To understand the acid propagation and the dissolution pattern development in carbonate acidizing, one has to tackle with numerous problems. For instance, the length of the wormholes has to be determined, information has to be gathered on the shape of the wormholes, i.e. single versus multibranch wormhole. Finally, when using high velocity condition, the extension of the pitted zone should be determined.

A series of acidizing experiments have been conducted with X-Ray Computed Tomography visualization to study the local dissolution mechanisms in limestones on a millimeter scale during acid injection. The acid is injected at constant flow rate through the core. The pressure variation curve, monitored during the experiment is analysed in relation to the dissolution pattern followed as a function of time by the CT scannings. The two methods allowed an analysis of the development of the wormholes in space and time. For comparison purpose, an experiment with a tangential circulation of the acid on the face of the core, designed to mimic the acid leak-off in acid fracturing, is reported.

Influence of core length, acid interstitial velocities, heterogeneity and limestone nature are considered. CT observations are interpreted in terms of initiation time, wormhole and multiwormhole facies, depending on the conditions. Wormhole propagation has been measured. It is shown that, assuming infinite permeability within the region containing the wormhole, the wormhole length can be back calculated by measuring pressure drop across the core in a constant flow experiment – experimental validation of the effective length concept. For the flow conditions applied – i.e. effective flow velocities encountered in acid frac leak-off – branching occurs only at the tip of the current wormhole.

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