Control of acid fluid loss during fracture acidizing treatments is recognized as being a dominating limiting factor in the effective use of such treatments. This paper will discuss the importance of adequate acid viscosity, proper acid concentration, and proper particle size distribution for maximizing acid fluid loss control. The recommendations are based on laboratory testing which utilizes a severe acid fluid loss test. The test involves the use of a hollow core reactor under conditions of several hundred psi radial pressure drop across about a 5/8 inch thick wall. The pressure drop was from the inside of the limestone core to the outside. Parameters investigated included acid viscosity, acid concentration, and fluid loss additive particle size distribution. It was discovered that simply viscosifying the acid provided a remarkable improvement in acid fluid loss control. This enhancement was most pronounced in very low permeability limestone cores. The nature of the viscosifying agent was found to be a secondary effect with polymeric materials being more effective than surfactant type viscosifiers. A viscosity of 20 cp at 511/sec at the treating temperature was found to be an optimum value. It was also shown that the higher acid concentrations caused poorer fluid loss control. The effect of particle size distribution of a silica fluid loss additive was studied and indicated the particles with sizes larger than 80 mesh were key to successful acid fluid loss control. Silica flour was found to provide little enhancement of acid fluid loss control. The principles learned from the laboratory tests can be used to design improved fracture acidizing treatments. Indeed, these principles have been used successfully.

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