Molecular simulation study on the rheological properties of a pH-responsive clean fracturing fluid system

The rheological properties of a pH-sensitive clean fracturing fluid system formed by the anionic surfactant N-hexadecyl iminodiacetic acid (HIA), which has two carboxyl groups, and salt-free cationic hexadecyl trimethyl ammonium bromide (CTAB) have been carefully studied. Viscoelastic wormlike micelles (WLMs) were prepared in a HIA/CTAB hybrid system. The increased electrostatic interactions between the heads of HIA and CTAB lead to a structural transition from spheres to WLMs as a function of pH. Large-scale coarse-grained molecular dynamics (CGMD) revealed that the formation of WLMs can be described in three stages: oligomer-to-sphere fusion; sphere-to-rod fusion; and 'rod-to-rod fusion. The simulation results show that electrostatic repulsion occurs among the carboxyl groups of HIA and that HIA is screened by the positively charged heads of CTAB, which leads to an increase in solution viscosity. The excellent rheological properties and pH sensitivity of the clean fracturing fluid significantly improve the seaming capacity and reservoir protection of the fracturing fluid, thus helping to increase the productivity of the well after fracturing and control the construction cost.