Mineral dissolution of carbonate

Chou, L, R.M. Garrels, and R. Wollast (1989), "Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals", Chemical Geology 78, 269-282. 

Carbon dioxide has a dominant effect on the dissolution of carbonate minerals, such as calcite and dolomite (Table 2.1). If a carbonate mineral dissolves in water that is equilibrated with a constant source of CO, then the concentration of dissolved carbonic acid remains constant and high. However, when calcite dissolution is accompanied by consumption of carbonic acid and a continuous source of CO is not maintained, the reaction proceeds further to achieve equilibrium. 

Dissolution of carbonate minerals does not lead to mineral trapping of C02 (Gunter et al. 1993). However, carbonate dissolution, and other mineral precipitation-dissolution reactions can impact sequestration capacity by altering the permeability of the aquifer near the injection site. 

Rate of Dissolution of Carbonate Mineral Matrix in Oil Shale by Dilute Acids... 

Given the above considerations and the fact that waters overlying most shelf and upper slope sediments are substantially supersaturated with respect to calcite and aragonite, little or no dissolution of carbonate minerals generally is thought to occur in these environments (e.g., Broecker and Takahashi, 1977). However, as will be discussed later in this chapter, this assumption may be incorrect. 

Additional in-stream processes can affect the fi G of DIG (Figure 10). Assimilation of DIG by aquatic organisms through photosynthesis produces organic material with a fi G 30%o lower than the carbon utilized (Rau, 1979 Mook and Tan, 1991), resulting in an increase in the fi G of the remaining DIG. Dissolution of carbonate minerals in-stream will also tend to increase the 6 G of DIG. In contrast, precipitation of calcite will cause a decrease in the fi G of the remaining DIG, due to the equilibrium fractionation between calcite and DIG of 2%o. 

Carothers and Kharaka (1980) reported 5 C values of inorganic carbon dissolved in oil-field waters from California and Texas, and discussed the sources and reactions that yield 5 C of —20%c to 28%c. Dissolution of carbonate minerals and the oxidation of reduced carbon both produce bicarbonate as a by-product. Emery and Robinson (1993) reported a range from — 60%o to 10%o, depending on the source of the carbon and the fractionation factors accompanying the production of HCOj". 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

The acid generated through sulfide oxidation reacts with the nonsulfide gangue minerals within the mine wastes. The most significant pH-buffering reactions in mine settings are the dissolution of carbonate minerals, aluminum hydroxide and ferric oxyhydroxide minerals, and aluminosilicate minerals. 

Dissolution of carbonate minerals has the potential to raise the pH of the pore water to near neutral. Dissolution of carbonate minerals releases calcium, magnesium, manganese, iron, and other cations that are present as solid-solution substitutions or as impurities, and increases the alkalinity of the water. At many sites, the mass of carbonate minerals contained in the mine wastes exceeds that of the sulfide minerals, and... 

Alexandersson, E.T., 1975. Etch patterns on calcareous sediment grains petrographic evidence of marine dissolution of carbonate minerals. Science, 189 47—48. 

It is a well-known fact that the rate of dissolution of carbonate minerals is considerably enhanced under strong acid conditions. In fact, the rate of the chemical reaction becomes so fast at low pH that the rate of dissolution is limited by the transport of the reacting species between the bulk of the solution and the surface of the mineral (Berner and Morse, 1974). The rate can then be described... 

Under most of the natural conditions, the rate of dissolution of carbonate minerals is far less than that expected for rate control by diffusion. The chemical reaction at the water-mineral interface is then assumed to be the rate-determining step. This reaction consists in the attachment or interactions of reactants with specific surface sites where the critical crystal bonds are weakened, which, in turn, allows the detachment of anions and cations of the surface into the solution. 

TABLE 5. Rates of Dissolution of Carbonate Minerals and Limestones... 

More recently Smith (8), and Hill and Lake (14) studied cation exchange as it affected the behavior of micellar slugs in typical reservoir cores. These authors found that cation exchange in cores was quite complex, but that calcium and magnesium could, for all practical purposes, be treated as a single species. Moreover, they found that pre-flushing of a core reduced surfactant losses in most cases. Hill and Lake found that surfactant adsorption in cores was reduced by dissolution of carbonate minerals and by converting the clays to their sodium form. 

Well blowout. Injection of acid into carbonate formations will cause the dissolution of carbonate minerals and produce CO2. When the pore pressure is high enough, fluids and wastes can be forced up the injection well and cause well blowout. 

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