The dissolution of calcite in hydrochloric acid was studied with the aid of a rotating disk system at 800 psig in the temperature range-156-25°C. At 25°C the dissolution process is mass transfer limited even at high disk rotation speeds whereas at-156°C both mass transfer and surface reaction rates limit the dissolution rate. The multicomponent coupled ionic diffusive fluxes of reactants and products were defined by using the gradient of the electrochemical potentials as driving forces for the diffusion. The activity coefficients used in calculating the multicomponent diffusivities of the diffusing species were estimated by Hamed's rule. The concentration protiles of the ions in the boundary layer were then determined by numerically integrating the system of coupled convective diffusion equations. The effects of variable density, viscosity, and high mass fluxes on the fluid velocity in the boundary layer were taken into account. The rate of the surface reaction was found to be proportional to the 0.63 power of the surface hydrochloric acid concentration. Analysis of the experiments suggests that the absorption of hydrogen ion (described by a Freundlich adsorption isotherm) on the solid calcite surface and subsequent reaction of the adsorbed hydrogen ion with the solid calcite matrix is the reaction mechanism. lNTRODUC'IlON

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The experimental variables that atfect the acidixation of sandstone cores in a permeameter are discussed. It was found that as HCl/HF acid mixtures are injected into porous sandstone cores a reaction front between selective minerals and the acid is formed. This reaction front and a corresponding permeability front move through the core with a constant axial velocity. The time for the permeability front to move through the core is detined as the breakthrough time. The breakthrough time is directly proportional to the core length but inversely proportional to the HP acid concentration and the rate of injection, 0 dimensionless time, t/T p fluid viscosity, centipoise 4 porosity v stoichiometric coefficient 7 space time (time to fill one pore volume of core), min RFJEIWNCES

An extensive theoretical and experimental analysis of the acidization of linear sandstone cores has been carried out. The relative rates of reaction of common minerals found in sandstone, which were determined from rotating-disk-experiments, are presented and discussed along with the underlying presented and discussed along with the underlying reaction mechanisms. A lumped-parameter mathematical model developed from this information predicts the movements of acid and permeability predicts the movements of acid and permeability changes through the porous sandstone cores as a function of acid concentration and flow rate. Two acid core flood experiments are needed to determine the characteristic parameters of the model (the Damkohler number and the acid capacity number). However, this paper also shows how, in the absence of these experiments, one can extend the model to make predictive calculations for various sandstones from a porosity measurement and an X-ray analysis of the sandstone. In carrying out this latter analysis, a comparison of the experimental results of other investigators with the model shows the theory and experiment to be in excellent agreement. Introduction The stimulation of petroleum wells by acid injection is accomplished by selective dissolution of part of the formation rock to reduce the resistance to fluid flow in the vicinity of the wellbore. Because of the radial flow geometry, the productivity of the well may be increased greatly. Field and laboratory data have been gathered through years of applying the process to allow the design of an acid treatment for a particular formation. The complexity of the porous media and the reactions occurring during the acidization have made it difficult to predict a priori the exact results of the stimulation. A large step forward in the technology was made by the introduction of permeameters to permit the testing of cores at permeameters to permit the testing of cores at conditions similar to those existing in the reservoir. A series of experiments then can be carried out using different acid strengths, volumes, and flow rates to determine the best conditions for acidization. In recent years, the acidization process has been investigated in detail experimentally and theoretically. This has led to a better understanding of acidization and the development of mathematical models of the process. The application of a model allows a successful acidization treatment to be designed on the basis of few or no laboratory experiments. Even though differences exist in the manner by which the treatment is carried out in practice, the basic processes occurring during practice, the basic processes occurring during acidization are the same: homogeneous reactions in the fluid phase, heterogeneous reactions between the fluid and the solid phase, mass transfer, fluid flow, and changes in the pore structure of the porous media. porous media. This paper is primarily concerned with matrix acidization of sandstones, but the results will be of interest for other forms of acidization. Although emphasis is placed on the results of studies by the authors, it is seen that by comparing these results with the work of other investigators a uniform theory of acidization may be proposed. HOMOGENEOUS REACTIONS In the acidization of sandstones, and also of carbonates, a large number of product species are released to the acid solution. Since these may react with each other and with the acid, it is necessary to understand the equilibria between the different chemical species to determine the reaction stoichiometry. Carbonic acid formed by the dissolution of carbonates may participate in several reactions. However, since strong acids or acid mixtures are generally used, these side reactions usually may be neglected. SPEJ P. 248