Unlike other acid systems, such as gelled and viscoelastic surfactant-based (VES) acids, where the mobility of hydrogen ion controls the overall rate of the reaction, emulsified acid/calcite reaction involves the transport of acid droplets in the diesel phase to the rock surface, breaking of acid droplets, and then the actual reaction on the surface. A limited number of papers have been published on the reaction kinetics of emulsified acid. However, none of the published work considered the effect of acid droplet size on the reaction of emulsified acid. The objective of this work is to examine the effect of the acid droplet size on the reaction rate of emulsified acid with calcite.
The acid was 15 wt% HCl emulsified in diesel with 70:30 acid/ diesel volume ratio. The emulsifier concentration was varied from 1 to 10 gpt. All emulsions were characterized by measuring the droplet size distribution, viscosity, and thermal stability. Diffusivi-ties were measured using the rotating disk device. The experiments were carried out at 25, 50, and 85°C, under 1,000 psi pressure, and disk rotational speeds from 100 to 1,000 rev/min. Samples of the reacting acid were collected and analyzed to measure calcium concentration in the reactor.
The effect of the acid droplet size on the overall reaction rate was significant. The diffusion rate of acid droplets to the surface of the disk was found to decrease with increasing emulsifier concentration because of higher viscosities and smaller droplet sizes. The effective diffusion coefficient of emulsified acid was found to increase linearly with the average droplet size. Emulsions with low emulsifier concentrations (1 gpt) had average droplet sizes of nearly 13 μm. These emulsions were found to have high effective diffusion coefficients (5.093 × 10-9 cm2/s) and low retardation. On the other hand, emulsions with high emulsifier concentrations (10 gpt) had smaller average droplet sizes (nearly 6 μm) and found to have low effective diffusion coefficients (4.905 × 10-11 cm2/s) and high retardations.
The new sets of data can be used to determine the optimum emulsified acid formulation to yield deeper acid penetration in the formation. It is suggested that droplet size can be adjusted to produce the desired diffusion coefficients for acid fracturing treatments.