Naim A. Mumallah

Hydrochloric acid diffusion coefficients at acid-fracturing conditions

Abstract The reaction of a 15% HCl solution with Indiana Limestone was investigated at 93 °C and 34.5 MPa using an annular flow reactor that was erected vertically. The laminar flow conditions were such that both free and forced convections contributed to the overall mass-transfer rate. The experimental results correlated well with theory when the forced and free convections were in the same direction (assisting flow) and when free-convection mass transfer dominated the mass-transfer process. Deviation from theory, however, occurred when free convection was in the opposite direction to forced convection (opposing flow). Correlating the experimental results with theory provided a method for the determination of the effective diffusion coefficient of HCl at the high temperature, pressure and acid concentrations encountered in acid-fracturing treatments. An HCl effective diffusion coefficient of 5.25 × 10 −5 cm 2 /s was found from correlating the present experimental results with theory. Recent trend in designing acid-fracturing treatments is to use computer models to predict and optimize the etched length of the created fracture. The accuracy and reliability of such computer model predictions depend heavily on the accuracy of the values used for physical parameters such as reaction rate and diffusion coefficient of HCl. The annular flow reactor and the correlations discussed here provide a way to obtain reaction rates and diffusion coefficients at practical conditions for use in computer models and design of acid-fracturing treatments.

Reaction rates of hydrochloric acid with chalks

Abstract The reaction rate of hydrochloric acid with carbonate rock at acid-fracturing conditions depends on controllable variables such as injection rate and acid concentration, and variables inherent to the carbonate rock (permeability, mineral composition, etc.). In a previous study, the effects of some controllable variables on reaction rates were investigated and the results correlated with theory. This paper presents the results of an investigation of the influences of four rock properties (porosity, permeability, hardness and acid solubility) on reaction rates. Samples of chalk were selected from five North Sea reservoirs. Due care was exercised to insure that the selected samples spanned a wide range of each of the four rock properties investigated, in order to generalize the findings so that they will apply to a variety of conditions. Whenever possible, the four properties were determined for each sample prior to acidification. All reaction rate experiments were performed using an annular flow reactor under the same constant experimental conditions of 93°C, 34.5 MPa, and a flow rate of about 87 mlxa0min −1 . The results revealed that reaction rate is positively correlated with both carbonate content of the chalk and core permeability. The reaction rate also appeared to increase as the porosity increased, and to decrease as the hardness increased. The data, however, scattered so widely that the relationship between reaction rate and porosity or Brinell Hardness Number (BHN) did not appear to be statistically significant. The effects of permeability and acid solubility were incorporated into an overall correlation that combines the effects of controllable and uncontrollable variables.

Effective HCl Diffusion Coefficients From Correlations of HCl-Limestone Reactions

Hydrochloric acid solutions were reacted with limestone core plugs using an annular flow reactor at acid-fracturing conditions. The obtained reaction rates were correlated with theory using a correlation that was recently verified to be suitable for correlating HCI reaction rate with mass transfer parameters. Applying the correlation to the present data produced effective diffusion coefficients for HCl from the different acid solutions at acid-fracturing conditions. The results showed that the diffusion coefficient of an acid solution made in distilled water was larger than the diffusion coefficient of an acid solution made in synthetic North Sea water. The diffusion coefficient of a retarded acid was found to be an order of magnitude smaller than the diffusion coefficient of neat acid.