[21].  As temperature increases, relative permeability to oil increases and relative permeability to water decreases at a given saturation while residual oil saturation decreases and irreducible water saturation increases.  This is shown in Figure 2‑92 and Figure 2‑93 below.

Figure 2‑92 and Figure 2‑93 show the effects of temperature on high and low-tension systems respectively.  In both figures, the cross-over point shifts to higher S_w as temperature increases.  In general, as temperature increases oil is able to flow much easier than water can.  These results suggest an increase in water wettability of sandstone with temperature.

The figures also show that temperature effects are more significant for low tension than for high -tension systems.  The residual oil saturation decreases significantly at higher temperature, but only small changes in irreducible water saturation ( S_wir ) are seen in the high-tension systems.  At any temperature, S_wir for the low-tension system is smaller than that observed for the high-tension system.  Relative permeability to oil and water both increase with increasing temperature up to 100°C for the low-tension system.

For the low tension system, the changes in S_wir with temperature are not significant, but S_wir values for low tensions were lower than those at high tensions.  This implies that the increase in temperature and decrease in IFT have opposite effects on S_wir; increasing temperature increases S_wir but as IFT is decreases, S_wir decreased.  Therefore it is possible that S_wir in the low-tension system at high temperatures is controlled by both the IFT and wettability changes due to temperature.

For both low IFT and high IFT systems, k_rw / k_ro decreases with increasing temperature at a given saturation (as temperature increases, k_rw decreases and k_ro increases, thus k_rw / k_ro decreases).  Thus k_rw / k_ro shifts toward higher water saturations with increasing temperatures.

Sufi et al.

Sufi et al. found that in the temperature range of 20°C to 85°C, the relative permeability of oil and water obtained from unconsolidated clean Ottawa sand does not change[22].  In a separate experiment, they saw that as temperature increases, Sor decreases.  This observation is consistent with Handy et al.  Sufi et al. also said that as temperature increases, the viscosity ratio of the oil water system decreases.  This reduction in the viscosity ratio is responsible for the change of the fractional flow curve as seen in the second experiment. Thus a change in S_or is not necessarily due to a change in wettability.

They also found that the irreducible water saturation increases as the temperature of the system increases.  This observation is similar to that of Handy et al.  The changes in irreducible water saturation can be illustrated to depend only on the change in the viscous force, irrespective of whether it is caused by a temperature or a rate change.  Figure 10 demonstrates this:

Figure 2-94: Effects of Viscous Force on S_wir (After 22)

From this figure, it can be seen that the change in Swi due to temperature or flow rate follows the same pattern.  This indicates that changes in irreducible water saturation with temperature can be caused by changes in viscous forces not necessarily changes in wettability.

Other Theories

Nakornthap et al. believed that the adsorbed layer of polar components in crude oil on rock surfaces is thermodynamically unstable, thus it can be desorbed at high temperature and the rock may become more water wet[23].

From the analysis of three-phase relative permeability data, Maini and Okazawa also found that the studies of cleaned sand shows that relative permeability does not change as temperature changes.  They mentioned that Polikar et al. reported that the temperature effects could be controlled by the characteristics of the porous medium[24].

References

[21] Torabzadeh, S.J. and Handy, L.L., “The Effect of Temperature and Interfacial Tension on Water/Oil Relative Permeabilities of Consolidated Sands”, SPE 12689.

[22] Sufi, A., Ramey, H. And Brigham, W., “Temperature Effects on Relative Permeabilities of Oil-Water Systems”, SPE 11071.

[23] Nakornthap, K. And Evans, R.D., “Temperature-Dependent Relative Permeability and Its Effect on Oil Displacement by Thermal Methods”, SPE 11217.

[24] Maini, B. and Okazawa, T., “Effects of temperature on heavy oil/water relative permeability of sand”, Journal of Canadian Petroleum Technology, May-June 1987, 33-41.

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