Diverting mechanism of viscoelastic surfactant-based self-diverting acid and its simulation

• Diverting mechanism of VES acid in carbonate acidizing is successfully revealed.
• Sharp pressure drop at breakthrough is due to the uneven distribution of viscosity.
• dpmax/dp0 increasing with k0 is due to increase of spent acid zone and viscosity.
• Quantify the changes in viscosity of spent acid with pH and concentration of Ca2+.
• Successfully simulate the diverting behavior of VES acid with numerical model.

In carbonate acidizing, acid selectively flows into the high permeability zones to dissolve the rock, leaving the low permeability zones untreated. Many techniques including mechanical ones and chemical ones are found to do their best to avoid the uneven distribution of acid. Among these techniques, injecting the viscoelastic surfactant-based self-diverting acid (VES acid) is an effective method to divert acid to other low permeability zones. The VES acid is able to be self-viscosified with its consumption through the gelling of the surfactant. Since the high permeability zone receives more acid, the gelling process occurs first in the zone so that the acid is diverted to other low permeability zones. However, to the best of our knowledge in the open literature, the diverting mechanism of the VES acid has not been understood in spite of many successful applications in oil and gas fields. In this paper, we thoroughly study the diverting mechanism through the analyses of the former experimental results and our own experimental observations. According to the diverting mechanism, a new model which describes the diverting behavior of the VES acid is developed. The results show that the diverting mechanism is dependent on both the spent acid zone and the viscosity of the spent acid. The spent acid zone and the viscosity of the spent acid keep increasing with the injection of VES acid, which leads to the continuous increase of the pressure difference. However, the spent acid zone is thinner for low permeability zone than for high permeability zone. As a result, the maximum pressure ratio follows a linear increase relationship with the permeability in the log–log coordinate system. In addition, the non-uniformly distributed viscosity of the spent acid leads to the sharp drop of the pressure difference in the vicinity of breakthrough. The new model couples the two-scale continuum model describing the conventional acidizing with three equations describing the concentration of Ca ions, viscosifying behavior and reaction rate. Through a series of simulations, the results are in good agreement with the experimental ones, which means that the new model captures the diverting mechanism of the VES acid.