Measuring Diffusion Coefficients in Acid Fracturing Fluids and Their Application to Gelled and Emulsified Acids

Abstract

ABSTRACT To predict the length obtained from an acid fracturing treatment, acid fracturing models require that diffusion coefficients values be included in the equations describing the transport of the acid to the surface of the rock. Numerous data have already been published for hydrochloric acid, but very often these data were not generated under conditions representative of the field. For instance, values for diffusion coefficients in straight acid are only available at low normality. In reality, acid fracturing models should use diffusion values for spent acid at a higher concentration. Gelled and emulsified acids are also widely used in acid fracturing treatments leading to increased fracture length and conductivity due to decreased leakoff and a "retardation effect." Unfortunately, very few diffusion data are available for these fluids. The diffusion coefficient of an ion in a fluid is a physical property which can be obtained from direct measurement. This paper presents measured diffusion coefficients for straight and gelled acids using both a diaphragm diffusion cell and a rotating disk apparatus. In addition, diffusion coefficients for emulsified acids, as obtained from the rotating disk apparatus, are also presented.