K. Prasad Saripalli

Changes in hydrologic properties of aquifer media due to chemical reactions: A review

Hydrologic properties that govern fluid flow through the subsurface are porosity, permeability, relative permeability, fluid-fluid and fluid-solid interfacial areas, pore and particle size distributions, which may change due to dissolution/precipitation of minerals, fine particle release and capture, ion exchange, and clay swelling. Provided here is a review on the change of hydrologic properties in subsurface media due to chemical processes, and the modeling of such changes. Precipitation and dissolution processes affecting the hydrologic properties, their kinetics and the effect of hydrodynamic factors on such processes are discussed. Precipitation in carbonaceous, siliceous, alkaline and acidic environments, and the role of dissolution and clay swelling in formation damage are reviewed. Changes in properties of unsaturated and fractured media were also discussed. Traditionally, different approaches were used to model various physico-chemical processes and their effect on the hydrologic properties. A detailed review of these methods, including the geochemical equilibrium and kinetic models, chemical divide pathway models, flow and transport models, precipitation/dissolution wave theory, network models, porosity and permeability reduction models, is presented. Recommendations are provided for the assessment of changes in the hydrologic properties of subsurface media attributable to chemical reactions, and modeling flow and transport in their presence. Further, research needs on the changes in hydrologic properties and constitutive relationships among such properties in unsaturated media are identified.

Flow and solute transport around injection wells through a single, growing fracture

Abstract During deep-well injection of liquids, the formation around an injection well is often fractured due to an imbalance between the injection pressure and the minimum horizontal rock stress opposing fracturing. The resulting fractures can grow during injection, which may span over several months to years. Earlier studies reported on solute transport in a single fracture in low permeability fractured media, assuming that transport into the formation perpendicular to the face of the fracture is mediated by diffusion alone. This may be valid for flow under natural gradients through fractured formations of low permeability. In contrast, due to the high rates of injection through a fractured injection well, both advection and dispersion play an important role in the spread of contaminants around a fractured injection well. We present a model for the flow and reactive solute transport profiles around fractured injection wells, through a single, two-winged vertical fracture created by injection at high rates and/or pressures and growing with time. The fracture, of constant height and infinite conductivity, serves as a line source injecting fluids into the formation perpendicular to its face via a uniform leak-off, resulting in an elliptical water flood front confocal with the fracture. Flow and solute transport within the elliptical flow domain is formulated as a planar (two-dimensional) transport problem, described by the advection–dispersion equation in elliptical coordinates including retardation and 1st order radioactive nuclear decay processes. Results indicate that transport at early times depends strongly on location relative to the fracture. Retardation has a more pronounced influence on transport for the cases where advection is significant; whereas 1st order radioactive nuclear decay process is independent of advective velocity. Flow and transport around an injection well with a vertical fracture exhibits important differences from radial transport that neglects the presence of the fracture, and also from transport from a fracture of constant length. The model and findings presented have applications in the calculation of the fate and transport of contaminants around fractured injectors and modeling the resulting contaminant plumes down stream of the wells. Further, the model also serves as a basis for modeling enhanced remediation of contaminated rock via injection well fracturing, a recently demonstrated technology.

Implementation of biofilm permeability models for mineral reactions in saturated porous media

An approach based on continuous biofilm models is proposed for modeling permeability changes due to mineral precipitation and dissolution in saturated porous media. In contrast to the biofilm approach, implementation of the film depositional models within a reactive transport code requires a time-dependent calculation of the mineral films in the pore space. Two different methods for this calculation are investigated. The first method assumes a direct relationship between changes in mineral radii (i.e., surface area) and changes in the pore space. In the second method, an effective change in pore radii is calculated based on the relationship between permeability and grain size. Porous media permeability is determined by coupling the film permeability models (Mualem and Childs and Collis-George) to a volumetric model that incorporates both mineral density and reactive surface area. Results from single mineral dissolution and single mineral precipitation simulations provide reasonable estimates of permeability, though they predict smaller permeability changes relative to the Kozeny and Carmen model. However, a comparison of experimental and simulated data show that the Mualem film model is the only one that can replicate the oscillations in permeability that occur as a result of simultaneous dissolution and precipitation reactions occurring within the porous media.