# Investigating the Adsorption Behavior of Zwitterionic Surfactants on Middle Bakken Minerals
## 1 Introduction  
Surfactants are widely used in chemical-enhanced oil recovery (cEOR) techniques as they can modify wettability and/or reduce interfacial tension (IFT) [1, 2]. Surfactants are molecules that are made up of a hydrophobic tail and hydrophilic head [1, 3]. Depending on the polar head group's charge, surfactants can be classified into four categories: anionic surfactants (such as carboxylates and sulfonates), cationic surfactants (such as pyridine halides and quaternary ammonium salts), nonionic surfactants (including polyethers, polyhydric alcohols, and polyoxyethylene derivatives), and zwitterionic surfactants (including betaine, amino acid, and imidazoline types) [4]. The modification of wettability by surfactantsprimarily occurs through three fundamental mechanisms: physical/chemical adsorption, ion pair formation, and micellar solubilization. These mechanisms depend on the specific characteristics of the surfactants, crude oil, and solid constituents involved [5]. However, surfactant flooding may not be efficient and economical due to the loss of surfactants caused by adsorption on reservoir formation surfaces [6; 7; 8; 9]. To ensure the success of the chemical-enhanced oil recovery (cEOR) process, it is critical to minimize surfactant adsorption. Studies suggest that keeping surfactant adsorption densities under 1 mg/g-rock can reduce the desired interfacial tension (IFT) and shift in wettability, which could otherwise render the process impractical. Therefore, it is required that excessive surfactant adsorption be avoided to achieve effective cEOR. Researchers in academia and industry experts have taken a keen interest in zwitterionic surfactants for their exceptional properties. These surfactants boast biodegradability, high water solubility, and compatibility with various surfactants, making them increasingly popular. Moreover, they offer durable interfacial properties that persist even under harsh conditions, making them a better option for surfactant flooding applications in enhanced oil recovery [10]. For example, [PERSON] et al. [11] have formulated zwitterionic and anionic surfactants to enhance oil recovery in non-alkaline conditions. Through rigorous screening, they identified excellent surfactant compositions and evaluated their oil recovery efficiency using core flooding tests. Similarly, [PERSON] et al. [12] have created a pH-responsive control mechanism that uses an anionic surfactant (sodium dodecyl sulfate) and a zwitterionic surfactant (erucic amide carboxylic betaine) to precisely control the protonation degree of erucic amide carboxylic betaine by adjusting the pH. This approach presents opportunities for exploring chemical EOR strategies that aim to improve oil recovery efficiency. In research conducted by [PERSON] and [PERSON] [13] the impact of zwitterionic surfactants on carbonate and sandstone adsorbent was investigated, with a focus on their adsorption behavior and influence on wettability. To understand the complex interactions between zwitterionic surfactants and rock surfaces, the researchers performed spontaneous imbibition tests and Zeta potential measurements and fitted the adsorption experimental result with various adsorption isotherm models. The findings revealed that zwitterionic surfactants exhibited low adsorption on sandstone and carbonate adsorbent. They discovered that differences in alkalinity and salinity significantly influence surfactant-induced wettability changes on rock surfaces. To be considered for enhanced oil recovery (EOR), surfactants must meet a variety of criteria, including compatibility with reservoir fluids, thermal stability, minimal adsorption, affordability, and the ability to decrease interfacial tension [14; 15; 16; 17; 18]. In recent years, substantial study has been undertaken on the adsorption of anionic, nonionic, and cationic surfactants onto reservoir formation, considering crucial parameters such as temperature, salinity, and ionic composition [19; 20; 21; 22; 23; 24].