Application of citric acid for damage removal in both clastic and carbonate rocks has historically not received widespread acceptance due to formation damage concerns regarding precipitation of calcium citrate. In addition, usage of citric acid in place of hydrochloric acid has been avoided due to 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 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 differential 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.