A high-pressure, high-temperature acid permeameter constructed to investigate the reaction of HCl-HF on sandstone cores at reservoir conditions is providing new insight into the reservoir and treatment variables that control the acidizing process. Introduction The literature has contained much discussion of both the theory and the practice of acid stimulation of oil-bearing formations. Of the hundreds of published papers dealing with the hydrochloric acid-limestone papers dealing with the hydrochloric acid-limestone reaction, a few notable ones are listed as Refs. 1 through 3. More recently in vogue is the matrix stimulation of sandstone reservoirs with hydrochloric and hydrofluoric (HCl-HF) acid mixtures. The reaction of HCl-HF on a sandstone matrix was studied in the laboratory by Smith and Hendrickson. Also, laboratory data were used to improve field techniques of matrix acidizing. Though these works contributed significantly to acidizing technology, there remain many unexplored facets of this complex stimulation mechanism. Our purpose here is to describe an apparatus developed to investigate the reactions of HCl-HF on the sandstone matrix at reservoir conditions and to discuss the application of experimental results to field treatment design. Description of Apparatus The high-pressure, high-temperature acid permeameter was designed to handle concentrated acid permeameter was designed to handle concentrated acid mixtures at temperatures to 250 degrees F. The working limits for overburden pressure and injection pressure are 10,000 psi and 4,000 psi respectively. The equipment was fully automated to reduce the hazards of handling hot HCl-HF under pressure and because of the long duration of some experiments. The corrosivity of the acids required that parts of the equipment be constructed of Hastelloy B. The apparatus was designed to have both research and technical service capabilities. Experimental versatility was incorported into the design. The apparatus can be programmed to perform a number of sequential steps (listed in Appendix A) -measure initial oil permeability at irreducible water saturation for the untreated core sample,measure damage of the core sample by drilling mud,evaluate the degree of damage by measuring return permeability to oil,evaluate treatment with acid, and finally,measure return permeability to oil following stimulation. Data are monitored automatically during an experiment and are processed and presented graphically by means of a high-speed digital computer and a plotting program. plotting program. The high-pressure, high-temperature acid permeameter shown in Fig. 1 is located in a special building permeameter shown in Fig. 1 is located in a special building that is divided into two sections, a control room containing low-pressure control and data-logging equipment and a room containing the high-pressure equipmentData that are logged automatically are processed by computer programs. One program plots the permeability data as a function of cumulative throughput permeability data as a function of cumulative throughput and labels the steps of interest. A typical plot is presented in Fig. 2. presented in Fig. 2. JPT P. 433