2019 AAPG Annual Convention and Exhibition:

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Dual Pore-connectivity & Wettability and Fluid-accessible Pathways of Shale

Abstract

Nearly 30 years of American experience in shale hydrocarbon development indicates a steep initial production decline and low overall recovery of hydrocarbons, which can be implicated in the complexity of nanopore structure (geometry and connectivity) and “dalmatian” wettability of shale formations. Working with several fluids (polar water, non-polar n-decane, and bi-polar isopropyl alcohol) and approaches (fluid invasion, small-angle neutron scattering), we systematically studied the characteristics, as well as the interplay, of pore connectivity and wettability of leading shales (Barnett, Bakken, Eagle Ford, Niobrara, and Wolfcamp). The approaches include pycnometry (liquid and gas), pore and bulk volume measurement after vacuum saturation, porosimetry (mercury injection capillary pressure, low-pressure gas physisorption isotherm), imaging (field emission-scanning electron microscopy, Wood’s metal impregnation), scattering (ultra- and small-angle neutron, small angle X-ray), and the utility of both hydrophilic and hydrophobic fluids as well as fluid invasion tests (imbibition, diffusion, vacuum saturation) followed by laser ablation-inductively coupled plasma-mass spectrometry imaging of different nm-sized tracers. A shale can have a good pore connectivity for a wetting fluid, but poor connection for a non-wetting fluid. Furthermore, pore spaces of different wettability are very likely associated with the compositions of shale, which also have different pore sizes. In other words, oil-wetting pores are smaller (~10 nm) and yet well-connected, while water-wetting pores are larger (~50 nm) but sparsely-connected. The low pore connectivity of shale also creates extensive dead-end pore complexes that are connected to the transport “backbones” to the sample edge, and this steep decline of edge-accessible connected pore spaces is implicated in steep initial production decline and low overall recovery.