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Fluid-Rock Interaction and Hydrocarbon Migration: Quantifying Wettability-Affected Advection and Diffusion Processed in Various Reservoir Rocks

Abstract

Darcy-type advection is the dominant transport mechanism in sandstone and carbonate reservoirs, while diffusion (driven by concentration gradient) can be the main process to transport hydrocarbons inside low-permeable shale matrices. These advection and diffusion processes affect fluid flow and hydrocarbon migration, with their rates difficult to quantify in rocks with μm to nm-scaled pore networks. Microscopic characteristics of porous materials - pore shape, pore-size distribution, pore connectivity - influence macroscopic behavior of fluid flow and hydrocarbon migration. The pore structure (both geometry and topology) effect is further complicated by fluid-wet characteristics of reservoir rocks. Using customly designed tracer recipes in hydrophilic, hydrophobic and zwittering fluids followed with micro-scale mapping of laser ablation-inductively coupled plasma-mass spectrometry, this work presents experimental approaches to quantifying the rates of advection and diffusion in different reservoir types (sandstone, carbonate, and shale). Results show that reservoir rock possesses a range of pore structure (with a wide range of pore sizes atμm to nm ranges, as well as different connectivity) to control the behavior and rates of imbibition and diffusion processes. Chemical diffusion in sparsely-connected pore spaces is not well described by classical Fickian behavior; anomalous behavior is suggested by percolation theory, and confirmed by results of our imbibition tests. Imbibition into a fluid-wet rock with well-connected pore spaces leads to mass uptake proportional to time1/2, while sparsely-connected pores exhibit an imbibition exponent of 1/4, with a much lower rate and anomalous behavior. Overall findings for organic-rich shale indicate that the pore connectivity and “Dalmatian” wettability of organic and inorganic compositions are implicated with the entanglement of nano-sized molecules in ~5-10 nm-sized pore spaces.