No abstract

Degradable fiber is widely used to assist gelled acid diversion and reduce acid leak-off in the acid stimulation of carbonate hydrocarbon reservoirs in Sichuan Basin of China. The rheological properties of an acid system will affect the geometry of the acid-etching fracture. However, the effect of fiber on the rheological properties of gelled acid is not yet clear. This paper investigates the rheological properties of gelled acid with various fiber concentrations at different temperatures. The results show that when the temperature is less than the degradable temperature of the fiber, the apparent viscosity of gelled acid rises gradually with an increase in fiber concentration, while the fiber has no significant effect on the viscosity of gelled acid at the degradable temperature. The dissolution process of fiber in gelled acid experiences none-dissolution, surface dissolution, dissolution and fining, and a complete dissolution stage from low to high temperatures, which all have different effects on gelled acid viscosity. The fiber links more gelling agent molecules of gelled acid together to form a quasi-network structure between the fiber and fiber and the fiber and polymer, which results in a rise in the viscosity of gelled acid. The acid system also shows a strong shear thinning property under different temperatures and fiber concentrations. However, the power-law index n of this acid system always maintains a steady average value of about 0.181, while the change pattern of consistency index K is similar to the change in viscosity with varying fiber concentrations and temperatures. The research results are useful for acid fracturing treatment design in carbonate reservoir.

Laboratory measurement of acid fracture conductivity is the most direct method for acid system selection during acid fracturing in carbonate reservoirs. However, the fracture in the laboratory test is too short compared with the in-situ fracture. Thus, the acid concentration and viscosity variation and their impacts on fracture morphology and conductivity along the fracture are seldom considered, leading to inaccurate estimation of conductivity and providing uncertainty in acid selection, especially for ultra-deep carbonate reservoirs with high temperatures. To fill this knowledge gap, a series of acid-etching experiments were performed utilizing the viscoelastic surfactant-based (VES) acid and gelled acid. Fresh acid with 20 wt % and spent acid with 15 wt % concentration were used to represent the acid at the fracture-wellbore interface and that in the inner fracture. A 3D laser scanner was applied to collect fracture morphology data, which were further used to build the 2D fracture-surface roughness model via the fractal theory. Based on the model, the distribution of acid-fracture width and its roughness were exhibited, while the conductivity was tested under different closure stress. Results indicate that in comparison with gelled acid, the fresh VES acid with low viscosity exhibits strong corrosion of rock and results in the heaviest dissolved-rock weight and the maximum acid-fracture surface roughness, but the situation is just the opposite for spent acid. Conductivity shows obvious segmentation characteristics because the acid fracture narrows with the increase of closure stress. Three contact modes of acid-fracture surface cause the loss of conductivity, including longitudinal strip-like, transverse strip-like, and longitudinal and transverse flaky modes. The VES acid is selected in the YSW block based on the experimental results. The field application proves that VES-acid hybrid fracturing could result in a large increase in well production. This study gives a new horizon to selecting reasonable acid systems by testing acid-fracture conductivity.