This paper discusses the development of a unique in-situ crosslinkable acid system that uses a blend of HCl/formic acid as the base acid and a synthetic polymer gelling agent. The ability to in-situ crosslink an organic acid blend is novel. In addition, an unexpected result of the fluid development was the discovery of its unique rheological properties.

Historically, both gelled and in-situ crosslinked acids have been used for fluid loss control during fracture acidizing and for diversion in matrix treatments in carbonate formations. Various synthetic polymers are used to gel the acid. Past research indicates that ~20 cp base gel viscosity is required as the first step in fluid loss control. In-situ crosslinking allows very high viscosities to be generated as the acid spends. The crosslinked gel creates a permeability barrier and subsequent fluid stages are diverted to other sections of the zone. When the acid fully spends, the gel breaks, giving a low viscosity fluid.

HCl is the most common base acid used for carbonate stimulation. Combinations of HCl and organic acids have been used because of their high dissolving power and relatively low rates of corrosion at elevated temperatures. In extreme cases, combinations of organic acids are used. While HCl/formic acid blends have been utilized in the past, the unique rheological properties of these blends have not been fully explored.

The chemistry and rheology of gelled and in-situ crosslinked HCl/formic acid blends equivalent to 28% HCl will be described and compared with traditional gelled acid and in-situ crosslinked acid.

You can access this article if you purchase or spend a download.