The effect of phase saturation conditions on wormhole propagation in carbonate acidizing

Matrix acidizing is a commonly used well stimulation technique in which acid is injected into the formation in order to dissolve a portion of the rock and therefore recover or enhance the permeability in the near-wellbore region. In carbonates, when acid is injected, selective dissolution of the rock takes place, creating highly permeable flow channels, called wormholes. In general, for a matrix acidizing treatment in carbonates, the deeper the wormholes penetrate into the formation, the better the outcome, as characterized by a lower skin factor. Thus, the deeper the wormhole penetration achieved with a given volume of acid, the more efficient the treatment. In this paper, we present laboratory results that show how wormhole propagation is affected by the presence of immiscible phases (gas or oil) when the acid is injected into the rock. As wormholes are created, not all of the acid reaches the wormhole tip, with a significant portion of the acid being lost as fluid loss to the surrounding matrix. If this fluid loss can be controlled, then more of the acid can reach the wormhole tip, and therefore the penetration of wormholes can be increased. This paper investigates the effect of having gas or oil saturation in the carbonate rock prior to the injection of acid. By reducing the relative permeability to the acid in the matrix surrounding the wormhole, the presence of an immiscible phase can reduce the fluid loss from the main wormhole, thus allowing for deeper penetration of wormholes with a given acid volume. It was found that gas injection prior to acid injection does significantly reduce the volume of acid required to propagate wormholes through cores. This effect is observed both at room temperature and at higher temperatures. The presence of gas in the core reduced the acid volume needed for wormhole propagation through the core by a factor of up to 3. The paper presents an extensive set of experimental results for both gas- and liquid-saturated carbonate cores subsequently treated with strong HCl solutions. We show that the acid volumes are generally smaller for the nitrogen-saturated cores, and that the wormholes created are narrower and less branched than in the case of water-saturated rocks. We also show that the presence of oil saturation at residual water saturation in the core has a similar beneficial effect on wormhole propagation to gas injection. On the other hand, oil present at its residual saturation had little effect on the acidizing process. All of these results confirm the strong influence that fluid loss from a propagating wormhole has on the efficiency of the acidizing process in carbonates.