Hydration of Transitional Shale and Evolution of Physical Properties Constrained by Concurrent NMR and Acoustic Observations

Abstract

Hydration state is a key factor affecting the pore structure and mechanical properties of shale that directly impacts the migration and then production of geofluids. We quantify the impact of the hydration state on physical properties through spontaneous imbibition experiments constrained by concurrent nuclear magnetic resonance (NMR) and acoustic observations as diagnostics of response. The long-duration experiments (24 days) are on samples from the Permian Shanxi Formation of the Ordos Basin, complemented by measurements of composition and physical properties. These shales primarily comprise quartz and clay minerals with intraparticle pores present within the clay minerals. The NMR T 2 spectra indicate that mesopores (2–50 nm) comprise ∼80% of the total pore space. The imbibition rate decreases by a factor of 3 from 0.08 g/h to a stable rate of 0.03 g/h from the 1st to the 24th hour. This is accompanied by a 10% decrease in the deformation modulus from 44 to 40.3 GPa over 24 days. The reduction in the modulus is consistent with observations of the shedding and spalling of hydrophilic quartz particles from the unconstrained surface and the generation of fractures. Reduction in the imbibition rate is consistent with the swelling followed by squeezing of clay minerals and the corresponding reduction in pore diameters in the confined core of the sample, together with the reduction in saturation gradients with the progress of imbibition. Understanding the mechanisms and effects of hydration on the pore structure and mechanical properties is important in understanding the shale hydration process and defining diagnostic (acoustic and NMR) signatures that may be used in reservoir surveillance.

Assessment of Pore Connectivity and Representative Elementary Volume Size in Marine-Continental Transitional Shale

“…The samples were collected from wells LX2–48 (sample V1) and LX-152 (sample V2), part of the Carboniferous-Permian Taiyuan Formation. The Taiyuan Formation is mainly barrier islands, tidal flats, and lagoon sedimentary environments . This formation is composed of a set of interbedded sediments including coal seams, shales, and tight sandstones. , The burial depths of the collected samples are1998.18 and 1761.30 m, and the shale in the study area is transitioning within the mature to postmature hydrocarbon generation stage .…”

“…(a) Location of the Ordos Basin in the North China Craton (modified with permission from ref . Copyright 2018 Springer Nature); (b) tectonic divisions of the Ordos Basin (modified from ref . Copyright 2023 American Chemical Society); (c) lithologic section and depositional environment in the Linxing area.…”

Correlation between Natural Seepage and Ionic Diffusion in Shales

“…Currently, the studies for core experiments mainly focus on the comparison of self-absorption and backdriving, determination of mechanical properties, exploration of microstructure, and proppant reflux law. − For example, Duan (2021) proposed a new diffusion model through the dual means of modeling and experimentation to achieve quantitative calculation of shale gas diffusion flow under given reservoir conditions, which meets the needs of engineering to accurately adjust the production system . Chen (2018) demonstrated that gypsum crystallization during shale–fluid reaction can induce fracturing to increase production by means of chemical experiments, and found that mineral crystallization or growth-induced spontaneous microfracture extension effect .…”