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It is generally assumed that the reaction of acid with limestone reservoir rock is much more rapid than acid reaction with dolomite reservoir rock. This work is the first to show this assumption to be false in some cases, because of mineral impurities commonly found in these rocks. Trace amounts of clay impurities in limestone reservoir rocks were found to reduce the acid dissolution rate by up to a factor of 25, to make the acid reactivity of these rocks similar to that of fully dolomitized rock. A rotating disk instrument was used to measure dissolution rates of reservoir rock from a deep, dolomitic gas reservoir in Saudi Arabia (275°F, 7,500 psi).More than 60 experiments were made at temperatures of 23 and 85°C and HCl concentration of 1.0 M (3.6 wt%). Eight distinctly different rock types that varied in composition from 0 to 100% dolomite were used in this study. In addition, the mineralogy of each rock disk was examined before and after each rotating disk experiment with an environmental scanning electron microscope (ESEM) using secondary and backscattered electron imaging and energy dispersive X-ray (EDS) spectroscopy. Acid reactivity was correlated with the detailed mineralogy of the reservoir rock. It was also shown that bulk anhydrite in the rock samples was converted to anhydrite fines by the acid at 85°C, a potential source of formation damage.

Oxidative coupling of methane by a mixed manganesemagnesium oxide is governed in part by the solid state structure and chemistry.

It is generally assumed that limestone reservoir rocks will react much more rapidly with acid than dolomite reservoir rocks. This work is the first to show this assumption to be false in some cases, due to mineral impurities commonly found in these rocks. Trace amounts of clay impurities in limestone were found to reduce the acid dissolution rate by nearly an order of magnitude, to make the acid reactivity of these rocks similar to fully dolomitized rock. A rotating disk instrument was used to measure dissolution rates of reservoir rock from a deep, dolomitic gas reservoir in Saudi Arabia (275°F, 7,500 psi). More than 60 experiments were made at temperatures of 23 and 85°C and HCl concentrations of 1.0 M (3.6 wt%). Eight distinctly different rock types that varied in composition from 0 to 100% dolomite were used in this study. In addition, the mineralogy of each rock disk was examined before and after each rotating disk experiment with an Environmental Scanning Electron Microscope (ESEM) using secondary and backscattered electron imaging and energy dispersive X-ray (EDS) spectroscopy. Acid reactivity was correlated with the detailed mineralogy of the reservoir rock. It was also shown that bulk anhydrite in the rock samples was converted to anhydrite fines by the acid, a potential source of formation damage. Introduction A study of acid reaction rates and reaction coefficients of a dolomitic reservoir rock was recently reported by Taylor et al.1 In that work, it was found that reaction rates depended on mineralogy and the presence of trace components such as clays. This paper examines in detail the relationship between acid reactivity and mineralogy of a deep, dolomitic gas reservoir rock. An accurate knowledge of acid reaction rates of deep gas reservoirs can contribute to the success of matrix and acid fracture treatments. Many studies of acid stimulation treatments of formation K, a deep, dolomitic gas reservoir in Saudi Arabia, have been published.2–5 It is generally assumed that limestone reservoir rocks will react much more rapidly with acid than dolomite reservoir rocks during acidizing treatments. However, much of the reported data was obtained with pure limestones, dolomites, and marbles. These include calcite marble (CaCO3),6–8 dolomite marble (CaMg(CO3)2),9,10 Indiana limestone,11 St. Maximin and Lavoux limestones, 12 Haute Vall e de l'Aude dolomite, 13 Bellefonte dolomite,10 San Andres dolomite,14 Kasota dolomite14 and Khuff dolomite reservoir cores.2 The effects of common acid additives on calcite and dolomite dissolution rates were reported in detail by Taylor et al.15 and by Frenier and Hill.7 The effects of impurities such as clays on rock dissolution has not been reported. Lund et al.6 studied the dissolution of both pure calcite marble and pure dolomite marble9 with the rotating disk instrument. According to Lund et al., 6 at the solid-liquid interface calcite marble reacts with 1 M HCl approximately 650 times faster than dolomite marble at 25°C. Calcite and dolomite marbles make excellent standards, but do not occur in oil and gas reservoirs. Alkattan et al.12 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 and 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 showed that dissolution rates of pure forms of dolomite are not significantly affected by mineralogy.10 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.10 The single crystal of dolomite was of hydrothermal origin. The sedimentary dolomite was from the upper Bellefonte formation, Pennsylvania, USA. It was composed of 80 wt% microcrystalline dolomite (grain size approximately 10 µm), 20 wt% interstitial quartz and traces of feldspar. The coarse-grained dolomite marble (grain size approximately 100 m) was from the Fauske formation in Norway and was composed entirely of dolomite.