## 1 Introduction  
Coalbed methane (CBM) is a high-quality, efficient, and clean energy source; however, it also presents considerable risks to coal mines [1]. It is therefore important to ensure the efficient extraction of CBM in order to address energy shortages and reduce the incidence of accidents in coal mines [2]. The permeability of coal beams is a critical factor in determining the efficiency of CBM extraction [3]. Hydraulic fracturing is an effective measure to improve coal-seam permeability, for which fracturing fluids (FFs) play a pivotal role [4]. Compared with conventional polymer-based FFs, viscoelastic surfactant FFs (VESFFs) have the characteristics of being economical and environmentally friendly. Thus, VESFFs have become the main substitute for traditional-polymer based fluids in the exploitation of unconventional reservoirs and are used industrially [5, 6, 7, 8]. [PERSON] et al. [9], [PERSON] et al. [10], [PERSON] et al. [11, 12], [PERSON] et al. [13], and [PERSON] et al. [14] have pointed out that VESFFs are also suitable for enhancing coal-seam permeability. However, as the depth of CBM extraction increases, the majority of CBM reservoirs exhibit traits associated with elevated gas content, low permeability, and high ground temperature [15, 16]. This situation has prompted the pursuit of novel VESFFs with augmented performance. The addition of VESs to the solution can lead to the formation of self-assembling aggregates. This causes the solution to thicken and gives the FFs sufficient viscoelasticity and sand-carrying capacity. VESs have amphiphilic structures, which are reflected in their molecular head chains (hydrophilic groups, HLGs) and tail chains (hydrophobic groups, HBGs). There are many types of HLGs that can promote contact between water and coal seams. HBGs are composed of hydrocarbon chains and their molecular formula contains mainly -CH\({}_{2}\)- subunits. Recently, many researchers have directed their attention toward enhancing the performance of VES-based fluids [17]. It has been highlighted that the optimization of the hydrophobic chain length of VESs and the combination of different ionic VESs represent opportunities for the enhancement of VESFF performance. [PERSON] et al. [18] have demonstrated that an increase in the hydrophobic chain length of VESs is conducive to micellization. [PERSON] et al. [13] investigated the impact of the length of the hydrocarbon chain in surfactants on the viscoelasticity, temperature resistance, and wettability of VESFFs on coal surfaces. [PERSON] et al. [19] found that the mixing of cationic and zwitterionic VESs had a weakly synergistic effect, which could make the micelles more stable. Therefore, VESFFs with varying hydrocarbon chain lengths of VESs and a combination of different VES types as the main agent are studied in this paper. The physicochemical properties, counter ion type, and solution environment of VESs determine different physical and chemical effects that VESFs can produce on coal. [PERSON] et al. [20], [PERSON] et al. [21], and [PERSON] et al. [22] noted that VESFFs can affect the pore structure of coal. [PERSON] et al. [23] explained the reasons why VESFFs change the pore structure of coal in terms of clay mineral expansion, mineral dissolution, and coal particle movement. The current work focuses on revealing the mechanism by which VESFs act on coal nanopores from the perspective of zeta potential, surface functional groups, and macromolecular structure. Coal is a structurally complex medium composed of molecules, atoms, and electrons, exhibiting certain surface electrical characteristics. The zeta potential on the coal surface can be altered by the action of ionic VESs. [PERSON] et al. [24] explored the effects of zeta potential on coal particle agglomeration and concluded that the control of zeta potential can optimize the agglomeration of coal particles. [PERSON] et al.