Fluid space reformation law of liquid nitrogen fracturing coal based on NMR T1–T2 spectra and inspiration for coalbed methane production

The low porosity and permeability characteristics of coal seams have become the main bottleneck restricting high-efficiency extraction of coalbed methane. At present, enhanced fracturing and permeability enhancement technologies are often used to transform the pore structure of coal reservoirs, aiming to improve the permeability of coal bodies. In order to study the evolution law of coal pore and pore fluid occurrence space under the action of liquid nitrogen (LN2) cold shock, this paper takes lignite in saturated and centrifugal state as the research object. After freezing and thawing of LN2, the coal samples were tested by nuclear magnetic resonance. Through the T1-T2 spectrum and 3D peak map, the change process of coal pore fluid occurrence space was quantitatively characterized, and the change of T2 distribution and T2 spectrum area was used to reflect the coal pore evolution process. The results shows that after the same coal sample is frozen for 5, 50, and 150 min, the free water storage space expands by 86.7%, 101.4%, and 120.1%, respectively, the effective porosity ratios are 62.5%, 66.7%, and 80.4%, respectively, 5, 15, and 25 cycles after freeze–thaw cycles, the free water storage space was expanded by 146.9%, 170.6%, and 220.1%, respectively, the effective porosity ratios were 71.5%, 75.4%, and 83.3%, respectively. Further analysis shows that with the increase of LN2 freezing time and the number of freeze–thaw cycles, the occurrence space of free water in the pores increases significantly, and the occurrence space of irreducible water gradually decreases. This study can provide some basic support for the quantitative evaluation of the pore structure of LN2 fracturing coal in field applications.