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abstract

Recently, an abundant concentration of short-chain aliphatic acids has been recognized in oil field waters. This has led to an interest in organic acids as a means of creating secondary porosity in sandstones through dissolution of carbonate and aluminosilicate minerals. This paper compares stoichiometric and thermodynamic reactivities of organic acids to those of carbonic acid. The stoichiometric reactivity is presented in the form of hydrogen ion donor capacity, and the enhanced dissociation in terms of solubility of the corresponding calcium salts in water. The hydrogen ion donor capacity of organic acids is calculated through the dissociation constant data and is shown to be 6 to 350 times higher when compared to that of carbonic acid.

Porosity preservation is discussed in terms of solubility of corresponding calcium salts of the organic acids. The solubility of calcium acetate is approximately three orders of magnitude higher than that of calcium carbonate. Thus, organic acids will keep more calcium ions in the solution, preventing cementation by calcium carbonate.

Gibbs Free Energy of dissolution of feldspars and calcite in the presence of carbonic and organic acids shows a better thermodynamic drive when the dissolution agent is an organic acid. This paper proposes dissolution-precipitation mechanisms involving organic and carbonic acids that can be tested through measurement of carbon isotopes of cements and the host rock, such as limestone and sandstone.

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