Designing effective sandstone acidizing treatments through geochemical modeling

This work reports on the application of a complex kinetic geochemical model to explore key issues in sandstone acidizing.(1) The issues include the im portance of the minerals initially present, the consequences of mineral pre cipitation, and the effect of acid formulation and injection rate. The mode l's capabilities exceed those of its predecessors that have greatly simplif ied the reaction chemistry or have ignored kinetics altogether. No assumpti ons were made as to the nature of precipitation that could ensue; all poten tial precipitates were considered. An important advance is the use of a new permeability prediction model take n from the work of Panda and Lake.(2) This model relates the permeability o f a permeable medium to the porosity, grain size distribution and the amoun ts and identities of all detrital minerals present. Coupled with the reacti on chemistry, this model allows predictions that are relevant for specific treatments. The use of this model also gives quite realistic productivity i mprovement predictions. We study three cases of generic mineral assemblages: high quartz, high clay , and high feldspar. Model results indicate that the precipitation of react ion products is both inevitable and substantial. Hence, the optimal matrix stimulation is a compromise between maximizing the dissolution of the damag ing minerals and minimizing secondary precipitation. Given such competitive effects, our results indicate that high rate stimulations tend to give the largest productivity improvements even though these usually do not cause t he largest removal of damaging minerals.