--> ABSTRACT: Dissolution Kinetics of Permian Blaine Gypsum, by Michael A. Raines; #91020 (1995).

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Dissolution Kinetics of Permian Blaine Gypsum

Michael A. Raines

The physical and chemical characteristics of gypsum (CaSO4) are important for the understanding of such environmental, energy, and agricultural topics as sink hole development in karst regions, hydrologic studies, scale development, localized permeability increases through cation exchange, depositional environment interpretation, soil reclamation, and crop damage. Therefore, an understanding of the controlling factors of gypsum dissolution is also important.

When solids and liquids interact, five processes may occur: transport of the reactants to the surface of the solid, adsorption on the surface, reaction with the solid, desorption of the products, and transport of the products to the bulk solution. The slowest step determines the rate at which a reaction progesses. Gypsum dissolution can be approximated as a transport controlled reaction. The rate limiting step is the transfer of Ca and SO4 ions through a diffusive boundary layer to the bulk solution.

Transfer through the boundary layer is affected by thermodynamic and physiochemical conditions in the system, i.e. ionic strength and saturation state of the solution, surface geometry, and thickness of the diffusive film. Film thickness is affected by hydrologic conditions in the system, including flow regime (i.e. laminar or turbulent).

Experiments have been performed at the University of Oklahoma on gypsum from the Permian Blaine Formation using a mixed flow/rotating disc reactor. Chemical and hydrologic conditions have been varied to show effects of ionic strength, degree of saturation, flow regime (as expressed by Reynold's number), flow velocity, gypsum surface preparation, gypsum texture, and lithologic variation. Gypsum dissolution rates are significantly increased as the system moves from laminar to turbulent flow. The sensitivity of rates to fluid flow velocities across gypsum surfaces lessens as reactant solutions approach saturation with gypsum.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995