Reaction of Calcite With Surfactant-Based Acids

AbstractSurfactant-based acids have been extensively used in matrix and acid fracturing treatments in carbonate formations. As the acid spends and the pH rises, the surfactant molecules aggregate and form long entangled micelles, which enhance the apparent viscosity of the solution. The high viscosity is required for proper acid diversion, which is needed in horizontal and vertical wells with long target zones. Successful field data were obtained with surfactant-based acid, however the impact of the viscoelastic surfactant used on the reaction of the acid with the rock was not systematically examined before. Therefore the objective of the present work is to examine in detail the effect of a viscoelastic surfactant (amphoteric) on the reaction of HCl acid with calcite.The reaction between calcite and surfactant-based acids was thoroughly investigated using the rotating disk apparatus. The effects of disk rotational speed, surfactant concentration and temperature on the dissolution rate were determined. All experiments were conducted using 20 wt% HCl and various combinations of corrosion inhibitor and surfactant concentrations. The system pressure was maintained at 1,000 psi and the reaction was allowed to proceed for 20 minutes only. Samples from the reaction vessel were collected at various times and were analyzed for calcium using ICP. The apparent viscosity of surfactant-based acids was measured as a function of shear rate, temperature, and surfactant concentration using a HTHP Brookfield viscometer.The results obtained indicated that the viscoelastic surfactant significantly reduced the dissolution rate of calcite with HCl. This trend continued as the concentration of the surfactant was increased to 4 wt%. The effect of temperature on the dissolution rate was similar to that of temperature on the apparent viscosity of surfactant solutions where there was a maximum in viscosity at 50-60°C. The dissolution rate was controlled by mass transfer of H+ to the surface of calcite.