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| EFFECTIVE STRESS LAW FOR PERMEABILITY OF TIGHT GAS RESERVOIR SANDSTONE |
| QIAO Liping，WANG Zhechao，LI Shucai |
| (Geotechnical and Structural Engineering Research Center，Shandong University，Jinan，Shandong 250061，China) |
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Abstract Tight gas refers to natural gas produced from reservoirs that have very low porosities(＜10%) and permeabilities(＜0.1 mD). Physical fracturing of these formations could enhance the overall formation permeability and improve tight gas extraction. One of the outstanding issues in rock fracturing is to determine the magnitude of applied effective stress. The general effective stress law is defined as： ，where and are total confining stress and fluid pore pressures，respectively，and is Biot?s coefficient. For soils，the Biot?s coefficient equals one，but for rocks，especially fluid-saturated rocks，the Biot?s coefficient is not one. It is concluded that the Biot?s coefficient is not only a particular material property，but also markedly sensitive to the magnitude of applied confining stress and pore pressure. The main objective of this study is to experimentally determine the Biot?s coefficient for permeability of Nikanassin sandstone. A series of permeability measurements were conducted on Nikanassin sandstone core samples from the Lick Creek region in British Columbia under various combinations of confining stress and pore pressures. The results show：(1) Tight gas reservoir sandstone has a very low permeability with an order of magnitude of 0.01–0.001 mD and there is an exponentional relation between permeability and pore pressure under the same confining pressure；(2) Biot?s coefficient is increasing with the permeability；(3) the average values of the two sandstone samples are 0.509 and 0.612，respectively.
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Received: 28 January 2011
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2011, Vol. 30 
