A series of laboratory experiments were performed in which a 2 percent KCl solution was pumped through simulated sand-packed fractures between core halves representing three different tight reservoir formations. During these fluid injections, the core assemblies were subjected to typical overburden pressures ranging from 500 to 9,000 psig, and effluent samples of particle-liquid mixtures were collected for analysis with a particle-counting apparatus (1).- These analyses typically include the particle concentrations as well as the means, modes, medians, and size ranges of these mixtures. This data, except for the particle concentrations, was obtained in the form of particle-size distribution curves. The degree of plugging of similar types of hydrocarbon-producing formations experienced in the field can be anticipated and then reduced by proper engineering techniques.