Summary
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.
Introduction
A study of acid reaction rates and reaction coefficients of a dolomitic
reservoir rock was recently reported by Taylor et al. (2004a). 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 (Nasr-El-Din
et al. 2001, 2002a, 2002b; Bartko et al. 2003).
It is generally assumed that the reaction of acid with limestone reservoir
rock is much more rapid than acid reaction with dolomite reservoir rock during
acidizing treatments. However, much of the reported data were obtained
with pure limestones, dolomites, and marbles. These include calcite
marble (CaCO3) (Lund et al. 1975; de Rozieres 1994; Frenier and Hill
2002), dolomite marble [CaMg(CO3)2] (Lund et al. 1973;
Herman and White 1985), Indiana limestone (Mumallah 1991), St. Maximin and
Lavoux limestones (Alkattan et al. 1998), Haute Vallée de l’Aude dolomite
(Gautelier et al. 1999), Bellefonte dolomite (Herman and White 1985), San
Andres dolomite (Anderson 1991), Kasota dolomite (Anderson 1991), and Khuff
dolomite reservoir cores (Nasr-El-Din et al. 2002b). The effects of
common acid additives on calcite and dolomite dissolution rates were reported
in detail (Frenier and Hill 2002; Taylor et al. (2004b; Al-Mohammed et al.
2006). The effects of impurities such as clays on rock dissolution have not
been reported.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
12 January 2004
- Revised manuscript received:
3 March 2006
- Manuscript approved:
21 June 2006
- Version of record:
20 December 2006
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