In-situ gelled acids with Zr4+cross-linkers utilize a solid breaker to reduce the viscosity of the gel at pH range of 4–5. A literature survey confirmed that solid breakers caused a premature reduction in the fluid viscosity resulting in a less than desirable productivity. Therefore, an effective liquid breaker that is based on tetrafluoroboric acid was developed. As pH increases, the liquid breaker releases fluoride ions, which will combine with the cross-linker, Zr4+. This study was conducted to evaluate this new breaker system under the following conditions: breaker concentration (0–200 ppm), and acid injection rate (0.5–10 cm3/min).
The effect of the new breaker on the rheological properties of in-situ gelled acid was examined using a HPHT rotational viscometer. A coreflood study was conducted to follow the propagation of acid and cross-linker inside the limestone core. Samples of the core effluent were collected and the concentrations of calcium, zirconium, and acid were measured. Permeability enhancement and location of the residual gel were determined using CT scans.
The pH profile during neutralization confirmed that the crosslinking of the polymer occurred at a pH value of 1.8. At a pH of less than 2, doubling the breaker concentration did not affect the viscosity of the acid. However at a pH of greater than 2, the viscosity of acid was reduced by 30%. At a breaker concentration of 0 ppm, the appearance of Zr in the core effluent sample was delayed by 0.25 PV compared to the reaction product, while at 100 ppm Zr was delayed by 0.75 PV. At 200 ppm breaker, no Zr ions were detected in the effluent samples. As the breaker concentration increased, more Zr remained inside the core, as ZrF4, which is water-insoluble. Increasing the breaker concentration from 100 to 200 ppm reduced the final normalized pressure drop by 50% at injection rate of 2.5cm3/min. Permeability reduction due to gel was reduced by increasing the acid injection rate. Therefore, the acid solution with the liquid breaker should be injected at high injection rates.