Nanopore Connectivity, Wettability and Fluid Migration in Mudrocks

Abstract:

Micro(nano)scopic pore characteristics (e.g., pore size, pore-size distribution, and pore connectivity) of mudrocks, implicated by their mixed wettability, control macroscopic fluid flow and hydrocarbon production. This work discusses various approaches to investigating pore structure (both geometry and topology) of several mudrocks of leading U.S. plays (Barnett, Bakken/Three Forks, and Utica), and the presence and connection of mixed wettability associated with compositional phases. Results show that these mudrocks have very limited edge-accessible pore spaces. This is shown from low pore connectivity behavior of fluid imbibition, steep decline of edge-accessible porosity from vacuum saturation, the heterogeneous presence of only trace amount of diffusing tracers beyond a few mm from a sample edge, and limited connected pathways from high-pressure injection of traced n-decane at 414 MPa. As mudrocks contain distinct hydrophobic organic materials (e.g., kerogen), as well as hydrophilic and/or hydrophobic minerals, different nano-sized tracers in two wettability fluids (API brine and n-decane) were developed to interrogate their pore spaces and connectivity. For two molecular tracers in n-decane with the sizes of 1.393 nm×0.287 nm×0.178 nm for 1-iododecane and 1.273 nm×0.919 nm×0.785 nm for trichlorooxobis (triphenylphosphine) rhenium, much less penetration was observed for wider molecules of organic-Re in these mudrocks with median pore-throats of around 5 nm, indicating the entangling of nano-sized molecules in nanopore spaces of mudrocks. The sparse nanopore space connection within the mudrock matrix, implicated by mixed wettability, could lead to limited movement of nano-sized hydrocarbon molecules and fracture-matrix interactions in fractured reservoirs, and consequently steep initial production decline and low recovery.