Heterogeneous structural responses of high-rank coal pores to hydraulic fracturing based on low-field nuclear magnetic resonance

Pores in coal are not only the main space for coalbed methane (CBM) occurrence but also the space to be opened during CBM recovery. Therefore, the analysis of the impact of hydraulic fracturing on coal pores, especially the change of adsorption pores before and after hydraulic fracturing, is of great significance to the evaluation of the hydraulic fracturing effect and CBM recovery. Hydraulic fracturing experiments and low-field nuclear magnetic resonance technology were used to analyze changes in the T2 curve, adsorption pore, and the effects of distance and in situ stresses on pore modification in coal samples of the Sihe (SH) and the Chengzhuang (CZ) mines before and after hydraulic fracturing. The results show that hydraulic fracturing can affect pores <10 nm. The CZ coal samples exhibit stronger heterogeneity than the SH coal samples after hydraulic fracturing, and the pore size distribution (PSD) anisotropy of the CZ samples is increased. For the SH coal samples with poor heterogeneity, the effect of hydraulic fracturing on pore transformation depends more on in situ stresses. The smaller horizontal in situ stresses difference facilitates the establishment of complex pore networks. After hydraulic fracturing, when the pore diameter is 30–100 nm, the pore volume proportions of the SH samples in the directions of maximum horizontal principal stress (σH), minimum horizontal principal stress (σh), and vertical stress (σV) increase from 43.73% to 64.84%, 59.79%, and 60.16%, respectively. Hydraulic fracturing increases the anisotropy of the PSD of the CZ samples.