Probing nanopore structure and confined fluid behavior in shale matrix: A review on small-angle neutron scattering studies

Although continued growth in unconventional oil and gas production is generally projected, its long-term growth potential and sustainability have significant uncertainties. A critical problem is the low hydrocarbon recovery rates from shale and other tight formations using the horizontal well drilling and hydraulic fracturing techniques: < 10% for tight oil and similar to 20% for shale gas. Moreover, the production rate for a given well typically declines rapidly within one year. The low recoveries and declining production of shale oil and gas reservoirs are apparently related to the small porosity (a few to a few hundred nm) and low permeability (10(-16) -10(-20) m(2)) of shale matrix, which make the enclosed hydrocarbon fluids difficult to access. Hence, to enhance the hydrocarbon recovery from shale matrix, it is essential to study its nanopore structure and confined fluid behavior. Small- and ultra-small-angle neutron scattering (SANS and USANS) have emerged as a powerful method for characterizing shale nanopore structure and confined fluid behavior. Owing to neutrons' high penetrating ability and high sensitivity to hydrogen (and its isotope, deuterium), SANS/USANS can probe inside shale samples to characterize nanopores from 1 nm to 10 mu m in size and be readily combined with sample environmental cells to examine the fluid (hydrocarbon and water - frack fluid) behavior at relevant pressure-temperature (P-T) conditions. In this review article, an introduction is first given on the characteristics of shale matrix and the uniqueness of SANS/USANS compared with conventional methods. Then current studies on shale nanopore structure and confined fluid properties using SANS/USANS are summarized. Finally, an outlook and perspective on future research in this emerging area will be offered.