Investigations of Gas Sorption-induced Strain for Sorptive Rocks Using a New Optical Method

Open Access
Zhao, Xu
Graduate Program:
Energy and Mineral Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 14, 2015
Committee Members:
  • Shimin Liu, Thesis Advisor
  • Coal
  • Shale
  • Sorption-induced Strain
  • New Optical Method
It was a well-known phenomenon that the adsorption of sorbing gas results in change of coal matrix volume due to the surface energy change. The changes of matrix volume would affect the permeability in coal during CBM production and have impact on CO2 coalbed sequestration. Shale, as a sorptive rock, is expected to have similar sorption-induced change of matrix volume. Besides its influence on permeability, the change of matrix volume would also change stress status in formation, thus affecting well stability. Therefore, a thorough understanding of sorption-induced strain in coal and shale is critical for the energy bearing subsurface characterization. In this study, an optical-based strain measurement apparatus was designed and established to measure the areal strain of specimens of San Juan coal, Hazelton coal and Marcellus shale with carbon dioxide and helium injections. For coals, compared with strain gauge, this optical method avoids the use of chemical glue and reduces the influence of the built-in cleats. At 5 MPa CO2, the linear strain of San Juan coal is 0.7% in current work but the value measured by pervious researchers using strain gauges is 0.37%, which is expected. The areal strain results for San Juan coal and Hazelton coal follows the Langmuir-type equation. Liu and Harpalani model was also used to model the pressure-strain data and the modeled results well matched with the measured strain data by choosing proper rock properties. The swelling of Hazelton coal with CO¬2 is greater than that of San Juan coal at low pressure (<2.48MPa), and lower than that of San Juan coal at high pressure (>2.48MPa), which means the swelling depends not only on rank but also on other petrophysical properties. For Marcellus shale specimens, a negative strain was observed with CO2 at low pressure (0.69MPa). This repeatable results might imply that the adsorption-induced swelling was fully offset by the mechanical compression at low pressure. With injection pressure increase, the adsorption-induced strain started to dominate the overall strain behavior and a maximum swelling strain of 0.18% was observed for the tested shale.