Abstract

Geological heterogeneities such as polymineralic assemblages may be important in the acidization of the Halfway Formation which contains both authigenic anhydrite and dolomite. It has been previously reported that anhydrite in the presence of carbonate may initially dissolve in HCl but subsequently reprecipitate in pore throats thereby diminishing the effectiveness of the acidization. Experimental and theoretical methods can be used to evaluate the potential significance of this problem in the Halfway Formation.

Preliminary simulations of Halfway Formation acidization using the coupled reaction-transport geochemical computer program, RFACTRAN, suggest that anhydrite precipitation may be a significant problem. This is consistent with previous acidization experiments in which permeability is reported to have decreased despite extensive carbonate dissolution. Experiments performed in the current study, however, fail to exhibit the suspected anhydrite precipitation.

The disparity between simulation and experimental results is resolved with more accurate input to the numerical model. In this case, a petrographic examination of the Halfway Formation reveals a heterogeneity in grain size between anhydrite and the carbonate which when input to REACTRAN results in a prediction of much less significant anhydrite precipitation than in the previous simulations. Though anhydrite precipitation may theoretically be a problem in some situations due to heterogeneous mineral assemblages it appears that it may be diminished in importance in the Halfway Formation due to grain size heterogeneity.

The use of numerical models such as REACTRAN can be helpful in investigating fluid-rock interactions such as occur in some formation treatments but the results should be periodically tested by some means such as experimentation.