基于Tight gas致密砂岩储层渗透率的有效应力特性研究

EFFECTIVE STRESS LAW FOR PERMEABILITY OF TIGHT GAS RESERVOIR SANDSTONE

Tight gas是一种非传统新型能源,其储层通常为砂岩储层,具有很低的孔隙率(<10%)和渗透率(<0.1 mD)。合理确定Tight gas储层岩石的有效应力,了解其储藏动态,为制定和调整开发方案及实施综合治理提供依据。研究表明,针对土提出的有效应力原理对于岩石,特别是储油或储气岩石不再适用。在其基础上,提出改进的有效应力原理,即σeff=σc?ασp,其中α称为Biot系数(又称有效应力系数)。因此,如何确定Tight gas储层岩石的Biot系数,成为合理确定有效应力的关键。在此研究中,通过测定多组不同外部控制压力和孔隙压力下岩石的渗透率,对试验结果用Cross-plotting方法得到Tight gas致密砂岩储层的Biot系数。结果表明:(1)Tight gas储层砂岩具有低渗透率,其数量级为0.010～0.001 mD,而且相同控制压力下,渗透率与孔隙压力近似成指数函数关系;(2)Biot系数随渗透率的变化而变化,渗透率越小,Biot系数越小,相反,渗透率越大,Biot系数也越大;(3)试验所取岩样的平均Biot系数分别为0.509和0.612。

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.