低渗砂岩地层因素的应力敏感研究

为探究有效应力变化对鄂尔多斯盆地低渗砂岩储层地层因素的影响，实验测试7块岩芯样品在围压（p_c）和内压（也称作孔隙压，p_p）变化下的地层因素。基于Biot有效应力定义，结合响应面函数对实验数据进行分析，发现采用响应面割线法得到的有效应力系数（α）更能准确地表征有效应力，同时发现α并非为1.00的常数，而是分布在0.04～0.60并与围压和内压呈现一定函数关系。明确了低渗砂岩储层地层因素随有效应力的非线性变化规律，发育的微裂缝是导致有效应力非线性变化的主要因素。基于岩芯微观结构建立等效微裂缝模型，结合岩电理论，推导出地层因素与有效应力间的函数关系，表征了低渗透砂岩储层非线性有效应力作用下地层因素的变化规律。

Study on Stress Sensitivity of the Formation Factors of Low-permeability Sandstones

To explore the influences of effective stress variations on the formation factors of the low-permeability sandstone reservoirs in the Ordos Basin, the formation factors of seven core samples under varying confining pressures (p_c) and internal pressures (also known as pore pressures, p_p) were obtained experimentally. Experimental data were analyzed based on Biot's definition of effective stress and response surface functions. It is found that the effective stress coefficient (α) obtained through the response surface secant method can characterize the effective stress more accurately. Also, α is discovered not to be a constant at 1.00, but be distributed within the range of 0.04～0.60, having certain functional relationships with the confining and internal pressures. It is confirmed that the formation factors of low-permeability sandstone reservoirs change non-linearly with the effective stress and that microfractures are the main cause of non-linear variations in effective stress. An equivalent microfracture model was established based on the microscopic structure of the cores. It was integrated with the theory of rock resistivity to deduce the functional relationships between formation factor and effective stress and to characterize the variation pattern of formation factors of low-permeability sandstone reservoirs under the action of non-linear effective stresses.