Influence of viscoelastic surfactant fracturing fluid on coal pore structure under different geothermal gradients

Deep coal bed methane generally has a higher formation temperature than shallower coal, which can have important consequences for hydraulic fracturing. In this study, the pore characteristics of coal samples treated by viscoelastic surfactant (VES) fracturing fluid as a function of temperature (293.15-353.15 K) were qualitatively and quantitatively analyzed using mercury injection capillary pressure tests (MICP) and low-temperature N(2 )gas adsorption (N(2)GA). The Angulo model and Frenkel-Halsey-Hill (FHH) model were used to describe the fractal characteristics of the coal pores. The results indicated that VES fracturing fluid was most effective for pore changes at temperatures below 323.15 K. The mechanism of VES fracturing fluid on coal pores at variable temperatures was analyzed and addressed from two aspects: (1) the ability of fracturing fluid to enter pores and contact the pore surface; and (2) the ability of fracturing fluid to scavenge minerals on the pore surface. The wettability of VES fracturing fluid on pore surface was analyzed from the surface tension, carboxyl and hydroxyl groups of the coal. The influence of temperature on the solubility of minerals (e.g., calcite, kaolinite, and other inorganic minerals) in VES fracturing fluid was analyzed. (C) 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.