Influence of viscoelastic surfactant fracturing fluid on permeability of coal seams

Hydraulic fracturing is an effective way of improving permeability of coal seams. Performance of the fracturing fluid is a key factor affecting the extent of improvement. Coal-seam permeability directly affects the difficulty of coal-bed methane extraction and is mainly related to the pore size. Comparison of the influence of water, which is widely used in underground fracturing, and viscoelastic surfactant fracturing fluid on the composition, pore structure, and gas permeability at different buried depths of coal samples were experimentally investigated. The X-ray diffraction and atomic emission spectrometry results showed that the viscoelastic surfactant fracturing fluid decreased the content of clay minerals that easily block pores and fractures in the coal. Scanning electron microscopy observations of coal pores were consistent with these changes of mineral composition. Low-temperature N-2 adsorption showed that the cumulative pore volume of coal samples processed with the viscoelastic surfactant fracturing fluid was 0.00092 cm(3)/g, which was an increase of 33.3% compared with that of coal samples processed with water. Analysis of the pore structure showed that the influence of fracturing fluid adsorption on gas permeability was mainly affected by its surface tension. Use of the viscoelastic surfactant fracturing fluid reduced the occupation of migration channels and improved the gas flow compared with water. Permeability contrast tests conducted on the coal samples processed with viscoelastic surfactant fracturing fluid and water showed that permeability of the former was greater by almost 178%, which verified the results of the above analyses. As the effective stress increased, the permeability decreased exponentially. (C) 2016 Elsevier Ltd. All rights reserved.