| Popis: |
Transport properties, such as permeability, are significant in evaluating the production capacity of petroleum reservoir rocks. When a new reservoir is discovered, the most crucial parameter that needs to be determined by reservoir engineers is the rock permeability. Because the permeability of rocks is controlled not only by porosity, but also, and perhaps more importantly, by pore size distribution (PSD) and pore connectivity, a quantitative understanding of the PSD in petroleum reservoir rocks is critical in the evaluation of reservoir capacity. For example, two sandstones having similar porosities can have different permeabilities because of the variance in the PSD, degree of cementation, and tortuosity (PoA) of the porous media, all of which are critical to determining hydrocarbon production of the rocks. Within this context, a new method combining digital image analysis with an empirical equation was used to evaluate the pore geometry in thin sections of ten sandstone samples as a function of pore size distribution in three dimensions (3D) and tortuosity in 2D. Comparing the results of the PSD (3D), tortuosity (PoA), and the degree of cementation for ten samples shows that the extensive calcite cement in sample 1 is the primary control on porosity and permeability of the sample. On the other hand, the dominant pore diameter (R2=0.73), the value of the PSD slope (pore population density/mean length of pore, R2=0.70), and PoA (R2=0.64) are the leading controlling factors in permeability of the remaining nine samples. In this study, the samples having similar porosities (samples 3 and 5, and samples 4 and 6) have different distributions of pore sizes and different pore tortuosity, resulting in significant differences in the dominant pore diameters (149.3 µm and 42.6 µm, and 52.9 µm and 138.2 µm, respectively), the values of the PSD slope (–11 and –44, and –36 and –13, respectively) and PoA (52 mm-1 and 140 mm-1, and 118 mm-1 and 62 mm-1, respectively). These data reveal that permeability increases with increasing dominant pore diameter and the PSD slope, while PoA decreases with increasing permeability. In addition to providing an estimation of permeability for the sandstones with similar porosity, this method can be extended to evaluate pore size distribution as a function of depth in a drill core, percent of pores in each class interval and pore types and pore geometry. |
