Calcite saturation index

Figure 3 shows a schematic diagram of the C02-E0R site, together with the calculated calcite saturation index for each produced water sample as a function of time and for each well. This figure also shows that there is a clear difference between those wells on the fracture trend and those off-trend. For most of the on-trend wells, the calcite saturation drops shortly after the onset of C02 injection, becoming negative. This is contrasted by the off-trend well behavior, for which the SI remains nearly constant, and positive through most of the period represented. The rapid decrease seen in a few wells... 

Fig-3. Schematic representation of the Penn West Cardium C02 EOR project together with the time evolution of the calcite saturation index. The horizontal line in each SI figure represents an SI of 0. The maximum SI on each figure is 0.8 and the minimum is -0.4, with the exception of well 08-11 with a maximum SI of 0.4 and a minimum of-1.6. 

Fig. 33 Calcite saturation index of complete and incomplete water analyses (calculated with PHREEQC after data by Merkel 1992)...

Fig. 71 Calcite saturation index and amount of calcite precipitated per year for a discharge of 0.5 L/s in a 400m long karst channel with a flow velocity of 0.25 m/s assuming turbulent mixing (P(C02) = 0.03 Vol%).

To display the secondary Y-axis for the calcite saturation index besides the primary Y-axis with the concentrations for Ca and C, Chart options / Show secondary y-axis must be chosen by click on the right mouse button in the graph. The result of the modeling can be seen in Fig. 72. The figure depicts a convergence to the calcite equilibrium. However, the saturation index shows that... 

Fig. 74 Dissolution of calcite, precipitation of gypsum and iron hydroxides and the development of the calcite saturation index for the AMD water shown in Fig. 73...

Surface water temperature is approximately 32°C throughout the year. Water temperature is 34.25°C at a depth of 51 meters (R.J. Hoffman, U.S. Geological Survey, 1990, personal communication). Water in Devils Hole is slightly supersaturated with respect to calcite (saturation index averages about 0.18) with calculated Pco2 values from 0.0123 to 0.0141 atm (Plummer et al., 2000). The water has been supersaturated with calcite for at least 500,000 years. 

Spring Creek, Virginia, they observed some calcite precipitation at SI, (calcite saturation index) values between +0.1 and -t-0.7. Major precipitation ofcalcite as travertine took place at a waterfall where the local turbulence facilitated COt oulgassing. The CO2 loss caused the water to exceed calcite saturation by up to 20 times SI, = +1.3). 

TABLE 6.6 Computed carbonate speciation, COj pressure, total carbonate Cj, and calcite saturation index, S4, of samples 1-8 from Slab Cabin Run... 

Simulations of marine-meteoric mixing (e.g. Plummer, 1975 Wigley Plummer, 1976) predicted calcite oversaturation in waters with 20-70% sea water. However, the saturation degree of the mixed waters varies depending on the initial calcite saturation index, C02 and temperature. Nevertheless, predictive models constructed by Frank Lohmann(1995) for low-Mg calcite precipitation in... 

Fig. 7. Modelled change in the calcite saturation index vs the relative increase observed in the calcium concentration. The modelled change in the calcite saturation index (81 3, ) refers to the SI,. , . calculated for pretreatment water minus the that was calculated based on predicted CO2 injection conditions. The relative increase in calcium concentration is the maximum post-treatment concentration observed divided by the original concentration.

Fig. 11. Calcite saturation index vs depth for Crooks Gap. The water analysis used for the calculations was the original water analysis plus the amount of added calcium and bicarbonate determined from Fig. 7.

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. 

Fig. 12. Modelling of Cole Creek water to predict well-bore scale. Upper graph shows modelled calcite saturation index (ST iJ for various modelling conditions. Lower graph shows anticipated calcium concentrations. Solid black is original, pre-C02 calcium concentration. Open circles are predicted calcium concentrations. Diamond is actual post-C02 concentration observed 4 months prior to intense well-bore scale precipitation in this well.

Table 3. Calculated values for the pH and calcite saturation index, when including the effects of the annulus gas ...

Calcium carbonate (such as calcite, siderite, dolomite, and aragonite) is typically present in marine deposits and Pleistocene sediments deposited in river-banks and practically absent in deposits from ancient rivers and deposits of Miocene age. Calcium is generally the dominant exchangeable cation (56-82%) followed by magnesium (6-26%). An important parameter for behavior during aquifer transport is the calcite saturation index (SI = log[Ca ][C03 J/KcacOj)-... 

---