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Determination of Stabilization Time During Stress-Sensitivity Tests

  • Author / Creator
    Yu, Miao
  • Recent research shows that, the stress sensitivity of tight sandstone formations is significantly larger than that of high-permeability sandstone formations. Also, it takes a longer time for the permeability of a tight sandstone to get stabilized when it is subjected to a change in the confining stress. Such phenomenon is referred as the delayed stress sensitivity phenomenon. To quantify the delayed stress sensitivity phenomenon, a term called stabilization time can be used. It characterizes how much time is required for a given core to reach an unchanging permeability level when the confining pressure is changed from a lower pressure to a higher pressure. In this study, we make a hypothesis that the delayed stress sensitivity can be correlated with one or more pore-structure properties of the reservoir rocks. Stress-sensitivity tests on twelve tight cores are studied to testify the hypothesis that the stabilization time of tight sandstone cores under tri-axial stress test conditions can be related to the pore-structure properties of the reservoir rocks. Twelve cores with different permeability levels are retrieved from two field (i.e., a gas field and an oil field). Mercury intrusion porosimetry (MIP) tests and tri-axial stress-sensitivity tests have been conducted on these cores. The purpose of the MIP tests is to measure pore structure parameters such as the pore size distribution. Using a trial-and-error approach, we develop an empirical method to split the pores in a given core sample into large pores and small pores based on the pore size distribution charts. Once the pores are split into large pore and small pores, we can further calculate the area ratio of the large pores to the small pores. When conducting the tri-axial stress-sensitivity tests, we apply a constant axial stress of 10 MPa, but change the confining pressure from 5 to 30 MPa. During each tri-axial stress-sensitivity test, we monitor the variation of permeability versus time. By analyzing the permeability variation data, we can determine the stabilization time, i.e., the time required for the permeability to reach a constant value when confining pressure changes to a higher level. We also record the cumulative stabilization time as a function of the confining pressure. The cumulative stabilization time (T) is found to linearly correlate with the logarithm function of the confining pressure (lnP). We discover that the slope of the linear relationship between T and lnP shows a strong correlation with the area ratio of large pores to small pores. Regression using an inverse exponential function results in a regression coefficient of R2=0.86. This indicates that a core sample with a larger area ratio of large pores to small pores has a shorter stabilization time, while a core sample with a smaller area ratio of large pores to small pores has a longer stabilization time. Such finding is in line with the physical understanding that the core sample with a smaller area ratio of large pores to smaller pores tends to have a steadier structure and deform more slowly than the one with a larger area ratio of large pores to smaller pores.

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  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.