Abstract

In-situ gelled acids are used in well stimulation to improve sweep efficiency during acid placement. These acids comprise a polymer, a cross-linker and a breaker in addition to chemicals used with regular acids. The gelation reaction in these systems occurs over a narrow range of pH (2–4). The gel forms once the acid reacts with the carbonate formation and the pH rises to a value above 2. The gel breaks once the pH rises above a value greater than 4. Propagation of the crosslinker plays a key role in the success of these acids. However, in a recent paper by Lynn and Nasr-El-Din,1 there was evidence of precipitation of the cross-linker in some systems. The cross-linker is typically a multi-valent cation (iron(III) or zirconium(IV)), which may precipitate at high pH values or react with hydrogen sulfide (in the case of sour wells) to precipitate damaging material. Coreflood experiments were conducted at 100°F in a linear mode to determine the rate of propagation of various cross-linkers in reservoir cores (mainly calcite). This temperature represents bottom hole temperature of seawater injectors in a carbonate reservoir in Saudi Arabia.

Propagation of the cross-linker in carbonate cores was examined by measuring the concentration of the cross-linker in the core effluent and comparing it with the chloride ion concentration (used as a tracer). The profile of the tracer was used to determine dispersion of the acid in the core, whereas the area between the tracer and cross-linker profiles was used to determine the amount of cross-linker precipitated or retained in the core. The effects of acid concentration, slug volume, injection rate, cross-linker type, and polymer loading on the rate of propagation of the cross-linker were examined in detail.

The following conclusions were made. 1. Viscous fingering effects that occur during pumping low-viscosity post-flush fluids affect the propagation of the cross-linker in reservoir cores. 2. The rate of propagation of the cross-linkers depends on the type of cross-linker used (iron, zirconium, etc.), and 3. Results of this study will help production engineers to design acid treatments with minimum damage due to precipitation/retention of the cross-linker.

Introduction

Hydrochloric acid (HCl) has been used to stimulate carbonate formations for more than seven decades.2 The acid, however, reacts very rapidly with the formation rock, especially at high temperatures. This in turn limits acid penetration into the formation, which can result in surface washout in matrix acidizing or poor etching patterns in acid frac treatments.3,4 In addition, straight or regular HCl acid has low viscosity, which can cause poor sweep efficiency during acid placement.5–11

Acid-soluble polymers (synthetic and biopolymers) have been used to increase the viscosity of HCl, and hence improve its performance.12–14 As the viscosity of the acid increases, the rate of acid spending decreases, and as a result, deeper acid penetration into the formation can be achieved.2

Addition of uncross-linked polymers to HCl improved acid penetration, however, acid placement was not significantly improved.7 Cross-linked acids were introduced in the mid 70's.15 These acids have much higher viscosity than regular acids or acids containing uncross-linked polymers. Two types of cross-linked acids are available. The first type consists of a polymer, a cross-linker, and other acid additives.13 The acid in this case is cross-linked on the surface and reaches the formation already cross-linked. The second type of crosslinked acid consists of a polymer, a cross-linker, a buffer, a breaker, and other acid additives, e.g., corrosion inhibitors and surfactants. The acid in this case reaches the formation uncross-linked, and the cross-linking reaction occurs in the formation.6,7,9,16–18

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