Acid-in-diesel emulsified acid has been used in the oilfield for many years. Emulsified acid systems are retarded systems that can be used effectively in stimulation of carbonate reservoirs. Emulsified acids have primarily been used 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 wormholes or the length obtained from an acid fracturing treatment, diffusion coefficient values need to be estimated.
This paper discusses the rheology, and reaction kinetics of emulsified acids formulated using a new emulsifier. The emulsified acid systems were prepared at 15 wt% HCl and 0.7 acid volume fraction. Emulsifier concentration was varied from 0.5 to 2.0 vol%. For all emulsions, viscosity was measured using an HPHT rheometer. Emulsified acid reaction rates, and hence acid diffusivity, were measured using a rotating disk apparatus at a temperature of 230 °F. Disk rotational speeds were varied from 100 to 1,500 rev/min. Samples of the reacted acid, from the reactor, were collected and analyzed using the Inductively Coupled Plasma, to measure calcium and magnesium concentrations.
Rheological measurements indicated that emulsified acid systems behaved as a non-Newtonian shear-thinning fluid, and this behavior can be represented by a power law model. The emulsifier concentration and temperature affect greatly the viscosity of emulsified acids. The new emulsified acid systems achieved low reaction and diffusion rates; as the emulsifier concentration increased, both reaction and diffusion rates decreased. Emulsified acid –dolomite reaction was mass transfer limited at low emulsifier concentration. The behavior was in the middle region between mass transfer limited and surface reaction limited for higher emulsifier concentrations. At high emulsifier concentration, the reaction appears to be surface reaction limited. These results can be used to optimize the design of carbonate acidizing treatments using emulsified acid.