A placement model for matrix acidizing of vertically extensive, heterogeneous gas reservoirs
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A critical issue in matrix acidizing of vertically extensive carbonate reservoirs is the acid distribution along the wellbore. This estimation is very important, especially for the case where the reservoir properties (permeability and damage distribution) vary along the wellbore. Several acid-placement models for oil reservoirs have been developed and applied in the field successfully. However, when evaluating acidizing in a gas well, the models are not adequate because of the viscosity contrast between the reservoir fluid (gas) and the injected fluid (acid) and because of relative permeability effects. We have developed an acid-placement model for acid injection into vertically extensive gas wells in heterogeneous carbonate reservoirs that includes gravity segregation in the wellbore, viscosity contrast, relative permeability effects, and a wormhole model. The effects of all these factors on the local injectivity during acid injection are incorporated into a dynamic-skin-factor model. To test this new model, we analyzed coreflooding data in the literature. The analysis showed that differential pressure across a gas-saturated core did not linearly drop during acidizing, but instead increased during a large part of the acid-injection period. From this analysis, we determined that the pressure drop in a spent-acid zone ahead of the wormholes dominates the overall pressure-drop behavior. Applying this model to typical field conditions, we find that a natural viscous diversion takes place in heterogeneous gas reservoirs because of the viscosity contrast between the gas and the injected acid. In particular, we have modeled thick carbonate gas reservoirs having a few thin, very-high-permeability thief zones. Before worm-holes break through the damage zone in the high-permeability parts of the reservoir, this viscous-diversion effect distributes acid more uniformly than expected. However, after wormholes break through the damaged zone, thief zones will take a large portion of the injected acid. Another important prediction with this model is that the injection pressure often increases during acid injection into gas wells. Without an understanding of the role of mobility contrast in the injection-pressure response, operators may conclude incorrectly that the injected acid is not stimulating the formation. Copyright 2010 Society of Petroleum Engineers.
