Measurement of Acid Reaction Rates with the Rotating Disk Apparatus
Authors: Taylor K.C. , Nasr-El-Din H.A. 
Published: March 2010

The authors have published several papers on the rotating disk apparatus (RDA) in recent years. The RDA is used to measure acid reaction rates, reaction order and activation energy of acidizing fluids with carbonate reservoir rock. This paper summarizes the different ways that acid reaction rates are measured, the factors that affect the results and why this is important when modelling acid stimulation treatments in the field.

In particular, experimental procedures used to obtain acid reaction rates vary widely and can produce very different results. These procedures are often not well-documented in the literature. The validity of different experimental procedures are carefully compared in this paper.

In addition to experimental concerns, the reactivity of reservoir rock can also vary widely. It was recently reported that the reactivity of reservoir rock toward acid is strongly affected by mineralogy and by mineral impurities. In particular, clay impurities in calcite rock were found to reduce the dissolution rate by nearly an order of magnitude. Some calcite rocks with clay content as low as 1 wt% showed acid reactivity similar to that of 100 wt% dolomite.

Acidizing additives such as polymers and corrosion inhibitors have been shown to significantly affect the way that acid reacts with the reservoir rock.

Introduction

The rotating disk instrument is widely used in the petroleum industry for kinetic studies of the reaction of acidic fluids and chelating agents with reactive rocks(1–11). This system allows the determination of rock dissolution rate, reaction rate constants, reaction order and diffusion coefficients(1, 12).

Taylor et al.(13) examined the effects of acidizing additives on acid reaction rates of calcite and dolomite rock. These additives were quaternary amines, polymer, surfactant, mutual solvent, iron chelating agents and dissolved iron (III).

Taylor et al.(14, 15) examined, in detail, the relationship between acid reaction rate and rock mineralogy for a deep, dolomitic gas reservoir. Their results showed that mineralogy significantly affected acid reaction rate, and that clay contents as low as 1 wt% could reduce the apparent reaction rate of limestone reservoir rock by up to a factor of 25.

There has been significant variability in acid reaction rate data of reservoir rocks(11). These variations have often been attributed to activation energies lower than obtained for pure calcite or dolomite rock(11). However, several detailed studies have contradicted this claim(8, 16).

Alkattan et al.(8) showed that the dissolution rates of calcite crystals, limestones and compressed calcite powders were the same within experimental error in the bulk solution pH range of ?1 to 3 at temperatures of 25, 50 and 80 °C. The limestones contained less than 1 vol% clays, but one type of limestone (St. Maximin) did contain 16 vol% quartz. This shows that the dissolution rates of pure forms of calcium carbonate are not significantly affected by different mineralogy.

Herman and White(16) showed that dissolution rates of pure forms of dolomite are not significantly affected by mineralogy. The dissolution rates of dolomites in the form of a single crystal, microcrystalline sedimentary rock and coarse-grained marble in aqueous carbonate solutions were also found to be similar(16).