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Silica Precipitation during Analcime Dissolution

Elizabeth A. Gorrepati and H. Scott Fogler. Chemical Engineering, University of Michigan - Ann Arbor, 2300 Hayward Ave,, Ann Arbor, MI 48109

It is estimated that 32% of matrix acidization treatments fail to increase permeability, and in fact, the treatments often-times decrease permeability of formations. One of the main reasons for these failures is cited as poor treatment fluid selection. An example of poor fluid selection occurred during acidization of Gulf of Mexico reservoirs that contain analcime, a common, naturally-occuring zeolite [1]. Analcime reacts with hydrochloric acid to form silicon reaction products, which subsequently polymerize, precipitate, and damage formations. Our studies focus on describing the fundamental reactions and phenomenon that govern analcime dissolution and the subsequent silicon precipitation.

We show that dissolution rate of analcime in hydrochloric, hydrobromic, or nitric acid is the same in the proton concentration range [H+] = 0.1 to 6M. That is, the anions Cl-, Br-, and NO3- do not affect analcime dissolution rate, and dissolution rate depends only on proton concentration. Thus, using HBr or HNO3 will not dissolve analcime any faster than using the standard hydrochloric acid.

We also show that silicon dissolved from analcime behaves macroscopically the same as a solution of sodium-metasilicate-nonahydrate. Therefore, we used the s-m-n to study silicon precipitation, rather than using analcime (the s-m-n is less costly, and the solutions are more well-defined). We show that the s-m-n forms a silicon “monomer”, which disappears extremely rapidly in HCl via a second-order reaction. The polymerization products are silicon oligomers, which then coalesce to form spherical silica particles. These particles grow exponentially in size, as measured by dynamic light scattering. The particle growth mechanism is conjectured to be flocculation or Ostwald ripening. We studied systems of 2, 4, and 8M HCl, and found higher acid concentration results in much higher monomer disapperance rates as well as particle growth rates. The monomer disapperance and particle growth rate constants were measured, and are hypothesized to be the same for silicon dissolved from analcime.

[1] Underdown, D.R., Hickey, J.J., and Kalra, S.J. "Acidiation of Analcime-Cemented Sandstone, Gulf of Mexico", 1990, SPE 20624.