The influence of surfactant on pore fractal characteristics of composite acidized coal

In order to study the influence of surfactant on pore fractal characteristics of composite acidized coal. In this paper, the coal samples were acidified by solution constituted with the Sodium Dodecyl Sulfate (SDS), hydrochloric acid (HCL) and hydrofluoric acid (HF). And the proximate and ultimate analysis, X-ray diffraction (XRD) testing, scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS) and nuclear magnetic resonance (NMR) testing were carried out. The fractal dimension of pore in coal was evaluated by fractal theory, and the evolution relationship between coal quality, NMR fractal features and coal pore structure parameters were characterized. The results show that the addition of surfactant has a great influence on the coal characters. The mineral content in coal is exponential positively correlated with the fractal dimension (D-s) of seepage pores. The relative content of ash is linear positively correlated with the fractal dimension. Conversely, the volatiles is linear negatively correlated with the fractal dimension. SDS synergistic acidification compared to without SDS, the fractal dimension (D-w) of the total pore distribution and the fractal dimension (D-s) of the seepage pores are both reduced, indicated that the addition of SDS promotes the improvement of coal pore connectivity by acidification treatment. The results showed that after SDS synergistic acidification, the proportion of seepage pore volume was increased by 5.99%, and the fractal dimension is linear negatively correlated with the porosity. The changes of these parameters are beneficial to the seepage, migration and extraction of coalbed methane. Therefore, SDS synergistic composite acid fracturing technology has practical guiding significance for improving acid fracturing effect and increasing coal seam permeability. The research results are of great significance for improving the efficiency of coalbed methane mining and reducing coalbed methane pollution.