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Polymer flooding is an enhanced oil recovery (EOR) method used to increase oil recovery from oil reservoirs beyond primary and secondary recovery. Although it is one of the most well-established methods of EOR, there are still continuous new developments and evaluations for this method. This is mainly attributed to the diverse polymers used, expansion of this method in terms of application, and the increase in knowledge pertaining to the topic due to the increase in laboratory testing and field applications.
Many studies have investigated the use of polymers for different applications in laboratory-based experiments. These experiments investigated either the ability of a specific or a newly synthesized polymer to improve oil recovery or the impact of a specific parameter on polymer performance and degradation [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. The majority of polymer flooding experiments conducted on polymer flooding EOR can be classified as core flooding experiments, polymer flooding in fractures, polymer application in microfluidics, or reservoir simulation of polymer flooding.
Core flooding is a broad term that refers to the injection of fluid into a confined core plug. The plug can be cylindrical or rectangular depending on the core holder. The dimensions of the plug can vary significantly depending on the core holder capacity and the type of experiment being conducted. Core flooding can include continuous cores or cores with fractures. Fractures involve two distinct modes: either continuous or partial fracture. Continuous fractures involve a fracture that covers the entire length of the core plug. A partial fracture is a fracture that propagates partially across the core [32][33][34].
Microfluidics studies the flow of the polymer through microchannels or porous media. The key difference between microfluidics and core flooding is the size of the observation, where microfluidics focuses on micro- and, sometimes, nanosized observations. Microfluidics is a key area of research with respect to the evaluation of the interactions occurring between polymers and rock, reservoir fluids, or both at the microscopic scale. Analysis of microchannels is usually conducted using imaging techniques such as magnifying microscopy, scanning electron microscopy, or transmission electron microscopy. There are several methods by which microfluidic channels can be created to model polymer flooding behavior in micropores. For all the methods summarized below, precision is key to producing a representative microfluidic rock sample for analysis [52][53][54][55][56][57][58][59][60][61].
Reservoir simulation of polymer flooding projects involves studying the ability of the polymer to increase oil recovery at the entire field scale rather than the small core scale. The polymer should be defined in the reservoir simulation process based on its properties and characteristics. This can be achieved by creating a new phase with unique properties, the most important of which is viscosity. It is important to note that many reservoir simulators do not take into consideration polymer degradation based on the change in conditions, assuming an ideal polymer [32][33][34][35][36][37][38][39].
Polymer dehydration is the process of the loss of water or fluid from the hydropolymer lattice. Loss of water can be due to several factors. An increase in temperature can result in significant and rapid polymer dehydration. The thermal stability of different polymers varies; therefore, dehydration should be tested to determine polymer compatibility with the reservoir. High pressure differentials can also result in polymer dehydration. This may be due to large depths, which require high injection pressure, or small pore sizes, which require high pressure for the polymer to propagate through the pores. Salinity is another significant factor that can impact polymer dehydration. When the formation water salinity is high compared to the hydropolymer water salinity, osmotic pressure causes the salt in the formation water to penetrate the polymer lattice. This results in polymer shrinkage and, therefore, loss of fluid. This can also result in polymer syneresis, which can damage the polymer structure and result in irreversible degradation [15][16][17][18][19][20][21][22][23][24].
Polymer degradation involves permanent damage to the polymer chains. Degradation can occur due to many factors depending on the polymer type and properties. Polymer degradation involves the weakening or destruction of the polymer chains, resulting in the polymer losing its high viscosity, which is attributed to the long chains and the high molecular weight of the polymer prior to degradation. Degradation is a strong function of the polymer limitations and can occur due to excessive temperature, pressure differentials, pH, salinity (including both monovalent and divalent cations), gasses such as carbon dioxide or hydrogen sulfide, pore size distribution in the formation, or shearing of the polymer during injection [57][58][59][60][61].
Polymer syneresis is the process of the loss of fluid from the polymer, which results in an increase in polymer rigidity and a significant reduction in polymer mobility. Syneresis differs from dehydration in that syneresis is usually associated with polymer structural degradation. This results in difficulty for the polymer structure in reabsorbing the lost fluid; therefore, the polymer degrades beyond return. In contrast, if the polymer dehydrates, it can be rehydrated by the addition of fluid, usually water. Polymer syneresis is impacted by the same factors that impact polymer degradation and dehydration.
During its injection, the polymer is subjected to extremely high shear rates. This shearing can damage the polymer chain and result in excessive polymer degradation. Some types of polymers can resist shearing more than others depending on their molecular structure. Vinyl acetate polymer can withstand some degree of shearing, for example, compared to weaker polymers such as acetic acid-based polymers. Although shearing cannot be entirely avoided, it can be reduced by proper design of the polymer injection on the surface. If the polymer is homogenously dissolved in the water, shearing tends to be significantly reduced. Decreasing the injection time and the injection pressure also tends to reduce polymer shearing [2][3][4][5][6][7][8][62][63].