New look at sandstone acidizing

The acid mutual solvent (AMS) technique is a 3-step process which involves a preflush, a mixed HF-HCl stage, and an afterflush employing the mutual solvent. The preflush is normally regular hydrochloric acid (15% HCl). This step is designed to serve as a buffer between formation water and hydrofluoric acid. Normally an adequate preflush is 50 gal of regular acid per ft of perforated interval. The mud-acid stage commonly consists of a mixture 3% HF and 12% HCl, although other concentrations may be used. Unfortunately this acid formulation is capable of producing by-products which as insoluble residues, reduce formation permeability or alter wettability in a way that lower relative permeability to oil. Effective treatments are sometimes conducted with as little as 10 to 20 gal of mud acid per ft of perforated interval. Third treatment stage, the afterflush, is composed of diesel oil containing 10% or more of a mutual solvent. A solvent of particular interest is ethylene glycol monobutyl ether (EGMBE). The EGMBE appears to improve cleanup to such an extent that an appreciable increase in well productivity is noticed. Tabular data show comparisons of multiple field treatments employing mud acid with and without the mutual solvent in the afterflush.