AAPG Geoscience Technology Workshop

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Pore structure, fluid flow and mass transport in porous and fractured media: Progressive challenges into the deep geological systems


Fluid flow and mass transport in porous rock are critical processes in the development of oil and gas reservoirs. Pore structure integrates both geometry and topology of the pore network, giving rise to emergent first-order effects on fluid flow and mass transport in the rock matrix. Pore connectivity, a largely overlooked topological characteristic of pore structure, is oftentimes more important than the geometrical aspects, especially for low-permeable formations in deep geological systems. Low permeability media make it likely that flow and transport is limited by pore topology (e.g., density of connections) rather than geometry (e.g., radius). In addition, the maturation process of organic-rich shale leads to a complicated interplay of pore structure and mixed wettability. Aided with integrated characterization approaches of mercury intrusion capillary pressure, low-pressure gas sorption, scanning electron microscopy, small angle neutron scattering, droplet contact angle measurement, the development of nano-sized tracers in hydrophilic, hydrophobic and zwittering fluids as well as their usage in imbibition, diffusion, and edge-only accessible porosity tests, followed with micro-scale mapping of laser ablation-inductively coupled plasma-mass spectrometry, we have systematically studied how pore wettability and connectivity are associated with compositional phases and pore-size spectrum for leading shales in the United States (Barnett, Bakken, Eagle Ford, Spraberry-Wolfcamp, and Niobrara) in consideration of their different geological characteristics (deposition, TOC, maturity, mineralogy), as well as preliminary studies on carbonate reservoirs in Tarim Basin in China. These innovative and complementary approaches indicate the shale-unique dalmatian wettability at a scale of microns, limited connectivity for shale matrix at >400 microns from sample edge, disparity of well-connected hydrophobic pore network (~10 nm) and sparsely connected hydrophilic pore systems (>50–100 nm), and anomalous fluid imbibition and tracer diffusion. In summary, pore structure characteristics and associated flow and transport behavior will dictate how low-permeability media in deep geological systems will perform in the oil and gas development.