Acidization--VI : On the equilibrium relationships and stoichiometry of reactions in mud acid

A method is presented for determining the distribution of reaction products and the stoichiometric coefficient for the reaction of mud acid (HF/HCl) with various minerals. To illustrate the techniques, the dissolution of two common alumino-silicates, potassium feldspar and kaolinite, in mud acid is investigated for various temperatures and acid concentrations. After determining the product distribution from the ionic equilibrium relationships involving the various fluoride ion complexes, the stoichiometric coefficient [moles HF consumed per mole mineral dissolved] is arrived at through a numerical solution of the coupled equilibrium and mole balance equations. The HF acid stoichiometric coefficient was found to decrease significantly with increasing temperature and HCl acid concentration. This information is of great importance in determining design conditions for the matrix acid stimulation of oil reservoirs. In the above analysis we have found how the HF stoichiometric coefficient in the reaction between mud acid and two alumino-silicates varies with acid concentration and temperature. The HF stoichiometric coefficient is defined as the moles of HF acid consumed per mole of mineral dissolved. The information obtained in this study is of great importance in the design of the acid stimulation of well reservoirs. It was found that the same stoichiometric coefficient can result for two different sets of mud acid concentrations. Consequendy, one could evaluate which set of mud acid concentrations would be most economically feasible. In addition to the economical considerations, the prediction of the overall stoichiometric coefficient for the dissolution of a sandstone is of vital importance in determining the acid capacity number. From this number, one can calculate the penetration radius of the permeability front resulting from matrix acid stimulation of the reservoir. In calculating the stoichiometric coefficient we first determined the distribution of the aluminum-fluoride and silicon-fluoride complexes resulting from the dissolution of the mineral using the ionic equilibrium relationships. A numerical solution of the coupled mole balance and equilibrium equations was then carried out to determine the stoichiometric coefficient with respect to HF and HCl for the specified mineral dissolution. These results were found to agree satisfactorily with previously reported experimental values. The results of this analysis showed that the HF stoichiometric coefficient decreases with increasing temperature. In addition it was found to decrease during the course of the mud acid-mineral reaction. The most important results of this study are related to the variation of the HF stoichiometric coefficient with HCl concentration. This coefficient was found to be virtually independent of HCl concentration below concentrations of ca. 5 × 10−2 M HCl. Above this concentration it was found to decrease significantly with increasing HCl concentration. A technique for estimating the overall stoichiometric coefficient for a sandstone from the coefficients of the individual minerals has been found to be in satisfactory agreement with the numerical technique involving the ionic equilibria for the sum of all minerals present which had to be carried out on the computer, for each sandstone composition.