A high-pressure, high-temperature acid permeameter was used in laboratory studies of sandstone formations to learn the characteristics of a formation that make it a good or bad stimulation candidate. A single correlation was developed to relate reservoir properties such as permeability, porosity, and mineral composition to the acid response expected for a given sandstone. Introduction Using hydrochloric-hydrofluoric acid (HCl-HF) mixtures to stimulate the matrix of sandstone formations is an important method for increasing the productivity of oil and gas wells. However, the choice of acid type, strength, and volume for a particular formation, or the decision of whether a formation would be a good candidate for matrix stimulation, is often based on guesswork or inflexible established practices. The situation would be greatly improved if the characteristics of a formation that make it a good or bad stimulation candidate were known. Matrix stimulation means the dissolution and removal or relocation of minerals comprising the original formation matrix. This is distinguished from stimulation of a formation that has been damaged by mud invasion or fines migration, in which case the acid reacts with the pore-plugging material as well as the rock matrix. The pore-plugging material as well as the rock matrix. The stimulation and the corresponding production increase of a damaged formation is generally greater than that in the undamaged case. However, the object of this study is to describe the response of the formation itself to acid treatment. Significant contributions to understanding the reactions of HCl-HF mixtures with sandstones have been made by earlier investigators. Smith and Hendrickson studied the reaction rates of HF with sand (represented by quartz) and with clay (represented by flat-drawn glass). Their results were later applied in the field for designing acid jobs., Gatewood et al. developed a method for designing acid stimulations based on rate studies of the reaction of HF with silica and selected silicates. The kinetics of the reaction of HCl-HF mixtures with the minerals found in sandstones have also been investigated. Farley et al described the design and use of a high-pressure, high-temperature acid permeameter for investigating, under flow conditions, the reaction of HCl-HF with sandstone cores at reservoir conditions. They also demonstrated the applicability of the permeameter-test results to acid jobs in the field. permeameter-test results to acid jobs in the field. Since that time, many other producing formations have been studied with the permeameter. These studies are a valuable source of data describing how sandstone formations respond to matrix acid stimulation. This paper demonstrates how those data were used to derive paper demonstrates how those data were used to derive generalized criteria for predicting the stimulation of a sandstone formation according to its properties. Testing Procedure The tests carried out in these studies are categorized as peripheral analyses and tests, and high pressure-temperature peripheral analyses and tests, and high pressure-temperature acid permeameter (HPTAP) tests. The peripheral analyses characterize the formation in terms of standardized laboratory methods. HPTAP tests describe the response of the formation to acid stimulation in the laboratory under simulated reservoir conditions of temperature and pressure. JPT P. 633