The influence of mean grain size on unconfined compressive strength of weakly consolidated reservoir sandstones

The effects of mean grain size on unconfined compressive strength (UCS) of weakly consolidated reservoir sandstones are studied by series of unconfined compressive tests performed. According to previous studies, the influence of grain size on rock strength is not clear and inconsistent results are reported. In this study, categorizing the rocks studied in previous researches into two classes of non-porous crystalline and porous granular rocks, it was found that the compressive strength of all investigated crystalline rocks is inversely related to the mean grain size, while in the case of granular porous rocks (e.g. sandstones) different results are obtained by different researchers. In current investigation, the obtained experimental results indicated that grain size alone is not a suitable indicator of the compressive strength of porous sandstone. The reason for this is that except grain size, other textural characteristics such as pore spaces and cementation, play important roles in the strength of sandstones and should be considered along with the grain size. Based on these results, an empirical model was developed to estimate the unconfined compressive strength of porous sandstones based on grain size, porosity, and cement content. The validity of the suggested empirical model for natural sandstones was checked against data reported in the literature.