FOREWORD a. The surfactant evaluation tests were compiled by a task group of the API Southern District Study Committee on Well Stimulation with Surface-active Agents@ of the Committees on Drilling and Production Practice@ membership of which is given on the preceding page. As defined by the committee@ oil-well conditions which may be susceptible to surfactant treatment are the following: A. Fluid block@ which may be corrected by: 1. Reduction in water saturation. 2. Increased deformability (lower interfacial tension) of blocking droplets. 3. Reduction in viscosity or breaking of emulsions. 4. Alteration in wettability to redistribute fluid phases. B. Particle block (particularly clays)@ which may be corrected by: 1. Reduction in amount of water associated with the particle. 2. Removal or redistribution of the particles. b. The methods herein described are tentative@ pending further evaluation and improvement. They represent procedures currently used in the laboratories of several major oil and service companies@ and range in character from complex laboratory installations to simple qualitative tests capable of application in the oil field. It is conceded that information is generally lacking in the area of well stimulation@ particularly as regards subsurface conditions. Accordingly@ these tests are not represented as absolute methods for determining well-stimulation procedure. Rather@ they are to be considered as an aid in comparing the many products currently available for well stimulation.