How feldspars are affecting pH at different salinities

A pH study, that have been conducted previously byAndersen (2015), was done using new and optimized mineral samples. Andersen (2015) was using the old preparation procedure to study

highest at low salinities and then pH increment decreased at higher salinities and temperatures.

To confirm the results obtained by Andersen (2015), and also the test the reproducibility of the optimized mineral samples, the same pH study was performed using mineral samples prepared with new preparation procedure. Previous pH studies was performed at different temperatures on both albite, microcline and anorthite. In this thesis only anorthite has been tested, at ambient temperature.

Figure 31 show how anorthite is influencing pH and how the reaction is dependent on salinity.

The results from the new preparation procedure are a bit lower, which was expected, but they are following the same trend as the previous results, figure 37. The chemical mechanism behind the observed increase in pH is caused by cation exchange between the mineral surface and protons in the brine, equation 29

CaAl₂Si₂O₈+H₂OHAl₂Si₂O₈+Ca²⁺+OH⁻ (29) Increase in pH is highest in DI water and then as salinity increase the pH increment decrease, figure 31. When there are no N a⁺ in the brine, an exchange of Ca²⁺ ions by H⁺ ions happens.

This cation exchange releases hydroxide ions into the solution and an increase in pH is observed.

Dissolved cations in solution affect the kinetics of feldspars very significantly. The dissolution rate of feldspar decreases when dissolved alkali ions such as N a⁺ and K+ are added to solution due to the competition of ions with protons on the surface (G¨ulg¨on¨ul et al., 2012). In the NaCl brines there areN a⁺present that will compete in the ion exchange, thus less protons will participate and we observe a decrease in pH increment. Higher salinity results in lower increase in pH, which was observed in the experiments, the results are in line with theory.

In previous pH study the results from anorthite did not behave fully in accordance with theory at ambient temperature, figure 37. A high increase in pH was observed with DI brine, but as salinity in the brine increases the pH increment is not decreasing accordingly. The large amount of crushed very small particles caused by extremely high centrifugal forces in the planetary ball mill that were present in the anorthite samples, figure 33, could have significant impact on the analysis results. This could explain why it previously was observed only a minor change in pH for the different salinities at ambient temperature. Results from present pH screening study, figure 31, show a steady decrease in pH increment as the salinity is increased, which is more in line with what we expect will happen. The results obtained by the optimized samples behave more in accordance with theory and are probably more representative than results obtained using samples from the old preparation procedure.

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pH

Salinity (ppm)

Anorthite, pH after 24h

Initial pH of brine Ambient temperature

60 deg.C 130 deg.C

Figure 37: Results from the static pH screening test for anorthite at different temperatures (An-dersen, 2015)

Comparing the pH screening result from anorthite with the results from other feldspars (An-dersen, 2015), figure 38, anorthite gives the largest ∆pH, then albite and microcline. This order can be explained by the relative stability of the different feldspars, figure 5, where microcline is the most stable feldspar. Stability of feldspar minerals:

M icrocline > Albite > Anorthite

Anorthite is the least stable feldspar, and are not very abundant in sandstone reservoirs. Mi-crocline however are more stable and therefore more frequently found in sandstones. All feldspars are following the same trend, highest ∆pH at low salinity then increment in pH decreases as the salinity increases. Because of the higher stability, microcline will exchange less cations with protons compared to anorthite. Less hydroxides will be released into the solution, resulting in a lower pH increment.

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pH

Figure 38: feldspar pH test results at ambient temperature, (Andersen, 2015)

An IC analysis was done on the brine that had been in contact with anorthite, to evaluate the chemical composition of the brine and to confirm results obtained by Andersen (2015), figure 39.

From the IC of DI water analysis performed in the present study, it was possible to see traces of ions in the solution, figure 32. In the DI test, only very little ions were detected in the solution, and in the LS and HS tests noCa²⁺ ions were detected. This is because of the dilution of the salinity brines, which was necessary to do prior to IC analysis of the brines. The pH screening, figure 31, shows a steady decrease in pH as the salinity is increases, suggesting less hydroxides released into the brine. Less cations are exchanged and there will be less Ca²⁺ ions released into the solution.

A decrease in pH from 9 - 8 is a concentration of 10⁻⁹M −10⁻⁸M, which is a very little amount of ions. When the solutions are diluted 500 times, it will not be possible to see any of this tiny concentrations, and therefore the IC results are in line with what is expected.

The DI test on the other hand was not diluted, and the trace ions present in this solution verify that there is ion exchange between the mineral surface and the brine.

The results from the optimized samples behave in accordance with the results obtained in previous tests. From the DI test at ambient temperature, it was possible to see traces ofN a⁺,K⁺, and Ca²⁺ ions in the solution, showing that the anorthite sample is not 100 % pure. Both studies confirm that there is an ion exchange between the mineral and the brine which will give an increase in pH as the protons are exchanged from the DI water.

0,74

DI water analysis after contact with anorthite

Ambient temperature T=60°C

T=130°C

Figure 39: Plot of IC results for anorthite (Andersen, 2015)

6.2.4 Effect of feldspars on initial wetting and wettability alteration processes in