysis of the transport of reactive solutes. Even less work has been devoted to the influence of reactive solutes on the physical and chemical properties of the fracture networks and the surrounding porous matrix. This is in part due to the highly complex interaction between the mass transport mechanisms and the changing properties of a fractured porous medium.Precipitation and dissolution can significantly modify the physical and chemical properties of fractured media. Physical changes of fracture aperture, tortuosity, and thus effective mass diffusivities and permeabilities are coupled to the subsequent fluid flow and solute transport and the precipitation and dissolution reactions. Chemical changes of the fractures, such as sorption and reaction capacities, also have a significant effect on solute transport. As such, a variety of closure patterns can be observed in the field, which range from fully filled (mineralized) fractures to those with pockets or irregul...

Étude des processus chimio-hydro-mécaniques dans un massif rocheux fracturé perturbé par une exploitation minière :

“…La dissolution de la calcite adoucit la topographie de la fracture et par conséquent permet une diminution de la tortuosité de l'écoulement; l'écoulement se concentre dans un seul chenal. Ce type d'équation est appliqué le plus souvent dans des études de modélisations numériques (Dijk et Berkowitz 1998;Bekri et al 1997) et dont les résultats ne concordent pas toujours avec ceux des études expérimentales utilisant les mêmes équations.…”

Dissolution and precipitation of fractures in soluble rock

“…The rate law describing the speed on the reaction was assumed to be linear, and coefficients were taken from the quartz system. Dijk and Berkowitz (1998) showed that precipitation occurs under a wide range of conditions. 15 Dijk et al (2002) examined the dissolution patterns in an artificial halite fracture in the laboratory.…”

“…A sequence of deposition-dissolution cycles lasting several hundred hours each reveal periodic changes in flow velocity, until a clogging of 10 the fractures stops the evolution. Dijk and Berkowitz (1998) have modelled transport through fractures of different dimensions with different flow velocities assigned under laminar flow conditions. The transport equation used accounts for a source term representing dissolution or precipitation of dissolved matter.…”

Pore Scale Modeling of Reactive Transport Involved in Geologic CO2 Sequestration

“…Because of its importance, the general problem of reactive transport at the pore scale has been studied extensively using various approaches under different simplifying conditions [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, only recently has the ful!…”