# A New Compound Staged Gelling Acid Fracturing Method for Ultra-Deep Horizontal Wells
## 2 Results and Discussion
### Acid Fracturing Technology for the Ultra-Deep Reservoir
It can be seen from Figure 13 that the width of the acid  
Figure 10: Acid corrosion fracture wall after injecting acid with different viscosities. (**a**) Viscosity ratio 3:1; and (**b**) Viscosity ratio 8:1. Figure 9: Fingering form of the acid solution under different viscosity ratios (red represents gelling acid and blue represents crosslinked authigenic acid). (**a**) Viscosity ratio 1:1; (**b**) Viscosity ratio 3:1; (**c**) Viscosity ratio 8:1; and (**d**) Viscosity ratio 12:1.  
corrosion fracture changes greatly after a three-stage alternating injection of acid fracturing. The multiple alternating injection of the crosslinked authigenic acid and gelling acid intensifies the interface effect between liquids and promotes the generation of the acid fingering phenomenon. Under the condition of keeping the injection displacement, injection stages, and total amount of liquid unchanged, the fracturing software was used to simulate the acid fracturing fracture extension under different liquid injection ratios. It can be seen from Figure 13 that, with the decrease in the amount of crosslinked autogenous acid, the length of acid  
Figure 11: Acid corrosion fracture parameters under different injection series. Figure 12: Simulation diagram of the acid corrosion fracture width under the three-stage alternating injection. (Red represents fractures with a width of 7–9 mm, yellow represents fractures with a width of 5–7 mm, and green represents fractures with a width of 3–5 mm). fracturing fracture decreases, and the conductivity of acid etching fracture increases steadily. When the volume ratio of the crosslinked authigenic acid and gelling acid is 1:3, the length of acid etching fracture is close to 100 m, and the dimensionless conductivity of fracture is close to the maximum. The acid fracturing design of the Sichuan carbonate gas reservoir takes into account the two parameters of fracture length and conductivity, and the volume ratio of the injected crosslinked authigenic acid and gelling acid is preferably 1:3. As can be seen from Figure 14, when the injection volume ratio of the crosslinked autogenous acid and gelling acid is 1:3, the acid liquid creates non-uniform corrosion on the rock wall, the acid liquid creates a groove corrosion with a certain height difference on the rock wall, and the non-corrosion rock creates a support resembling a pier, which improves the effectiveness of the crack under high closure stress. Acid strength refers to the amount of acid used per meter of the reservoir during acid fracturing. The acid corrosion fracture length and dimensionless conductivity when the acid strength is from 0.7 to 1.9 m\({}^{3}\)/m were simulated by fracturing software. It can be seen  
Figure 14: Wall morphology of the acid corrosion fracture. Figure 13: Acid corrosion fracture parameters under different liquid injection ratios. from Figure 15 that, with the increase in acid strength, the acid corrosion fracture length and dimensionless conductivity increase rapidly because increasing the amount of acid can ensure that more acid liquid reacts with the rock, so as to improve the acid fracturing effect. When the acid strength increases to 1.6 m\({}^{3}\)/m, the growth rate of the acid corrosion fracture length and dimensionless conductivity slows down. When the acid strength is increased to 2.0 m\({}^{3}\)/m, the length of acid corrosion fracture is 2.8 m longer than that of 1.6 m\({}^{3}\)/m. The optimal acid strength of the Sichuan carbonate gas reservoir is 1.6 m\({}^{3}\)/m, which can not only ensure a better fracture length and conductivity, but also reduce the excessive consumption of acid liquid and improve the economy of the acid fracturing stimulation.