Pore characterization of shales: A review of small angle scattering technique

To better evaluate the reservoir quality and optimize shale gas extraction, it is necessary to quantitatively characterize pore structure in shale reservoirs using complementary methods. Conventional tools that have been used extensively for pore characterization include gas physisorption, mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR). Small- and ultra-small-angle scattering (SAS and USAS) techniques use neutrons or X-ray beam to penetrate shales and obtain information on its pore structure by measuring the intensity of scattered radiation within a range of scattering angles. SAS and USAS measures total pores in a size range from 0.5 nm to 20 μm. The petrophysical parameters such as porosity, pore size distribution (PSD), fractal dimension, and pore connectivity of shale reservoirs can be obtained by analyzing the SAS/USAS data using recently established mythology. The difference in the results of porosity and PSD between the scattering technique and fluid intrusion or physisorption techniques can provide unique information on closed pores. As reported in the literature, contrast-matching small-angle neutron scattering (CM-SANS) experiments can effectively characterize the accessibility of pores to various fluids to assess the pore connectivity of shale reservoirs. Future perspectives of utilizing SAS/USAS techniques are also put forward in the review.