Research on the Effective Stress Coefficient for Porosity in Sandstone Reservoirs Considering Clay Filling Patterns

ABSTRACT Porosity attenuation from effective stress changes during sandstone mining restricts reserve assessment accuracy. The traditional Terzaghi theory assumes that the effective stress coefficient is always 1, ignoring the heterogeneous influence of clay minerals on the pore structure. This article proposes two types of clay filling modes: clay shell‐shaped filling model and clay granular filling model. Through theoretical modeling and physical experiment verification, the regulation mechanism of clay content on the effective stress coefficient for porosity is revealed. The research results show that (1) the effective stress coefficient for porosity in the granular model is significantly higher than that in the shell‐shaped model, and the difference stems from the mechanical action mode of the clay. (2) The effective stress coefficient for porosity shows a trend of first increasing and then decreasing with the increase of clay content. The peak position is controlled by porosity and is positively correlated with the Poisson ratio of the skeleton and negatively correlated with the Poisson ratio of the clay minerals. (3) Experimental verification shows that the chlorite‐dominated samples are more in line with the shell‐shaped model (error