Acidizing Sandstone Reservoirs Using HF and Formic Acids

Abstract

Mud acid, which is composed of HCl and HF, is commonly used to remove the formation damage in sandstone reservoirs. However, many problems are associated with HCl acid, especially at high temperatures. In this study, formic acid was used to remove carbonate minerals as a preflush and with the main HF stage. A series of formic acid and HF mixtures with different ratios and concentrations were tested. Sandstone cores featured by different minerologies with dimensions of 1.5 in. x 6 in. were used in the coreflood experiments, which were run at a flow rate of 5 cm3/min and temperatures from 77 to 350°F. The cores were analyzed by CT scan before and after the acidizing to investigate the effect of the acid. The core effluent samples were analyzed to determine concentrations of Ca, Mg, Fe, Si, and Al by ICP. 19F NMR was utilized to follow the reaction kinetics and products. Zeta potentials of clay particles (kaolinite, illite, and chlorite) were measured in various acid solutions Formic acid (9 wt%) damaged sandstone cores. Zeta potential measurements indicated that formic acid can trigger fines flocculation. Addition of 5 wt% ammonium chloride helps to shield negative charges on clay surface. Analysis of core effluent samples indicated that there was CaF2 precipitate in the core when a small volume of preflush was used. Coreflood tests highlighted that formic acid/HF caused loss of core permeability. This paper will discuss the detailed chemical reactions occurred within cores and were followed by chemical analysis of core effluent samples and 19F NMR. Secondary reaction between clay minerals and HF became faster at higher temperature, and decreased the ratio of Si/Al. It was also found that different clay minerals react with HF offering very different concentrations of Al and Si in spent acid.

A New Technique to Increase the Performance of Organic Acids to Stimulate Carbonate Reservoirs at High Acid Concentrations

“…The organic-HF mixtures also can cause damage by precipitating AlF 3 . Similar work confirmed these observations (Yang et al 2012).…”

Acidizing Sandstone Formations Using a Sandstone Acid System For High Temperatures

“…Both calcium acetate and formate salts have limited solubility, so the maximum concentrations of acetic acid and formic acid are below 13 wt% and 9 wt%, respectively. Yang et al (2012) compared the effectiveness of 9 wt% formic acid in removing carbonates in Berea sandstone reservoirs at various temperatures. But there is no study to show the effectiveness of formic acid in removing carbonates in Bandera sandstone reservoirs, especially at high temperatures.…”

Sandstone matrix acidizing knowledge and future development

“…(2016) applied nuclear magnetic resonance analysis on core samples obtained after core flooding using HCl and CH 3 COOH:HCl and found the later one more effective, hence validated the results of previous analysis using this combination. Furthermore, to prevent the hydrated silica precipitation, HCl or an organic acid like citric acid, formic acid or acetic acid are added in the main acid stage with HF acid as reported by Yang et al (2012). McLeod presented the basic guidelines for sandstone acidizing mentioned in Table 1.…”

“…These problems become more severe at high temperatures. Therefore, Shuchart and Gdanski (1996), Shuchart (1997), Shuchart and Buster (1995), Al-Harbi et al (2011) and Yang et al (2012) applied organic-HF in sandstone matrix acidizing to overcome the problems. Due to less corrosion rate and retarded nature of organic acids, these acids provide better results especially at elevated temperatures and considered as an excellent alternative compared to mud acid in sandstone acidizing.…”