The effect of grain size, porosity and mineralogy on the compressive strength of tight sandstones: A case study from the eastern Ordos Basin, China

To better understand how textural properties and mineralogy control compressive strength, this work performed the grain size analysis, porosity measurement, X-ray diffraction, microscopic observation, and mechanical tests on deep-buried reservoir sandstones in the eastern Ordos Basin, China. The results suggest that uniaxial compressive strength (UCS) declines with increasing grain size. Meanwhile, the UCS is reduced by grains >250 μm and enhanced by grains <250 μm. Sandstones with higher fractal dimensions are poorer-sorted and have platykurtic distribution, while those with low fractal dimensions are better-sorted and show leptokurtic distribution. The former tends to have larger UCS than the latter because they have more smaller grains to distribute the load over larger grains. Both the UCS and confined compressive strength (CCS) exponentially decline with porosity. Internal friction angle and influence coefficient decrease with increasing porosity, suggesting that the compressive strength of low-porosity sandstones is more sensitive to the variation of confining pressure. With regard to mineralogy, there is a weak negative relationship between UCS and quartz content. It is because textural properties control the UCS, dominating over the effects of mineralogy, or alternatively stress concentrations and the initiation of cracks preferentially occur in brittle quartz grains rather than softer materials.