The efficiency of the matrix acidizing process in carbonates depends strongly on the wormholing phenomenon—if wormholes are formed, the effects of near-wellbore damage can be overcome with relatively small volumes of acid. Numerous previous studies have shown that wormhole patterns can be placed in these general categories: compact dissolution in which most of the acid is spent near the rock face; the wormholing pattern; and uniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing. We have developed a theory of the wormholing process, which predicts when the wormholing pattern is most efficiently created as a function of the acid flux and other treatment variables.
By testing this theory with several independent sets of laboratory data, we can now demonstrate the important roles that surface reaction rate and fluid loss play in the wormholing process. This theory accurately predicts the optimal flux (that which leads to dominant wormholes with a minimum of branching, and hence, a minimum acid volume) for experiments with HCl in limestone and dolomite at several temperatures and with acetic acid in limestone. The surface reaction rate differs by several orders of magnitude in these experiments and is the only process variable that differs greatly among them. Paradoxically, though wormholes are formed because the overall reaction rate is controlled by mass transfer in the wormholes, diffusion rates play only a minor role in the wormholing process.
Fluid loss through the walls of the wormholes ultimately limits the distance to which wormholes can propagate. Because of this effect, laboratory linear corefloods will give optimistic predictions of wormhole penetration distances. We developed a cylindrical flow model to represent the flow field around a wormhole propagating from a wellbore which illustrates how to translate laboratory results to field conditions.
We have used these theories to predict optimal acid formulations and injection rates for field conditions. In general, the lower the reaction rate (such as at low temperatures in dolomites or with weak acids in limestone), the lower the injection rate required, making it easier to propagate dominant wormholes under matrix treating conditions in the field.