Abstract：
Acidizing is often used in naturally fractured vuggy carbonate reservoirs in the Tarim Basin to remove damage near
the wellbore caused by drilling and improve the productivity. The stimulation performance depends on acid flow, acid-rock
interaction, and rock dissolution in the reservoir. But the acid-rock interaction in naturally fractured vuggy carbonate reservoirs
has not been systematically studied. Aiming at this problem, this article uses numerical modeling on acidizing naturally fractured
vuggy carbonate reservoirs. Firstly, we use the method of sequential Gaussian simulation to generate a spatial distribution of
matrix and vugs. Then we use a probability distribution function to generate geo-statistical spatial distribution of natural fractures.
Combining the three distributions above generates a spatial distribution of pores, vugs, and natural fractures. Then, a mathematical
model of acid flow, acid-rock interaction and pore evolution is built by coupling a two-scale wormhole model with spatial distri
butions of pores, vugs, and natural fractures. Next the numerical model is developed. Based on the model, we conduct numerical
simulation of acidizing and analyze the effects of vugs and natural fractures on the wormhole pattern as well as acid penetration
distance. The results show that the natural fractures have a dominant effect on acid flow. The acid flows along some natural fractures,
and the acid flows fast in the fractures so that the acid can reach a longer distance. Since the porosity and permeability in the vugs
are high, when meeting a vug, the acid spreads out in the vug and forms a large area dissolution, which reduces the advance speed
of the acid. The dissolution pattern in the vugs is significantly different from the thin wormholes formed in the natural fractures or
matrix. Since the porosity and permeability of the matrix are much lower than those of natural fractures and vugs, little acid enters
the matrix. The distribution of natural fractures has a great influence on acid penetration distance, and the predicted distance is up to
25m under the simulated conditions. The model established in this paper provides an simulation tool for the acidizing of naturally
fractured vuggy reservoirs and a theoretical basis for the design of acidizing schemes.
