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Zeta potential in oil-water-carbonate systems and its impact on oil recovery during controlled salinity water-flooding

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ABSTRACT

Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the brine composition injected during recovery in a process termed controlled salinity water-flooding (CSW). However, CSW remains poorly understood and there is no method to predict the optimum CSW composition. This work demonstrates for the first time that improved oil recovery (IOR) during CSW is strongly correlated to changes in zeta potential at both the mineral-water and oil-water interfaces. We report experiments in which IOR during CSW occurs only when the change in brine composition induces a repulsive electrostatic force between the oil-brine and mineral-brine interfaces. The polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. A new experimental method is presented that allows this. Results also show for the first time that the zeta potential at the oil-water interface may be positive at conditions relevant to carbonate reservoirs. A key challenge for any model of CSW is to explain why IOR is not always observed. Here we suggest that failures using the conventional (dilution) approach to CSW may have been caused by a positively charged oil-water interface that had not been identified.

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Impact of brine composition on the zeta potential of water-wet carbonate samples.(a) Effect of varying composition from formation brine (FMB) to seawater (SW) to dilute seawater (dSW). Data are plotted against ionic strength on the lower horizontal axis and pCa on the upper horizontal axis, where p represents the negative logarithm. (b) Effect of varying calcium concentration at two different values of (constant) total ionic strength (2 mol·dm−3 and 3.5 mol·dm−3 comparable to FMB). Data are plotted against pCa.
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f2: Impact of brine composition on the zeta potential of water-wet carbonate samples.(a) Effect of varying composition from formation brine (FMB) to seawater (SW) to dilute seawater (dSW). Data are plotted against ionic strength on the lower horizontal axis and pCa on the upper horizontal axis, where p represents the negative logarithm. (b) Effect of varying calcium concentration at two different values of (constant) total ionic strength (2 mol·dm−3 and 3.5 mol·dm−3 comparable to FMB). Data are plotted against pCa.

Mentions: We begin by reporting the effect of the tested brine compositions on the zeta potential of strongly water-wet samples saturated only with the brine of interest (Fig. 2). The raw experimental data used to obtain these results are reported in the Results section of the Supplementary Information. The zeta potential of the sample in formation brine was positive, but became negative in contact with seawater, and increasingly negative as the seawater was diluted (Fig. 2a). These results are consistent with previous studies of natural carbonates40434547. Thus a typical low salinity waterflood yields increasingly negative zeta potential at the mineral-brine interfaces. Removing the calcium ions from formation brine also yielded increasingly negative zeta potential, while adding Ca2+ to NaCl brine yielded increasingly positive zeta potential (Fig. 2b). However, the change was smaller in magnitude than that observed when switching from formation brine to seawater and then dilute seawater. Moreover, removing Ca2+ from the formation brine failed to invert the polarity of the zeta potential.


Zeta potential in oil-water-carbonate systems and its impact on oil recovery during controlled salinity water-flooding
Impact of brine composition on the zeta potential of water-wet carbonate samples.(a) Effect of varying composition from formation brine (FMB) to seawater (SW) to dilute seawater (dSW). Data are plotted against ionic strength on the lower horizontal axis and pCa on the upper horizontal axis, where p represents the negative logarithm. (b) Effect of varying calcium concentration at two different values of (constant) total ionic strength (2 mol·dm−3 and 3.5 mol·dm−3 comparable to FMB). Data are plotted against pCa.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5120358&req=5

f2: Impact of brine composition on the zeta potential of water-wet carbonate samples.(a) Effect of varying composition from formation brine (FMB) to seawater (SW) to dilute seawater (dSW). Data are plotted against ionic strength on the lower horizontal axis and pCa on the upper horizontal axis, where p represents the negative logarithm. (b) Effect of varying calcium concentration at two different values of (constant) total ionic strength (2 mol·dm−3 and 3.5 mol·dm−3 comparable to FMB). Data are plotted against pCa.
Mentions: We begin by reporting the effect of the tested brine compositions on the zeta potential of strongly water-wet samples saturated only with the brine of interest (Fig. 2). The raw experimental data used to obtain these results are reported in the Results section of the Supplementary Information. The zeta potential of the sample in formation brine was positive, but became negative in contact with seawater, and increasingly negative as the seawater was diluted (Fig. 2a). These results are consistent with previous studies of natural carbonates40434547. Thus a typical low salinity waterflood yields increasingly negative zeta potential at the mineral-brine interfaces. Removing the calcium ions from formation brine also yielded increasingly negative zeta potential, while adding Ca2+ to NaCl brine yielded increasingly positive zeta potential (Fig. 2b). However, the change was smaller in magnitude than that observed when switching from formation brine to seawater and then dilute seawater. Moreover, removing Ca2+ from the formation brine failed to invert the polarity of the zeta potential.

View Article: PubMed Central - PubMed

ABSTRACT

Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the brine composition injected during recovery in a process termed controlled salinity water-flooding (CSW). However, CSW remains poorly understood and there is no method to predict the optimum CSW composition. This work demonstrates for the first time that improved oil recovery (IOR) during CSW is strongly correlated to changes in zeta potential at both the mineral-water and oil-water interfaces. We report experiments in which IOR during CSW occurs only when the change in brine composition induces a repulsive electrostatic force between the oil-brine and mineral-brine interfaces. The polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. A new experimental method is presented that allows this. Results also show for the first time that the zeta potential at the oil-water interface may be positive at conditions relevant to carbonate reservoirs. A key challenge for any model of CSW is to explain why IOR is not always observed. Here we suggest that failures using the conventional (dilution) approach to CSW may have been caused by a positively charged oil-water interface that had not been identified.

No MeSH data available.


Related in: MedlinePlus