The use of fracturing methods for stimulation of oil and gas wells in carbonates formations has become common in the petroleum industry (Anderson, 1991). In fracture acidizing, HCl is injected under high pressure in limestones or dolomite formations in order to generate fractures whose faces are then etched by the hydrochloric acid. The created conductivity of the fracture is then the result of the acid promoted dissolution of the mineral exposed faces. Difficulties encountered in predicting the simulation ratio resulting from a fracture acidizing treatment are due to a lack of proper control of the created fracture dissolution patterns, which depend on the formation conductivity and the acid flow conditions during the acidizing process. Therefore, an accurate modeling of the fracture acidizing technique requires a good knowledge of the acid reaction kinetics (Li et al., 1993).
In this study a rotating disk / mixed flow reactor has been developed to characterise dolomite dissolution kinetics. This new type of reactor allows to perform kinetic measurement both at a constant and selected chemical composition (and thus reaction affinity) of the solution (Compton and Daly, 1984), and under well-constrained hydrodynamic conditions at the mineral surface (Lund et al., 1973; Lund et al., 1975; Berger et al., 1994) The dissolution rates of various dolomite samples have been determined in the temperature range 30-70°C, at varying disk rotation speed (N = 210 to 1200 rpm) and at pH between 1 and 3.
Two samples used in this study are pure and stoichiometric dolomites. The Pampelune (Spain) dolomite is a coarse grained crystalline mineral with a maximum grain size of 0.5cm. The Eugui (Spain) dolomite consists in transparent centimetric crystals. The third sample, which contains a few percent of calcite (less than 5%), is from the Haute Vallée de l'Aude. It is a polycrystalline dolomite with a millimetric grain size. Dissolution rotating disks were made by setting a single crystal or a disk cut from the solid rock in a cylinder of Epoxy resin, leaving one face free. Prior to the dissolution runs, the free face of the disk was polished and rinsed with alcohol.
The rate of dissolution was calculated according to the classic equation of mixed flow reactor:
where Q represents the solution flow, S is the surface of the dissolution exposed face, and Cout and Cin stand for the output and input calcium or magnesium concentrations, respectively. Ca2+ and Mg2+ concentrations were measured via Atomic Absorption .
The results indicate that dolomite dissolution exhibits a mixed kinetic control, i.e. is influenced by both the hydrodynamic regime and the reaction rate at the dolomite-solution interface. The transport control of dolomite dissolution increases with increasing temperature and HCl concentration. The activation energy was found to depend on both the temperature and the hydrodynamic conditions.
All dissolution experiments exhibited stoichiometric dissolution with respect to Ca and Mg. No significant influence of the input concentration of Ca2+ or Mg2+ on the dissolution rate was detected. The rate of dissolution of the dolomites from Pampelune and the Haute Vallée de l'Aude were found to be greater than that of Eugui. This results from differences in the degree of crystallinity of the samples. The rate of dissolution increases as a function of time reflecting an increase of surface roughness due to the presence of grain joints or cleavage steps for the Pampelune and Eugui samples, respectively.
From the analysis of the data within the framework of the rotating disk model (Compton and Daly, 1984; Lund et al., 1973; Schott et al., 1994) the chemical rate constants of Pampelune dolomite at pH = 2 was found to be : kc = 9.2.10-7m/s and 3.6.10-6m/s at 30 and 50°C, respectively. The activation energy at pH = 2 and N = 210 rpm was found to be : Ea = 37.5kJ/mol and 31.5kJ/mol between 30 and 50°C and 50 and 70°C, respectively.
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