Abstract
Coalbed methane is stored mainly in coal pores, and pore changes by fracturing fluid affect the extraction effect of coalbed methane. To select the optimal formulation of viscoelastic surfactant (VES) fracturing fluids, we treated coal samples of different hardness with deionized water (A) and three different fluids that contained cationic surfactants (B), mixed cationic and zwitterionic surfactants (C) and mixed anionic and zwitterionic surfactants (D) as the main agents. The effect of the three VES fracturing fluids on pores was compared quantitatively and on a multi-scale. The pore characteristics were tested by field emission-scanning electron microscopy, mercury intrusion capillary pressure and low-pressure nitrogen physisorption. Image-Pro Plus 6.0 software was used to digitize information from the obtained images. The Angulo and Frenkel–Halsey–Hill models were used to calculate the fractal dimension by using mercury intrusion capillary pressure and low-pressure nitrogen physisorption data, respectively. The effect of VES fracturing fluids B and D on pores from 14 nm to 180 μm was greater than that for VES fracturing fluid C as the main agent. The effect of VES fracturing fluid C on the pores (2–300 nm) was most obvious.
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