Even though the proposed screening criteria for low salinity water flooding (LSWF) are fulfilled, improve recovery is not always obtained. The LSWF mechanisms are therefore still discussed. The objective for the study was to describe the brine-rock interactions at high and low salinity. Reservoir core plugs were flooded either by formation water, sea water and low salinity waters in succession or by low salinity water directly from initial water saturations. Effluent samples were analyzed for ionic concentrations and pH. Relative permeability (kr) and capillary pressure (Pc) curves were obtained at the core scale by history matching the experimental production and differential pressure across cores using a simulation tool. A developed two-phase model was used to predict the release of divalent cations from the rock during LSWF, and to relate this to the oil recovery. The high salinity water flood with formation water was found to give close to piston-like displacement, while the oil was produced over much longer periods in LSWF. The estimated kr- and Pc-curves indicated that the rock was water-wet in the high salinity floods and mixed-wet in the low salinity floods. The experimental results were in accordance with the modeling of the brine-rock interactions. When the formation water was replaced with the low salinity water, increase in the concentrations of divalent cations onto clay surfaces was predicted for the selected brine compositions. Higher concentrations of polar oil components can then be bonded to the clay surfaces by the divalent cations and make them less water-wet. It is concluded that the low salinity water altered the wetting state of the rock. The direction of alteration can be explained by ion exchange taking place on the clay surface. The low salinity water potential for improving recovery should be considered on a case by case basis based on the interactions between the formation brine, injected brines, oil components, and rock type.
Summary Wettability controls the fluid-phase distribution and flow properties in the reservoir. The ionic compositions of brine, the oil chemistry, and the reservoir-rock mineralogy have profound effects on wettability. Wettability measurement can be obtained from special core analysis (SCAL), but those data are not readily available, and the cost and time of analyzing different possible injection waters can be excessive. There is thus a need for early evaluation of wettability because it is crucial for selecting optimal field-development options. Information about wettability can be indirectly obtained from logging of other rock properties, but the uncertainty in the estimated wettability range is often high. In addition, wettability alteration by injection brines cannot be analyzed by logging. This study seeks to estimate the wettability by assessing the electrostatic interactions existing between the mineral/brine and the oil/brine interfaces using a surface-complexation model (SCM) supported with relatively simple and fast flotation experiments. The SCM is a chemical equilibrium technique of characterizing surface adsorption phenomenon. The SCM provides a cost-effective technique of characterizing the wettability of minerals at reservoir conditions. Ionic composition of the brine and the properties of the minerals were used as input to the model. In addition, the polar oil components in the crude oil were converted into their equivalent organic acid and base concentrations to be incorporated into the model. The electric-double-layer model that was used in the SCM was the diffuse-layer model. The SCM simulation is a fast and inexpensive wettability-characterization tool if reservoir cores and crude oil required in conventional wettability measurements are not readily available. From the flotation and SCM results, it could be concluded that the latter could capture the oil-adhesion tendencies of the former. Not only does the SCM predict the wetting tendencies of the minerals, but also it has the capacity to evaluate the mechanisms that led to their wetting preferences. For instance, the SCM results reveal that for negatively charged mineral/brine and oil/brine interfaces, divalent cations such as Ca2+ and Mg2+ can serve as a bridge between the two interfaces, thereby leading to oil adhesion. On the other hand, for positively charged mineral/brine interfaces such as calcite, direct adsorption of the carboxylic oil component was the dominant mechanism for oil adhesion. The SCM technique of characterizing wettability can be used to screen possible injection-water compositions to assess their potential to alter the wettability to more water-wet. Finally, the SCM technique could capture the trend of ζ-potential measurements from literature.
The potential of low salinity water flooding (LSWF) to improve oil recovery in sandstone cores has been well documented. The objective of this paper is further development and examination of a one-dimensional mathematical model for the study of water flooding laboratory experiments. The model describes how dissolution/precipitation of various carbonate minerals and multiple ion-exchange (MIE) will have an impact on the water-oil flow function. The model is formulated such that the total release of divalent cations from the rock surface will give rise to a change of the relative permeability such that more oil is mobilized. The combined effect of MIE and dissolution/precipitation was modelled to determine how this will affect pH and the total release of divalent cationsThe effects of solubility of carbonate minerals on the ion exchange process during the LSWF have been studied. Dynamic experiments were carried out using brines with different compositions and reservoir rocks. The ion concentrations and pH in effluent samples were determined.In experiments, pH was found to increase during the interaction between low-salinity brine and reservoir rocks. The ion composition of the low salinity brine was also found to change due to interaction with the reservoir rocks. The model developed successfully matched the experimental results. The simulation results show that dissolution of carbonate minerals can occur and this will then alter the composition of the injected low-salinity brine, the concentration of divalent ions on the rock surface and hence the potential of low salinity to improve recovery.It is important during the evaluations of the potential for LSWF to take into account that dissolution of minerals can change the composition of the injected low salinity brine. The potential should be evaluated using the composition that the brine will have in the main part of the reservoir.
Reduction of the salinity of injection water has been found to improve the oil recovery in some sandstone reservoir rocks, but for other sandstone rocks disappointing results have been obtained. No mechanism has been widely accepted as the reason for the improved oil recovery in low salinity water flooding (LSWF). Since alteration of wettability to more water-wet conditions has been reported in promising cases, desorption of crude oil components from the minerals surfaces should occur during LSWF. Retention of oil components onto sandstone reservoir rock saturated with brines of different compositions has been studied by injecting crude oils of different compositions. Results from experiments and simulation of rock-brine interactions have been compared. In the study it has been shown that the compositions of both low salinity water and the crude oil are important for the retention of polar oil components. For the original crude oil with the lowest base/acid ratio, the retention of oil components was found to vary with the brine composition even though the total salinity was the same. For this crude oil the measured decrease in the retention of oil components was found to be in accordance with simulated decrease in the total concentration of divalent cations on clay surfaces. This indicated that the retention of oil components were dominated by bonding of carboxylic groups to clay surfaces by cation-bridging. For treated crude oil with reduced amount of acidic components (highest base/acid ratio), the retention of polar oil components was not sensitive to the brine salinity/composition and was higher than for the original oil (with lowest base/acid ratio). The retention was for this oil probably due to direct adsorption of basic components. The retention was therefore found to be sensitive to the concentration of acids in the crude oil, and was increasing with increasing base/acid ratio. It has been shown the ionic composition of the low salinity brine and the composition of the crude oil are important for the retention of polar oil components and thereby the wettability. The optimum low salinity water composition will depend on the formation water composition, mineral composition/distribution of reservoir rock and oil composition, and for tertiary LSWF also the composition of earlier injection brine.
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