A placement model for matrix acidizing of vertically extensive, heterogeneous gas reservoirs Conference Paper uri icon

abstract

  • 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 of 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 the 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 wormholes 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 on the injection pressure response, operators may incorrectly conclude that the injected acid is not stimulating the formation. Copyright 2009. Society of Petroleum Engineers.

published proceedings

  • Proceedings - SPE Annual Technical Conference and Exhibition

author list (cited authors)

  • Nozaki, M., & Hill, A. D.

complete list of authors

  • Nozaki, M||Hill, AD

publication date

  • December 2009