Simultaneous Effects of Normal Stress and Sand Grain Size on Fluid Storativity, Transmissibility, and Structural Strength of Artificial Sandstones

Abstract Investigation of granular materials’ geomechanical and geological characteristics could provide invaluable information to researchers working in petroleum and civil engineering, geoscience, and petrophysics. In this study, the effects of normal stress and sand grain size on the petrophysical and geomechanical characteristics of artificial samples (made of a wide range of sand grains and cement) were investigated, while the chemical composition and mineralogy of all the samples were kept identical. The results showed that the rock porosity and permeability, the main criteria for evaluating rock storativity and transmissibility, respectively, were adversely affected by applying normal stress and the increase of grain size. Based on the results, there is a critical range of grain size (i.e., 0.1–0.2 mm) above which rock permeability decreases sharply with an increase in normal stress. The geomechanical outcomes revealed that improving the cementation process by applying compaction force, and thus, increasing the degree of hydration has a greater effect on ameliorating rock strength than reducing rock porosity. Besides, the uniaxial compressive strength (UCS) and Brazilian test results indicated that fine-grained samples fail at lower normal stress (i.e., at 38% and 66% of the coarse samples, respectively) compared with coarse-grained samples because of the large number of grain-to-grain boundaries. The fracture patterns showed a straight line for very fine and fine-grained samples, while an irregular shape appeared for medium and coarse-grained samples. In the end, a comparison of distinct petrophysical and geomechanical rock properties, such as X-ray fluorescence analysis, UCS, porosity, and permeability, between artificial and natural samples has been made to analyze the difference between them.