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Paper Number 107636-MS
Title

Investigation of a New Single-Stage Sandstone Acidizing Fluid for High-Temperature Formations

Authors

H.A. Nasr-El-Din, M. Al-Anazi, and A. Al-Zahrani, Saudi Aramco, and S.K. Kelkar, Schlumberger

Source

European Formation Damage Conference, 30 May-1 June 2007, Scheveningen, The Netherlands

Copyright

2007. Society of Petroleum Engineers

Preview

Abstract
Sandstone acidizing is very challenging because of the complex reactions that occur between the multiple-stage treatment fluids and the formation minerals. Such reactions are more likely to occur at elevated temperatures and can result in potentially damaging precipitation reactions. In conventional acid treatments, fluid is usually pumped in multiple stages of pre-flush, main fluid and over flush. The drawback of conventional sandstone acidizing treatments is that the success rate is generally low due to the uncertainty associated with the fluid-formation interactions.

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.

Introduction
Sandstone acidizing is a complex operation because the treatment involves flow and reactions in porous media where the reactive chemicals contact a wide range of minerals.1 The formation may contain various amounts of quartz, clays (aluminosilicates such as kaolinite or illite), or alkaline aluminosilicates such as feldspars, and zeolites, as well as carbonates (calcite, dolomite, ankerite) and iron-based minerals (hematite and pyrite). Recent studies on matrix stimulation have strongly emphasized the importance of secondary and tertiary reactions in determining the success of sandstone acidizing treatments.2-5 However, for acid-sensitive aluminosilicates, these reactions are especially important because they occur at much shorter time-scales than for the non-acid reactive minerals. The presence of acid-sensitive aluminosilicates may dominate treatment design considerations, even though they may be present in small quantities compared to other aluminosilicates.

Over the years, many different acidizing systems have been developed for specific applications. In general, the three main drivers for these developments are:6

  1. Retard the acid/mineral reactions for deeper acid penetration,
  2. Make the acid less aggressive to well completions, and
  3. Avoid undesirable reactions that could result in formation damage.

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