Abstract: 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.

Key words: effective pressure, formation factor, low permeability sandstone, microfracture