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Introduction

It is believed that a considerable amount of the world's proven hydrocarbon reserves are trapped in carbonate reservoirs. In spite of this, though, their recoveries are not even close to being high enough to be considered sufficient. Modifying the injected brine composition using a procedure known as low salinity water flooding (LSWF) or engineered water flooding (EWF) has been found to improve oil recovery from carbonate reservoirs in both Laboratory studies and field operations. The operation of EWF is quite similar to that of conventional waterflooding, and it typically has cheaper CAPEX and OPEX costs compared to other IOR/EOR processes. Furthermore, it is environmentally friendly and cost-effective because it contains no polymers or surfactants, which are common chemical additives (Nasralla et al., 2018). The main advantages of using EWF is the acceleration of oil production due to wettability alteration and the reduction of interfacial tension in the crude oil/brine/rock (COBR) system as compared to produced water re-injection (Mahani et al., 2015).

The underlying mechanisms by which engineered water flooding increases oil recovery are still the subject of controversy due to the existence of some complex processes in EWF (Sanaei et al., 2019). Due to this, there is uncertainty regarding the prediction of ideal low salinity water composition. Several underlying mechanisms have been suggested in the scientific literature to explain the enhanced oil recovery resulting from EWF. These mechanisms are either directly detected in sub-pore scale and core-flooding investigations or inferred from indirect observations. The primary mechanisms that have been postulated for wettability alteration can be categorized into two different categories. The first type is based solely on fluid-fluid interactions, such as an increase in pH that results in a decrease in interfacial tension (Mahani et al., 2018; Meng et al., 2015), variations in interface viscoelasticity, and an osmosis effect (Afekare & Radonjic, 2017). The second type is associated with rock-fluid interactions, such as surface charge change and double layer expansion (Ligthelm et al., 2009; Sohal et al., 2016), multicomponent ion exchange (Zhang et al., 2007), mineral dissolution (Hiorth et al., 2010), and fines migration (RezaeiDoust et al., 2009; Tang & Morrow, 1999). However, engineered waterflooding is a cooperative process in which multiple mechanisms act on different lengths and time scales. This indicates that focusing on a single-length scale is insufficient. Nevertheless, the field-scale to pore-scale processes responsible for enhanced oil recovery during EWF is still unclear, and there is currently no effective method for forecasting whether a given COBR system would respond to an EWF or for selecting the injection brine composition to optimize oil recovery (Aziz et al., 2019; Nasralla et al., 2018; Nasralla et al., 2016).

In the course of this research, an innovative and cutting-edge approach was developed to come up with a dynamic geochemical model. Our primary objective is to develop a mechanistic model that accurately represents the mechanisms of engineered waterflooding at small length scales and make a prediction about its performance in field-scale scenarios. This geochemical model takes into account the impacts of the composition and characteristics of water and oil on COBR interaction. In addition, this model covers all of the mechanisms that affect wettability alteration in carbonate rocks, such as oil/mineral surface interactions, brine/mineral surface reactions, mineral reactions, and equilibrium reactions. To accomplish this objective, we have taken a scientific solution by developing a model of wettability alteration. This model is based on the surface complexation reactions and calculations of bond product sum (BPS), and it has the capability to detect and quantify wettability alteration as a result of geochemical interactions in porous media.

Figure 1 shows the surface interaction at both the rock-brine and the oil-brine interfaces. The positively charged speciation sites at the rock-brine interface will link with the negatively charged speciation sites on the oil-brine interface while the negatively charged sites on the rock-brine are linked to the positively charged sites present on the oil-brine interface. This will form a set of electrostatic bond linkages influencing rock wettability. The product of the oppositely charged reactive sites on both the rock-brine and the oil-brine interface, which form the electrostatic bond linkages, is called the bond product. The summation of the bond product gives rise to the bond product sum (BPS), which can be used as a measure of the amount of oil-mineral electrostatic interactions contributing towards rock wettability.

Figure 1. A schematic of the electrostatic linkages present in a rock/water/oil system that lead to oil adhesion (Erzuah et al., 2019)

Summary

Overall, this study presents a novel model for predicting the alteration of wettability and the recovery of oil during engineered water flooding (EWF) in carbonate cores. This model considers all possible interactions between polar groups of oil and brine ions and provides a comprehensive explanation of the observed recoveries under EWF conditions in carbonate cores. A key contribution of this study is the development of a simulator that accounts for both the rock/brine and oil/brine interactions, which are often not considered in simulation methods. The simulator employs a method for measuring the electrostatic bond between oppositely charged surface complexes, a key indicator of wettability or the tendency of oil to adhere to the rock surface. The model also takes into account fluid flow and mass transfer in a porous medium. The simulator is implemented in MATLAB software, and the IPHREEQC module, one of the open-source modules in the geochemical package of the PHREEQC program, is utilized for modeling geochemical reactions, including surface complexation reactions and surface species concentration. The simulator is validated against both the Buckley-Leverett analytical solution and core-scale experiments and has the potential to be applied to analyze results from core flooding with low-salinity water and to develop new applications.

Conclusions

The study findings demonstrate the capability of the newly developed model in capturing the impact of surface complexation reactions, as well as the reactions among aqueous phases, mineral, oil, and brine chemistry (salinity and composition). By fine-tuning the model, it accurately predicts wettability alteration and resulting recoveries due to surface reactions. The proposed model was validated using published experimental works, and two case studies were presented to support the approach. In conclusion, the developed model provides a valuable tool for predicting wettability alterations in carbonate reservoirs and optimizing enhanced oil recovery processes. The main findings and results of this study can be summarized as follows:

• Wettability alteration and the performance of low salinity water injection can both be explained using calculating the BPS values by interpolating parameter (θ) for the calculation of modified relative permeability. The interpolating parameter (θ) is utilized in order to provide an explanation for the low salinity effect observed in carbonate cores by using the corresponding brine/rock and oil/brine surface complexation model.
• Since BPS, which indicates wettability, is calculated as a function of pressure, temperature, oil and brine compositions, and pH for a given COBR system, the results indicate that the BPS approach can accurately predict experimental results and can be effectively used to optimize water composition during EWF.
• A certain pH range is predicted to be optimal for the engineered water flooding process, where the salinity reduction causes the interactions between oil and rock to become repulsive. However, the acid and base numbers of the oil, the salinity level, and the composition of the injection water all play a role in determining the precise optimal pH values.
• The results indicate that the new BPS approach might be successfully used as an optimization tool to optimize the water composition during LSWF since BPS (which signals wettability) is calculated as a function of pressure, temperature, oil and brine compositions, and pH for a given COBR system.

Acknowledgments

We would like to express our heartfelt gratitude to Mr. Pouria Almasian, a Sharif University of Technology graduate, for his outstanding contributions to this project. His tireless efforts and invaluable assistance in all aspects of this work were critical to its success. We would also like to extend our sincere appreciation to Dr. Hassan Mahani, a faculty member of the Department of Chemical and Petroleum Engineering at Sharif University of Technology, for his invaluable guidance and support throughout the project. Without their contributions, this work would not have been possible.

References

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