Experimental study on removing water blocking effect (WBE) from two aspects of the pore negative pressure and surfactants

Abstract

The phenomenon that gas permeability decreases because of the water retention is known as water blocking effect (WBE), especially in the later period of gas drainage, the WBE is more serious. In order to improve the coal seam gas drainage performance as required in the real coal mine, experimental study was carried out for removing the water blocking effect (WBE) through adopting the nuclear magnetic resonance (NMR) method, and this study is conducted from two aspects of the pore negative pressure and surfactants. Results show that: with the increase of pore negative pressure, the variation of the T2 cutoff value (T2c) experiences three stages: “stable I-reduction-stable II”. When the pore negative pressure in the coal seam is less than 360.9 Pa, the external water in the coal seam can plug the coal pores and block the gas flow passage thus the WBE occurs consequently; When it gets larger than 769.68 Pa, the external water in the coal seam will be separated from the coal pores, conducive to the gas flow passage. The WBE in coal pores larger than 68.9 μm is called the temporary WBE, for which the water can be extracted, while the WBE in coal pores smaller than 68.9 μm is called the permanent WBE and the water cannot be removed by the pore negative pressure, unless the characteristics of fracturing fluid is changed. Anionic and nonionic surfactant can remove the WBE, which is beneficial to gas drainage. The cationic surfactant solution can inhibit the release of WBE, preventing gas migration in the coal seam and decreasing gas drainage efficiency. This study offers theoretical guidance for removing the WBE and enhancing gas drainage performance in the coal mine.