Non-uniformity of coal damage caused by liquid nitrogen freeze-thaw

Abstract

Liquid nitrogen freeze-thaw has been used in oil, shale gas, and coalbed methane exploitation as an efficient fracturing technology. To further reveal coal damage and cracking characteristics due to liquid nitrogen freeze-thaw, coal samples with different water saturations (0%, 48.2% and 93.1%)were classified into the inner and outer parts, which were simultaneously frozen in liquid nitrogen. Moreover, the coal samples were subjected to nuclear magnetic resonance tests before and after freezing in liquid nitrogen to obtain the internal and external damages through the T2 distribution curve. Results show that T2 amplitude and T2 spectral area of the coal samples with low water saturation (including dried coal) at the surface or exterior part are larger than those at the interior part. By contrast, the T2 amplitude and T2 spectral area of the coal samples with high water saturation are larger at the interior part than those at the exterior part. The degree of internal and external damages of coal with different water saturations differs. It is found that the pore water migrates from the surface to the interior of the coal because pore water in the surface is the first to freeze and volume expansion of water caused by phase change squeezes surface water to the interior of the coal. The transition from water to ice causes an increase in the internal water saturation, resulting in a serious internal damage. To better understand the results, stress distribution was analyzed using ANSYS simulation. The tensile stress at the surface was found to be greater than that at the interior part during the freezing process, leading to a more apparent external damage.