Hydrofluoric Acid Reaction with a Porous Sandstone

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Abstract
A procedure is developed for predicting changes in the porosity distribution in a sandstone resulting from reaction with hydrofluoric acid. This procedure is based on a theory for slow heterogeneous reactions in a porous solid where the solid matrix is consumed in the reaction Process. Reaction-rate data for use in this theory are obtained from experiments where acid is injected through short cores and effluent concentration measured using a fluoride specific ion electrode. This rate is found to be first order in hydrofluoric acid concentration. Variations in rate with temperature and quantity of rock dissolved are shown. Introduction Mixtures of hydrofluoric and hydrochloric acid are used to stimulate gas and oil production from sandstone reservoirs by increasing formation porosity and permeability near the wellbore. This porosity and permeability near the wellbore. This acid will react with almost all constituents of naturally occurring sandstones, such as silica, feldspar, clays, and calcareous material. In order to utilize this acid effectively, it is necessary to predict where acid reacts and changes that occur predict where acid reacts and changes that occur with reaction. Chemical reactions between hydrofluoric acid and silica or calcite in the rock matrix are simple, well known reactions. However, reactions with silicates such as clays or feldspars are complex since these minerals occur as three-dimensional lattices with only average empirical formulas. Examples are kaolinite ([A Fe +3 Mg]-Si O1.8 0.1 0.1 2 5 [OH]. Ca), montmorillonite (A Mg Si4 0.05 1.67 0.33 4 O [OH] . NA), and feldspars such as albite10 2 0.33 ([NaSi A ]). In the reactions shown below the3 8 reaction of sodium silicate is used to represent hydrofluoric acid reaction with silicates found in the matrix. REACTION WITH SILICA SiO + 4HF SiF + 2H O2 4 2 SiF + 2HF H SiF4 2 6 REACTION WITH SILICATES (FELDSPAR OR CLAYS) Na SiO + 8HF SiF + 4NaF + 4H O4 4 4 2 2NaF + SiF Na SiF4 2 6 2HF + SiF H SiF4 2 6 REACTION WITH CALCITE CaCO + 2HF CaF + H O + CO3 2 2 2 Reaction of HF-HCl mixtures with silicates and quartz has been the subject of studies by Blumberg, Blumberg and Stavinou, Gatewood et al. and Smith and Hendrickson. These studies indicate that the reaction is first order in HF concentration and that reaction rate with silicates is at least 10 times faster than reaction with silica. To dare, a reliable method for relating reaction data taken on finely ground silica, dispersed clays, or glass slides to the acidization process in a sandstone formation has not been developed Figs. 1 and 2 are photomicrographs of a Berea sandstone core illustrating the system in which acid reaction occurs. In these photomicrographs silica grains are black, and a few feldspar grains are apparent because of their internal porosity, which gives a streaked appearance. Unfortunately, clay or calcite cannot be differentiated from the pore space since all appear as an area shading from pore space since all appear as an area shading from gray to white. It is apparent that the heterogeneous nature of the porous material greatly complicates the reaction problem. For this reason, a theory including mass problem. For this reason, a theory including mass transport, surface kinetics, and a statistical representation for the porous material is required to describe acid reaction. SPEJ P. 306

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