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46419-AC9
Mechanistic Understanding of Wettability Alteration in Fractured Carbonate Reservoirs

Kishore K. Mohanty, University of Houston

The goal of this proposal is to develop a mechanistic understanding of the effect of potential determining ions, surfactants, oil composition, and temperature on the wettability of carbonate reservoir minerals. This knowledge can be used to alter wettability of initially oil-wet fractured carbonate reservoirs to recover oil from the matrix blocks. The results of wettability studies are reported here. The effect of temperature on wettability and core-scale studies are under progress.

Wettability alteration of a calcite plate can depend on parameters like brine salinity, oil composition, type of surfactant, concentration of surfactant, and temperature. Most of these experiments are done with a model oil (1.5 wt% cyclohexanepentanoic acid in n-decane). Figure 1 shows the dependence of the final contact angle on salinity at a fixed Alf-38 concentration. Sodium Chloride (NaCl) was used to change the brine salinity keeping sodium carbonate concentration fixed at 0.25 wt%. Na₂CO₃ was added to keep the brine pH above the point of zero charge of calcite. As pH increases, the calcite surface charge changes from positive to negative as it crosses its point of zero charge (PZC). Crossing PZC reduces the adsorption of anionic surfactants on carbonate rocks. The PZC for limestone is around 9.2 and the PZC for dolomite is about 7.4. Figure 1 shows that for a given Alf-38 concentration, the contact angle decreases and then increases with the increase in salinity. The salinity at which the contact angle is the minimum is termed as the optimal salinity for wettability.

Figure 1:  Effect of salinity on contact angle at a fixed 0.25 wt% Na₂CO₃ for varying Alf-38 concentration (using model oil)

Earlier studies have shown that for a given surfactant concentration there exits an optimal salinity which gives a minimum IFT.  To understand the relation between optimal salinity for a given surfactant concentration for wettability and optimal salinity for IFT, we measured both contact angles and IFT with the model oil for 0.1 wt % Alf-38 at a fixed 0.25 wt% Na₂CO₃ and variable NaCl concentrations, as shown in Figure 2.  The water to oil ratio (WOR) ratio was one in IFT experiments and very large (>100) in contact angle experiments. The results show that the optimal salinity for wettability and IFT coincides, at about 0.25 M NaCl. This result also supports the argument that optimal values for IFT and wettability occur at a salinity where surfactant has equal affinity for oil and aqueous phase. 

Figure 2: Contact angle and IFT values for 0.1 wt% Alf-38 at varying salinity with 0.25 wt% Na₂CO₃ (using model oil)

We investigated the effect of divalent ions like SO₄^2- and Ca²⁺ ions on the wettability of a calcite plate with the model oil without any surfactant. The equivalent monovalent ion concentration is kept constant at 0.576 moles per liter in all the brines. The base case magnesium, calcium and sulfate concentrations were taken as 0.045, 0.013 and 0.025 M, respectively. Brine MgCa2S has twice the base case sulfate; brine MgCa4S has four times the base case sulfate. Figure 3 shows the effect of sulfate ions in the presence of Mg²⁺ and Ca²⁺ ions on wettability of calcite with temperature. At room temperature, brines MgCa2S and MgCa4S could not alter the wettability with advancing contact angle ~150°. At 70°C, these brines alter the wettability of the plates to intermediate wet condition with contact angle values ~86°. At 92°C, MgCa4S was found to have advancing contact angle of 35° and MgCa2S had 54°, thus rendering the calcite surface to water-wet conditions. The SO₄^2- ions at the higher temperature tend to desorb cyclohexanepentanoic acid from the solid surface and occupy the sites freed.

Figure 3:  Effect of temperature on contact angle for brines MgCa4S and MgCa2S (using model oil; RT=room temperature)

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