Abstract: The f(^DgrG_r) Fundamental Role in the Modeling of Coupled Reaction-Fluid Flow

J. L. Mogollon, E. Manrique, A. C. Lasaga

This study ascertains the importance of the Gibbs Free Energy of reaction (^DgrG_r) in the understanding of chemical-mineralogical transformation of rocks in reservoirs. Recently, it has been found that the dissolution rate (R) of minerals, such as gibbsite, kaolinite and albite, follows a complex dependence with the solution saturation state ^Ogr, expressed as ^DgrG_r. Consideration of this finding, together with the well known effect on R of the mineral surface area, A_min; activation energy, E_a, and temperature, T; the activity of proton, a_H+, and other cationic or anionic species a_i in solution led Lasaga et al. (1994) and Mogollon et al. (1994) to propose a general rate law for the dissolution/precipitati n of minerals as:

[EQUATION]

where k₀ is the rate constant and f(^DgrG_r) accounts for the variation of the rate with ^Ogr.

Inclusion of f(^DgrG_r) in coupled reaction-fluid flow models allowed to predict the output Al³⁺ and H⁺ concentration in column experiments, with a difference less than 20%, under a range of flow rates and input pHs. The numerical simulation of the diagenesis of a sandstone, under fluid flow condition, can account for the timing of the mineralization inferred from textural relationship between quartz, feldspar and kaolinite. Consideration of the ^DgrG_r effect may explain the differences of up to 4 orders of magnitude between dissolution rates estimated from field measurements and those from laboratory test. The above results emphasize the need for accurate kinetic data on mineral dissolution/precipitation and the role of fluids controlling th non-equilibrium spatial as well as temporal evolution of subsurface rocks.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela