Quantitative Analysis of Reaction-Rate Retardation in Surfactant-Based Acids

Abstract

Fracture acidizing has been a dominant practice in the industry to enhance well productivity in low-permeability carbonate reservoirs. Many acid systems have been developed to improve this stimulation process. The most desirable characteristics for an acid system to be suitable for fracture acidizing are leakoff control and retarded reaction rate. These characteristics are required for deep acid penetration, so that when the fracture closes, long flow channels are etched on the fracture surfaces. Leakoff control can be achieved by pumping a pad containing a viscosifying agent or solid bridging agents to plug wormholes generated by acid dissolution. Reaction retardation is attempted usually by lowering the effective diffusivity of the hydrogen ion.It is well known that during an acid-fracturing operation, the overall reaction rate of hydrochloric acid (HCl) with limestone is mass-transfer-limited. Designing the treatment requires knowing the effective diffusivity of the hydrogen ion in the acid system, which, to the best of the authors' knowledge, has not been determined before. Because of their combined leakoff-control and retardation capabilities, surfactant-based acids have been used in acid-fracturing treatments. Because more carbonate reservoirs are treated by use of this acid system, it is important to obtain the effective diffusivity of H + .The rotating-disk device has been used to investigate the reaction kinetics between a reactive solution and carbonate rocks because the thickness of the boundary layer is uniform throughout the disk surface. This paper discusses the reaction-rate data generated recently for surfactant-based acid by use of a rotating-disk apparatus and presents the methodology used to determine the effective diffusivity from the measurements.The results obtained indicated that the viscoelastic surfactant examined (carboxybetaine-type) reduced the dissolution rate of calcite with HCl acid. The surfactant reduced the diffusion coefficient for H + . The effect of temperature on the diffusion coefficient did not follow the Arrhenius law.

Diffusivity of Citric Acid During its Reaction With Calcite

“…However, calcium citrate precipitation at low-initial citric acid concentrations (1 wt% and 2 wt%) was relatively minimal when compared to the precipitation at high-initial citric acid concentrations, (5 wt% and 7.5 wt%), which was evident from the amount of calcium citrate found in the solids collected by scraping the surface of the reacted calcite disk. Because of calcium citrate precipitation, it was expected that the pH value of the bulk solution would be above 3.0, based on Equations (5) - (7). However, the maximum increase in the pH value of the bulk solution was measured at 7.5 w% at 50°C, where the pH increased from 1.8 -2.3 because of the reaction, but no precipitate was noted in the solution.…”

“…With the exception of the citric acid system, these acids have been extensively studied and successfully employed in acid fracturing treatments (6)(7)(8)(9)(10)(11)(12)(13) .…”

A Procedure for Measuring Contact Angles When Surfactants Reduce the Interfacial Tension and Cause Oil Droplets to Spread

“…A US patent and the rheological properties for the VES were described by Caweizel (2007) and Li et al (2010Li et al ( , 2011, respectively. Field applications were described by Nasr-El-Din et al (2006a, 2006b, 2006c.…”

“…Laboratory and field studies suggest that significant amounts of VES are retained in the reservoir (Nasr-El-Din et al 2006a;Crews and Huang 2007;Huang and Crews 2008;Yu et al 2011). Retained VES can affect wettability and reservoir performance by affecting the displacement of spent acid or the production of hydrocarbons (Jennings 1975;Hinkel et al 2003).…”

Application of Citric Acid in Acid Stimulation Treatments

“…Except of citric acid system; these acids have been extensively studied and successfully employed in acid fracturing treatments (6)(7)(8)(9)(10).…”