Morphological changes of a lime reservoir pore structure that occurred during flooding with HCl solution (10 wt.%) are studied. A new method of measuring the lateral area of a wormhole is proposed for calculating the Damkohler number and consists of segmenting its boundaries into a digital model, calculating the surface area of resulting complicated shape of the flooding channel, and subtracting the area of its ends. The morphological sizes of the pore-channel structures through which the wormhole is subsequently formed differ considerably from the starting pore system by a simpler structure, greater connectivity, less tortuosity of the pore space, and larger pore channels, which are responsible for the initially high permeability. Pores in the starting sample are formed by contact of angular clastic grains. The surface of pore channels is uneven with many outcropping grains. The channels themselves are highly tortuous. After acid flooding, the channel surface evens out and the pore space becomes a dendritic structure of interconnecting tubes.