Application of citric acid for damage removal in both clastic and carbonate
rocks has historically not received widespread acceptance because of formation
damage concerns regarding precipitation of calcium citrate. In addition, usage
of citric acid in place of hydrochloric acid has been avoided because of lower
bulk carbonate dissolving power relative to hydrochloric acid. However,
environmental, safety, and corrosion concerns with using hydrochloric acid have
sparked renewed interest in citric acid for stimulation.
This paper reconsiders the use of citric acid as both an additive to
conventional hydrochloric acid systems and as a stand-alone acidizing fluid.
The paper reviews extensive laboratory work focused on understanding reaction
pathways and geochemical precipitation issues, and it takes a new look at
paradigms related to the calcium citrate issue.
Laboratory studies performed independently by two research laboratories
address two main areas of investigation: (1) Evaluation of potential
geochemical reaction paths and precipitation of calcium citrate and (2)
mechanisms for generation of preferential etching in two carbonate rock
lithotypes. The described mechanism for etching enhances conductivity in a
contrasting mode compared to hydrochloric acid.
Findings based on this work provide new insights into the applications,
usage, and potential limitations of citric acid. A newly proposed process for
downhole reaction is presented. This process differs greatly from long-held
paradigms built on a foundation of bench-top experimentation. Factors
controlling downhole reactions are disclosed, and the results of core testing
and fracture conductivity are presented.
Organic acids have been used to stimulate carbonate reservoirs. The two main
organic acids that are frequently used in the field are formic acid (HCOOH) and
acetic acid (CH 3 COOH). These acids are less reactive with carbonate rocks
than hydrochloric acid.
© 2005. Society of Petroleum Engineers
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