Fractal evolution under in situ pressure and sorption conditions for coal and shale

Coalbed methane (CBM) and shale gas become two most important unconventional natural gas resources in US. The fractal dimension, known as the degree of self-similarity or irregularity, is an important parameter to quantitatively characterize gas storage capacity and gas transport properties in pores of rock matrix. In this study, two coal and two shale samples were evaluated to estimate fractal dimensions using combined small angle X-ray scattering (SAXS), small angle neutron scattering (SANS) and low-pressure N-2 adsorption techniques. The results show that surface fractal dimension Ds of inaccessible pores is greater than that for total pores based on SANS results for all four tested samples. Ds of accessible pores estimated by N-2 desorption is greater than that for N-2 adsorption for each linear section of each tested sample. Based on in situ SANS results, Ds slightly decreases with increasing argon injecting pressure for San Juan coal. Ds decreases with increasing methane and CO2 injecting pressure for samples with high Ds. However, Ds significantly increases when CO2 became liquid phase for samples with low Ds. Furthermore, Ds almost didn't change after methane and argon penetrations for all these samples except Marcellus outcrop shale.