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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.
Gou et al. (Thu,) studied this question.