Some Frequently Overlooked Aspects of Reactive Flow through Permeable Media Aura Araque-Martinez and Larry W. Lake* Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, Texas 78712 Solid dissolution and precipitation are major reactions that occur in reactive flow through permeable media. Precipitated reaction products, in particular, can clog pore space and cause flow impairment, which is one of the reasons their simulation is important. Many calculations assume local equilibrium, ideal solutions, no aqueous reactions, and no supersaturation, but depending on the flow conditions, these assumptions can lead to a misrepresentation of the effects of the reactive flow. This paper evaluates the effects of these simplifying assumptions. We applied a method of characteristics model to run generic reactive flow cases consisting of a single mineral initially present and one possible precipitate. Results show that, under the conditions studied, the amount of precipitation predicted is less than that calculated by assuming equilibrium conditions; however, this precipitate can, nevertheless, cause flow impairment. The same conclusionsreduced precipitations appears to be true for the assumptions of no super- saturation, ideal solutions, and negligible flowing-phase reactions. Including these effects leads to no precipitation at all in some cases. It seems clear that including these effects is important for accurate geochemical modeling. This work is one of the many tangible consequences of Dr. R. S. Schechter’s distinguished work in geochemical flow modeling. This work has ranged from laboratory experiment to fieldwork to theoretical analysis to numerical simulation. The second author on this paper attributes much of his success to this diversity and to Bob’s unwavering devotion to scientific principles. We are pleased to be able to continue the pursuit of knowledge begun by Dr. Schechter and to honor him and his career at the same time. Introduction Simulation of reactive flow through permeable media can become complicated when accounting for all possible conditions that might affect the results. As a conse- quence, assumptions are often made to simplify the simulation. These assumptions include all or some of the following: (1) the medium and the flowing aqueous phase are in local thermodynamic equilibrium, (2) the flowing phase is an ideal solution, (3) no chemical reactions occur in the flowing phase, and (4) there is no solid supersaturation. However, there are always ques- tions about how these assumptions affect the results. We use a simplified method of characteristics (MOC)- based solution 1,2 that has proven to be as accurate and much faster than a complete numerical solution with little loss of generality. The results of these simulations will study the effects of the above assumptions on precipitation and dissolution reactions to determine the error involved when invoking them. Flow Regions under Nonequilibrium Conditions We use a time-distance diagram 8 in which the position of the fronts is shown as a function of dimen- sionless distance x D and dimensionless time t D . The x D - t D diagram is the flow domain in the following. The conventional dimensionless variables are for constant q in one-dimensional, linear flow. x D is the fractional position between the inlet (x D ) 0) and the outlet (x D ) 1) of a linear medium. t D is the cumulative fluid injected normalized by the pore volume of the medium. See the Nomenclature section for other defini- tions. Before illustrating the MOC solution to describe the flow regions under nonequilibrium conditions (NLEA), we will briefly discuss the equilibrium case (LEA). [LEA, the local equilibrium assumption, means that the solid and flowing phases are in equilibrium with each other at a given x D and all t D . NLEA, the nonlocal equilibrium assumption, means the solid and flowing phases are only in equilibrium initially.] Figure 1 sketches a simple dissolution problem for LEA flow that follows the reaction AX f A + + X - . AX is a generic solid initially present at uniform concentration in equilibrium with cation A + and anion X - . A solid initially present is a primary solid. The injected solution is undersaturated with respect to AX, which dissolves as flow progress. * To whom correspondence should be addressed. Phone: (512)471-8233. Fax: (512)471-9605. E-mail: larry•lake@ pe.utexas.edu. x D ) x L and t D ) qt AφL (1) Figure 1. Sketch of the mineral dissolution zonation under LEA conditions. 2717 Ind. Eng. Chem. Res. 2000, 39, 2717-2724 10.1021/ie990881m CCC: $19.00 © 2000 American Chemical Society Published on Web 06/29/2000