Reaction of Emulsified Acids With Dolomite

Abstract

Summary Emulsified acids have been used in the oil field for many years. They are retarded systems that can be used effectively in stimulation of carbonate reservoirs. Emulsified acids have been used primarily in acid fracturing and matrix acidizing. The delayed nature of emulsified acids is useful in generating longer etched fractures or deeper wormholes. To predict the penetration depth of the wormholes or the length obtained from an acid-fracturing treatment, diffusion-coefficient values need to be estimated. This paper discusses the reaction kinetics of dolomite disks with emulsified acids formulated using a cationic emulsifier. The emulsified acid systems were prepared using 15 wt% HCl and 0.7 acid volume fraction. The emulsifier concentration was varied from 0.5 to 2.0 vol%. Emulsified-acid reaction rates and, hence, acid diffusivity were measured using a rotating disk apparatus at 230°F. Disk-rotational speed was varied from 100 to 1,500 rev/min. Samples of the reacted acid were collected and analyzed using the inductively coupled plasma (ICP) mass spectrometry to measure calcium and magnesium concentrations. The dolomite core samples reacted slowly with emulsified acid. Emulsions with low emulsifier concentrations (0.5 vol%) had average droplet sizes of 8.118 lm and achieved a diffusion coefficient of 1.413x10-8 m2/s. While emulsions prepared with higher emulsifier concentration (2.0 vol%) had a smaller droplet size (2.82 lm), they achieved a diffusion coefficient of 8.367x10-10 cm2/s. Reaction of dolomite with emulsified acid at 230°F was found to be mass-transfer limited. Compared with calcite, the dissolution rate of dolomite in emulsified acids was lower by one order of magnitude, and the diffusion coefficient of acid was lower by two orders of magnitude.