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H.A. Nasr-El-Din, M. Al-Anazi, and A. Al-Zahrani, Saudi Aramco, and S.K. Kelkar, Schlumberger
European Formation Damage Conference, 30 May-1 June 2007, Scheveningen, The Netherlands
2007. Society of Petroleum Engineers
This paper presents the results of a laboratory investigation of a single-stage sandstone acidizing fluid designed to address some of the problems associated with conventional sandstone acidizing fluids. The application of the fluid system is sandstone reservoirs with bottom hole static temperatures greater than 200°F.
Core flow tests demonstrated that the single acid system minimized the potential for precipitation due to secondary and tertiary reactions. This system did not cause sand deconsolidation, and maintained the integrity of sandstone cores. Corrosion tests conducted for low-carbon steel and 13 Cr coupons demonstrated that the fluid had lower corrosion rates.
Over the years, many different acidizing systems have been developed for specific applications. In general, the three main drivers for these developments are:6
Traditionally, hydrofluoric (HF) acid-based systems have been used to dissolve aluminosilicates in sandstone formations. These formulations, typically referred to as mud-acid, are usually composed of hydrochloric acid (HCl) and HF at various concentrations. Examples of these traditional HCl:HF formulations include 6:1.5, and 12:3 mud acid systems. The use of 9:1 or even 13.5:1:5 mud acid systems has been advocated to allow greater dissolutions of secondary reaction products in low pH environments.2,3 In HCl sensitive formations, HCl is replaced with an organic acid such as acetic or formic acid.7
Various methods were suggested to retard the traditional mud acids including the use of buffered-HF systems,8 fluoroboric acid9 and mixtures of esters and fluorides to generate HF-in-situ by thermal hydrolysis.10 Reactions of some these acid systems with various clays were discussed by Al-Dhahlan et al.11
In general, during sandstone acidizing treatments, the following main precipitation reactions occur that can lead to formation damage.12,13