Summer graduation

Open Access
Author:
Fang, Zhicheng
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 14, 2017
Committee Members:
  • Shimin Liu, Thesis Advisor
  • Chaopeng Shen, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • Permeability
  • Pulse-decay experiment
  • Analytical solution
  • Numerical simulation
  • History-matching method
  • Pressure-dependent gas properties
Abstract:
For tight unconventional gas reservoir formations, such as shale or coal, the transient ‘pulse-decay’ technique is a time-effective method to experimentally estimate the rock permeability from the pressure versus time profile data. Currently the analytical solution of permeability that is derived based on the expression of simulated pressure profile has been widely used (Cui et al., 2009). However, this solution may lead to erroneous results because the assumption of constant gas properties is not always valid. Besides, the permeability solution is obtained by making simulated pressure profile and experimental pressure curve have the same late-time slope. In some cases, however, different pressure decay characteristics are observed demonstrating the huge differences between simulated and experimental pressure profiles and the invalidity of the permeability result. To overcome these limitations, in this study a new method of permeability measurement is designed. Finite difference method is used to solve the governing equation numerically and reproduce the experimental pressure profile. Pressure-dependent gas properties are incorporated in numerical simulation, and permeability is obtained when the differences between simulated and experimental pressure profiles are minimum. The minimum differences ensure similar pressure decay characteristics and can be quantified by the history-matching method. This new approach was tested by measuring permeability from pulse-decay experiments conducted on Illinois coal; the types of tested gases included helium, methane and carbon dioxide. The results show improved permeability values compared with the analytical solutions of Cui et al. (2009). Two factors contained in this proposed numerical approach result in the improvement: (1) be able to apply pressure-dependent gas properties; (2) using the whole pressure profile to capture the pressure decay dynamics so that improved gas permeability estimation can be provided. Finally, sensitivity analysis is carried out. It is found that the second factor is more influential in determining permeability in this study, and the numerically estimated permeability is not sensitive to porosity, Langmuir pressure and Langmuir volume, which is an advantage for the reliable permeability estimation through the pulse-decay technique.