Impact of Microstructural Heterogeneities on Fluid Transport in Unconventional Rocks

Wettability and fluid transport mechanisms in unconventional rocks have been a matter of continued debate, with broad implications on oil production from these reservoirs. The presence of microstructural heterogeneities makes the characterization of these rocks complicated. In this paper, the fluid transport properties in a tight sandstone and a silicaceous microfractured mudstone from the Permian Basin are quantified through spatial NMR imaging experiments on spontaneous imbibition of dodecane and water. The tight sandstone sample has a relatively homogeneous pore structure with mixed-wet pores and wettability features similar to conventional sandstones, leading to fluid transport governed by the fractional flow model. In contrast, the mudstone sample has bedding-parallel microfractures that are oil-wet due to the presence of organic matter, while the rock matrix is mixed-wet. The presence of these microfractures leads to nontrivial fluid transport features that require more advanced analysis. The microfractures enhance the transport of hydrocarbon fluids into the matrix by facilitating their movement along bedding planes. Hydrocarbon imbibition occurs along bedding planes, as well as normal to bedding planes from oil-wet microfractures to the matrix. In contrast, a similar transport normal to the bedding planes is not observed for water imbibition. Finally, transport rates along bedding planes are higher, implying anisotropy in transport parallel and perpendicular to the bedding planes.