Wormholing is a common phenomenon in carbonate acidizing. These macro-sized channels, which are several orders of magnitude larger in diameter than natural pores, provide a magnified flow path, and hence enhance productivity. As the acid mainly attacks the larger pores, acid can penetrate much farther into the formation and bypass the formation damage rather than dissolve it. There is experimental evidence that wormholing on a smaller scale can occur in sandstone formations. The purpose of this study was to determine the treatment conditions and formation characteristics required for such wormholes to be created in sandstone through experimental and modeling work.

We have conducted extended experiments with samples of varying properties at different treatment conditions. Meanwhile, a theoretical model was built to simulate the process and visualize the acid etching pattern in the laboratory core samples. The findings from this study show that there are many parameters that affect the etching pattern. The most important factor that determines etching pattern is the heterogeneity of the rock sample. The spatial variation in permeability, porosity, and mineralogy are the driving force for the acid to channel through certain paths, developing highly permeable channels or wormholes in sandstone. The simulation results show that high permeability channels result when the initial porosity distribution is non-uniform, especially with existence of high porosity streaks. Such wormholes are not likely generated in homogenous rocks, even with an extremely high concentration HF acid solution at a high temperature. Uniform dissolution patterns are created in such samples as supported from our experimental work.

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