Fundamental Investigation of Gas Storage and Transport in Shales

Restricted (Penn State Only)
Author:
Chakraborty, Nirjhor
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 13, 2019
Committee Members:
  • Zuleima T Karpyn, Dissertation Advisor
  • Zuleima T Karpyn, Committee Chair
  • Shimin Liu, Committee Member
  • Hamid Emami-Meybodi, Committee Member
  • Michael Charles Hillman, Outside Member
Keywords:
  • shale gas
  • supercritical fluids
  • nanopores and fluid confinement
  • gas densification
  • adsorption-desorption
  • surface diffusion
  • 3D X-ray CT Imaging
  • sub-resolution and multi-scale imaging
  • petrophysics and characterization
  • multi-phase multi-mechanistic modeling
Abstract:
Gas storage and transport in shales is very different from sandstones or limestones. This is not only due to their inherently lower porosity and substantially lower permeability, but also because more complex and fundamentally different physical mechanistic phenomena govern both storage and transport. We use gas injection porosimetry to measure the storage of several single-component gases at supercritical pressures and temperatures on whole core plugs of Marcellus, Haynesville, Mancos, and Bakken shales. We find that the storage capacities of all gases far exceed helium storage in most shales. This is indicative of densification of gas that is taken up by the samples. Possible mechanisms for this densification such as confinement induced-supercriticality, adsorption, and capillary condensation are evaluated and the case for each is presented. Assuming the excess storage, beyond helium derived pore or free-gas volumes, is adsorption, adsorbed methane gas is found to account for between 12-75% of total gas-in-place (GIP) and is more than 40% of GIP in most cases. Despite being a noble gas, argon storage is found to be almost the same as methane. Ethylene gas storage in the Marcellus sample is found to be over 96% of GIP. Closer analysis of the data in conjunction with pore surface area estimates from LPSA measurements indicates a multilayer adsorption mechanism. This raises questions on the applicability of the Langmuir monolayer-model to describe storage in shales. Compositional and textural characterization indicates that organic content is a moderately important factor controlling gas storage behavior. However, three-dimensional spatial maps indicate that high storage is not limited to organic-rich regions. Pore size, rather than composition, appears to be a better predictor of storage behavior, with storage being proportional to the prevalence of nanopores and to total pore surface area. Gas transport in shale is also multi-mechanistic and cannot be separated from the underlying storage mechanisms. A numerical model is developed accounting for free-gas and adsorbed-phase diffusion, as well as adsorption-desorption kinetics. The model is validated on dynamic in-situ gas concentration data obtained via x-ray CT imaging of the Marcellus Shale. Modeling results suggest that concentration-dependent surface diffusion is the dominant mechanism controlling gas transport in the Marcellus. It is observed that the surface diffusion coefficient can exceed the free-gas diffusion coefficient by up to ten times.