Microporosity in tight rocks: clues from neutron scattering experiments (Invited)

Recent advances in pore-scale flow physics and pore network modeling allow for the simulation of complex coupled reaction and flow processes. Simple simulations predict that surface roughness at the pore/mineral interface can impact flow or sorption of hydrocarbons and supercritical carbon dioxide in rocks. The current limitation in realistically extending these simulation techniques to systems like tight gas or carbon dioxide sequestration reservoir caprocks is our limited ability to describe the complex 3D pore structure of real porous rocks. Small angle neutron scattering (SANS) is a technique that can provide quantitative information about the structure and geometry of the pore network at length scales (1 nm to 0.3 μm) inaccessible by other techniques such as x-ray computed tomography. The volume and surface roughness of submicron pores was measured using SANS in geologic materials relevant to carbon dioxide sequestration studies (limestones, shales and sandstones). When a beam of neutrons is transmitted through a sample some of the neutrons are effectively scattered by interfaces between phases of contrasting scattering length density. Minerals have similar values of scattering length density that are very different than the value of pores; therefore in rocks neutrons scatter from interfaces between pores and minerals. The intensity of scattered neutrons, I(Q), with Q being the scattering vector, gives information about the number and volume of scattering objects (pores), their arrangement in the sample, and the chemicophysical composition of the scattering objects. The slope of the scattering data when plotted as log Q vs. log I(Q) provides information about the surface or mass fractality of the sample and related fractal dimensions. Like studies of other porous natural rock samples, all the samples in this study show fractal behavior permitting some interpretation of the distribution of pores and a measure of surface roughness. Results from these and further studies will be used to help constrain advanced pore scale simulation of multiphase fluid transport in natural systems.