土壤力學學理上最早認為圍壓與孔隙壓力之差為有效應力，即圍壓與孔隙壓力對有效應力之貢獻權重相等。然此一理論並不適用於岩石，故定義以有效應力係數表達孔隙壓力與圍壓對有效應力貢獻之權重關係。本研究欲測量砂岩之孔隙率/滲透率有效應力係數，並討論孔隙率/滲透率有效應力係數之差異。本研究選定多組砂岩為試體，並以氦氣作為孔隙流體，於不同圍壓及孔隙壓力下量測孔隙率/滲透率，藉此計算有效應力係數。結果顯示，利用孔隙率計算之有效應力係數χ範圍為 0.918 至 1.765，利用滲透率計算之有效應力係數κ範圍為 0.65 至 1.875。其中黏土含量較高的試體有效應力係數小於 1，黏土含量較低的試體有效應力係數大於 1。為了解此結果對石油工程關心之岩石孔隙率/滲透率-深度關係之影響，本研究使用實驗計算之有效應力係數，重新預測孔隙率/滲透率隨深度變化之結果，黏土含量較低的試體之初始孔隙率減小約 4%~18%，孔隙率之應力敏感係數減小約11%~30%，初始滲透率減小約 9%~81%，滲透率之應力敏感係數減小約0.8%~14%；黏土含量較高的試體之初始孔隙率增加約0.6%，孔隙率之應力敏感係數近乎不變，初始滲透率增加約 28%，滲透率之應力敏感係數增加約 0.6%。孔隙率隨深度變化考慮計算之孔隙率有效應力係數χ將造成孔隙率有 0%～9%之差異；滲透率隨深度變化考慮計算之滲透率有效應力係數κ後將造成滲透率有 50%～1900%之差異。;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.
