Porosity measurement of granular rock samples by modified bulk density analyses with particle envelopment

Abstract

Interest in assessing the extent of “isolated” pores in the total (both effective and “isolated”) porosity of tight rocks has increased in recent years due to the development of unconventional shale reservoirs. Porosity measurements of crushed granular and whole rock samples can be used to assess the proportion of “isolated” pores in the total porosity, which will help with reservoir characterization and understanding petroleum production. This study aims to measure the porosities of granular samples by independent measurements of bulk density (i.e., the density of the matrix and pore space combined) and particle density (i.e., the density of the matrix alone) provided by a modified bulk density method and helium pycnometry. The modified bulk density method uses quartz powder (<75 μm) to substitute for the original larger- and uniform-sized DryFlo of the GeoPyc 1365 method to measure the intra-granular density of granular rock samples in the size range 75–850 μm, thereby extending the sample size range for bulk density analyses of granular samples to 75–8000 μm. Using four rock samples as examples, the bulk density and particle density of crushed Woodford Shale (two samples with different pore connectivity), Paluxy Sandstone, and Austin Chalk were measured for the porosity calculation and assessment of “isolated” pores. The results show that (1) the modified bulk density measurement of granular crushed-rock could provide semi-quantitative results with acceptable repeatability and reproducibility between samples (of the same rock at different particle sizes or rocks of different origins), and (2) the changes in effective porosity as a function of sample size are related to the primary grain size for well-connected sandstone and chalk, and the extent of “isolated” pores for shales with poor pore connectivity.