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- Shale-water interaction plays a crucial role in many subsurface water-bearing rock engineering fields, including ground control in underground mines under humid environments, wellbore stability and formation damage in water-based post-fractured shale reservoirs, among many others. The effects of water on the poro-hydro-mechanical properties of shales include degradation in its strength/stiffness associated with alterations in seismic velocities, decreases in gas relative permeability and hydration-induced micro-structural alteration, among others. These processes are intimately associated with shale hydration when contacting with water - either as a liquid or as vapor. For example, sorption of water vapor occurs endemically in shales comprising the roofs of underground mines when exposed to humid environment. In addition, shale hydration also occurs through direct contact with liquid water/brine under subsurface reservoir conditions, driven by exceptionally strong capillary imbibition and influenced by the pore/fracture network and its wettability. During shale gas production, although initially associated with liquid water alone, long-term shale-water-gas interactions potentially involve the migration of water vapor in low-permeability shale matrix as driven by evaporation/condensation of water coexisting with desorbed gas within nanoporous shale matrix. Therefore, a fundamental understanding of the sorption and transport of water vapor in shale is key in predicting time-dependent shale degradation (in underground mines) and long-term gas/water production (in unconventional shale reservoirs). The following questions are addressed in this dissertation: (1) what is the structure of nanopores and nanopore networks in fine-grained shales and how do these ultra-fine pores impact water retention behavior? (2) how does shale hydration occur under dynamic water vapor conditions and what are the roles of surface chemistry and pore structure? (3) how does pore anisotropy, surface roughness, and pore volume evolve during condensation of water vapor? (4) how is water vapor transported in nanoporous shale and what are the roles of surface diffusion and viscous flow of the condensed water? These questions are addressed in the four Chapters of this dissertation. First, nanopores of four mine roof shales (6R, 5A, 6F, H6) and a fireclay (7F) are characterized in Chapter 2. Three complementary techniques are used, including small-angle neutron scattering (SANS), low-pressure N2 adsorption (LPNA), and high-pressure mercury intrusion porosimetry (MIP). The results show that overall distributions of pore volume obtained from SANS, LPNA and MIP techniques agree well between methods and over a wide range of pore size from ~1 nanometer to ~100 nanometers. Mercury porosities (7.3%, 7.8%, 8.3%, 12.3%, 4.6%) for the five ordered (7F, 6R, 5A, 6F, H6) samples are higher than the respective N2 porosities (5.0%, 6.3%, 3.8%, 8.2%, 2.5%), as attributed to the dilation of mesopores and compression of the grain skeleton induced by high pressure intrusion of mercury. The SANS porosities for samples 7F, 6R, 5A, 6F (4.0%, 6.2%, 4.1%, 8.8%) are in good agreement with their N2 porosities. Among all tested samples, H6 shale exhibits a relatively high SANS porosity (8.0%) but the lowest N2 (2.5%) and mercury porosities (4.6%). This can result from the fact that SANS detects both accessible and inaccessible porosity while the two fluid penetration methods only detect porosity accessible to N2 molecules and mercury. Based on LPNA, micropores (1.5-2 nm) and mesopores (2-50 nm) predominantly contribute to the total porosity (~77.8%-87.6%) for the five tested samples. Due to a large proportion of micro/meso-pores, a strong water retention capacity is developed with the matric suction reaching ~100-150 MPa at water saturation <3%, which is calculated according to the Laplace equation and the characterized pore size distribution. Besides, water adsorption capacity correlates positively with total porosity/specific surface area (SSA). Second, water vapor sorption behaviors are characterized and modeled in Chapter 3 through a combination of modeling analysis of sorption isotherms and the analysis of heat of adsorption. Modeling results show that shale hydration is controlled by surface chemistry at low relative humidity (Rh) through a strong intermolecular bonding, while is mainly influenced by the pore structure at high Rh (>0.9) through capillary condensation. This is consistent with the progressive decrease of isosteric heat of adsorption with water content, obtained by the Clausius-Clapeyron equation. Exceptionally, for the one shale containing 8.6% montmorillonite, mesopore condensation only accounts for 33% of the measured water adsorption even at Rh ~0.95 due to the limited external pores and the important role of clay swelling. The specific surface area defined by GAB analysis as available for water adsorption is larger than that available for low pressure N2 adsorption due to the complex surface chemistry. The one shale rich in expansive montmorillonite and with a large interlayer capacity for water but inaccessible to N2 molecules conditions this result. Among the other four shales, one with high kerogen content behaves the highest water adsorption, possibly due to the high content of oxygen-containing functional groups and the potentially high pore volume of kerogen. Chapter 4 probes the condensation response of two contrasting shales by in situ ultra-small/small angle neutron scattering (USANS/SANS) techniques under a vapor of contrast-matching water (D2O/H2O) conditions. SANS results on dry samples show that one shale with higher content of both kerogen and clay has rougher surfaces and higher anisotropy than the other shale (less clay and no kerogen) over length scales from 2.5-250 nm. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) analysis also confirms that the organic-rich shale presents more anisotropic microfabrics and higher heterogeneity compared to the other shale with less clay and no kerogen. USANS/SANS results show that water condensation effectively narrows the pore volume in the way of reducing the aspect ratio (or anisotropy) of non-equiaxed pores. For the shale with less clays and no kerogen under a relative humidity of 83%, a wetting film uniformly covers the pore-matrix interface over a wide range of length scale (1 nm - 1.9 [mu]m) without smoothing the surface roughness. In contrast, for the organic-rich and clay-rich shale with a strong wetting heterogeneity, condensation occurs at strongly-curved hydrophilic asperities (1-10 nm) and smoothens the surface roughness. Finally, the water vapor transport properties in nano-porous shales are explored in Chapter 5. We explore this response through dynamic vapor sorption experiments and modeling on two shale samples with differing fractions of hydrophilic clays and contrasting pore architecture. Measured diffusion coefficients of water vapor in the two shales are of the order of magnitude of 10 - 12 - 10 - 10 m2/s, increasing with Rh except at high Rh during adsorption process. The drop in diffusivity at high Rh during adsorption results from the impeding effect of capillary-occluding air bubbles and flattening of the pore-entry menisci. We also propose a model for water vapor transport accommodating surface flow of adsorbed water and viscous flow of capillary water -- with active mechanisms operational in different pore size populations. Actual pore size distributions (PSDs) are characterized by LPNA technique. Predictions from the proposed transport model, utilizing these measured PSDs, are consistent with the measured diffusion behavior during desorption, also replicating water uptake behavior across the full spectrum of 0<Rh<1. Observations and modeling illustrate that phase type and pore size significantly influence water vapor sorption and transport behaviors. Surface flow of the adsorbed phase contributes predominantly to the total flux over a wide range of Rh (< 0.96) while viscous flow of capillary water dominates only at very high Rh values (> 0.98). In terms of pore size effects, macropores (> 50 nm) contribute little to the total water adsorption but comprise more than of the 68% total water flux. Conversely, micropores (< 2 nm), contribute moderately to water adsorption (7%-40%) but insignificantly to the total flux. and Intermediate-sized mesopores (2-50 nm) play an important role in both total water adsorption and transport over the entire range of Rh. Sensitivity analysis of temperature (30-90 °C) indicates that transport of water vapor can be enhanced at higher temperature due to a lower viscous resistance of water to flow.
- Dissertation Note:
- Ph.D. Pennsylvania State University 2020.
- Technical Details:
- The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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