| Description |
Shale oil and gas has revolutionized the energy sector in the United States. Although most of the U.S.-produced oil and gas comes from unconventional (shale) reservoirs, a large fraction of the estimated oil-/gas-in-place cannot be recovered and remains in the subsurface during production. This is largely due to the extremely complex nature of shales and the lack of knowledge about various multi-physics processes that take place within these nanoporous rocks. Shales are inherently heterogenous across multiple length scales, and are characterized by nanometer-sized pores of complex geometries, which highly affect their petrophysical and/or geomechanical properties. Therefore, in this research study, various micro- and nanoscale-resolution two-dimensional/three-dimensional (2D/3D) imaging (and advanced image analysis) techniques were applied to study the Mancos, Vaca Muerta, Marcellus, and Woodford Shales - a few of the biggest shale plays in the word. A correlative multi-scale/-modal 2D/3D imaging workflow was developed and applied to image and characterize a Mancos Shale (core) rock sample, across multiple length scales, in order to capture pore and fracture networks, and investigate their role in the overall (total) porosity of shales. It was shown that both pores and micro-fractures contribute to the total porosity of shales; a similar workflow was applied to image and characterize a Vaca Muerta Shale rock sample. In addition, nanoscale-resolution focused ion beam-scanning electron microscopy (FIB-SEM) nano-tomography image datasets were used to generate two digital rock 3D models. These 3D models were then used to characterize and quantify their total and connected (effective) nano-pore systems (and microstructural features surrounding these systems) in order to investigate the role of different pore types in the porosity and permeability of shales. It was demonstrated that organic-hosted pores are characterized by their very small size and poor connectivity, and therefore, a large portion of the organic-hosted pores do not provide permeable flow pathways for the oil and/or gas migration, and hence have very little contribution to the hydrocarbon production; two additional digital rock 3D models of a Marcellus Shale rock sample were used for compression-permeability simulations in order to investigate the behavior of shales under realistic reservoir confinement conditions. Porosity and permeability of the pre- and post-compression digital rock 3D models were calculated and compared. A minimal effect of confinement on porosity and permeability of shales, at the microscopic level, was observed in the 3D models; Using the effective pore system of a different Marcellus Shale rock sample, for the first time, a digital (shale) rock 3D model was nano-3D-printed. This newly-developed protocol will be used next to microfabricate a microfluidics device - rock lab on a chip - to study fluid flow and transport phenomena in shales; finally, two unique uniaxial (unconfined) compression tests were developed and applied to crush millimeter- and micrometer-sized Woodford Shale rock samples, in order to investigate their geomechanical properties. Size-scale and compositional/structural heterogeneity effect on elasto-plastic deformation and failure behavior of shales were investigated. It was demonstrated that geomechanical properties of shales are scale-dependent and are strongly affected by compositional/structural heterogeneity of these rocks. |
