Carbonate minerals dissolution

The subsurface generally is an open system. The presence of CO and other gases in the atmosphere affects the partial pressure of gas constiments in the subsurface. For example, carbonate mineral dissolution in a system open to atmospheric COj does not achieve equilibrium. However, higher local subsurface CO concentrations can originate from biological activity and other oxidation processes. 

Prior to the onset of sulfate reduction extensive organic matter degradation can occur by bacterially-mediated oxygen reduction (see Chapter 4). The influence of benthic bacterial activity under aerobic conditions on carbonate mineral dissolution has been nicely demonstrated by Moulin et al. (1985) for pore waters from sediments of the Gulf of Calvi, Corsica. Under aerobic conditions, the oxidation of organic matter may be written ... 

One of the most important aspects of carbonate diagenesis is the net movement of carbonate minerals. This mass transfer can be accomplished on a small scale by diffusive transport or on a large scale by the flow of subsurface waters. It is the basic process by which secondary porosity is created and cementation occurs. In most cases, it involves carbonate mineral dissolution at one site and precipitation at another. While this can be simply accomplished where mineralogic transformations from a metastable phase to a more stable phase are involved, more complex mechanisms may be required within mineralogically homogeneous carbonate bodies. 

The rate at which metastable phases dissolve or are replaced is an important problem in carbonate diagenesis. Carbonate mineral assemblages persist metastably in environments where they should have altered to stable assemblages. The question is "what are the time scales of these alterations" They are certainly variable ranging from a few thousand to a few hundreds of millions of years. Even calcites in very old limestones show chemical and structural heterogeneities, indicating that the stabilization of these phases is not complete. Unfortunately, it is difficult, but not impossible, to apply directly the lessons learned about carbonate mineral dissolution and precipitation in the laboratory to natural environments. 

Walter L.M. and Burton E.A. (1986) The effect of orthophosphate on carbonate mineral dissolution rates in seawater. Chem. Geol. 56, 313-323. 

Carbonate mineral dissolution may also play a role in the observed lag time between well installation and the initiation of sulfide oxidation. Sulfide oxidation in carbonate-hosted sulfide deposits may not produce low pH and... 

A closed carbonate system is defined as one in which the carbon dioxide (carbonic acid) initially present in the water is not replenished as it is consumed in carbonate mineral dissolution. This may simply reflect that soil moisture/infiltration is charged with CO2 chiefly in the A horizon of the soil, whereas carbonate mineral dissolution by H2CO3 takes place at greater depths in the soil C horizon or below the water table in the absence of further sources of carbon dioxide. (There may be, however, sources of additional CO2 at depth, including pollution. See Chap. 5.)... 

The same equations and general approach have been used to constnict Fig. 6.14 in which the no CO2 added curves show the pathways of carbonate mineral dissolution for closed-system conditions, and the fixed CO2 pressure lines indicate the trend of open-system dissolution of the carbonates. 

The potential for well-bore scale during these CO2 treatments is determined by first predicting how much carbonate mineral dissolution will take place. After a calcite saturation index has been calculated for a water analysis for both pre- and post-C02 treatment, then Fig. 7 can be used to determine how much calcium (and therefore bicarbonate) will have been added to treated waters from calcite dissolution. These calcium and bicarbonate values can be added to the original formation water, which is then used to calculate a new calcite saturation index for the new water. 

Small differences in mineral/water ratios in the reservoir can result in large differences in carbonate mineral dissolution rates. 

Some of fhe chemical processes that influence groundwater chemistry in coal-bearing strata include carbon dioxide production, silicate hydrolysis, pyrite oxidation, carbonate mineral dissolution, cation exchange, sulfate reduction, and the precipitation and dissolution of secondary minerals leading to contamination of the groundwater (Banaszak, 1980 Groenewold et al., 1981 Powell and Larson, 1985). 

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