| Summary: | 碩士 === 國立中央大學 === 應用地質研究所 === 107 === In theoretical soil mechanics, the difference between confining pressure and pore pressure was considered as effective stress, implying that confining and pore pressure have the same contribution weight to effective stress. However, this theory does not apply to rocks. Therefore, effective stress coefficients have been defined to express the relationship between the weight contributions of pore pressure and confining pressure to effective stress. This study investigated the effective stress coefficients of sandstone porosity and permeability and discussed the difference between the effective stress coefficients of porosity and permeability. Using multiple groups of sandstone as samples and helium gas as the pore fluid, this study measured the porosity and permeability under different confining pressure and pore pressures to calculate the effective stress coefficients. The result showed that the effective stress coefficient χ calculated using porosity ranged between 0.918 and 1.765; the effective stress coefficient κ calculated using permeability ranged between 0.65 and 1.875. The effective stress coefficient for the sample with high clay content was smaller than 1, whereas that for the sample with low clay content was larger than 1. To understand the influence of this result on the relationship between rock porosity/permeability and depth that is concerned by petroleum engineering, this study used the effective stress coefficients obtained in the experiment to reestimate the changes of porosity and permeability with depth. For the sample with low clay content, the initial porosity decreased by 4%–18%, the stress sensitivity coefficient for porosity decreased by 11%–30%, the initial permeability decreased by 9%–81%, and the stress sensitivity coefficient for permeability reduced by 0.8%–14%. For the sample with high clay content, the initial porosity increased by approximately 0.6%, the stress sensitivity coefficient for porosity remained almost the same, the initial permeability increased by 28%, and the stress sensitivity coefficient for permeability increased by 0.6%. The effective stress coefficient of porosity χ calculated considering the change of porosity with depth caused the porosity to have a 0%–9% difference. The effective stress coefficient of permeabilityκcalculated considering the change of permeability with depth caused the permeability to have a 50%–1900% difference.
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