Thermodynamic Limitations in Organic-Acid/Carbonate Systems
Authors: Chatelain J.C. , Silberberg I.H. , Schechter R.S. 
Published: December 2007

Abstract
Organic acids used for acidizing do not generally react to completion with carbonate formations under reservoir conditions. The extent of reaction is thermodynamically limited. A correlation is presented that permits the fraction of unreacted acid to presented that permits the fraction of unreacted acid to be calculated as a function of both temperature and initial acid strength. Experimental measurement of the extent of reaction of several organic acids with limestone and dolomite was required to establish the parameters used in the correlation. Results showed that chloroacetic acid reacts almost to completion at the temperatures and concentrations tested. Formic, acetic, and propionic acids exhibit decreased extents of reaction, in that order.
Since the precipitation of calcium-acid salts is a possibility and the transfer of carbon dioxide to the possibility and the transfer of carbon dioxide to the residual oil phase is a probability, the method bas been extended to take both into account. A simple approach to the latter problem using CO2 solubility data was tested against experimental data and found to be adequate.

THERMODYNAMIC FORMULATION
Because weak organic acids react more slowly with calcium and dolomite than does hydrochloric acid, they are sometimes used to stimulate carbonate reservoirs. These weak acids do, however, have the disadvantage of reacting incompletely at reservoir conditions. This paper presents a method by which the extent of acid reaction can be predicted as a function of the pertinent variables, including the acid strength, the residual oil, and the reservoir temperature and pressure. The designer is thereby provided with a means for estimating the appropriate provided with a means for estimating the appropriate acid volumes to be used in a stimulation and with information regarding the acidity of the returned fluids.
The predictive method is centered around thermodynamic arguments that provide the mathematical structure of the correlation. However, because ionic activities in complex, concentrated mixtures are not now predictable, the final working equations, which neglect certain ratios of ionic activity coefficients, cannot strictly be regarded as general thermodynamic results.
The reaction of a weak organic acid with calcium carbonate may be thought of as the result of a series of ionic reactions. The first step is the reaction of the hydrogen ion with calcium carbonate (limestone).
(1)
A similar equation can be written to represent the reaction with dolomite. However, as will be seen, the dolomites can be included without considering MgCO3 separately in the present analysis.
The second and third ionic steps involve the dissociation of the acid and the association of the acid anion and the calcium cation. Thus,
(2) and(3)
Independent investigations have concluded that, generally, the two calcium ions, Ca++ and CaA+, are present to the exclusion of the neutral species, CaA2. These same investigations have attempted to measure the constant of the equilibrium shown in Eq. 3 for the acetate anion, but no thermodynamic data for the other anions studied here have been reported.
SPEJ
P. 189