Pitfalls in using fractal dimension to characterize the heterogeneity of pore throat structure of tight sandstone reservoirs

Fractal geometry theory is often used to characterize the heterogeneity of pore throat structure. While, some pitfalls that are often ignored will lead to incorrect conclusions. To provide insight into these issues, the mercury intrusion porosity experimental data of Chang 2 sandstone reservoirs in Jingbian area, and Chang 8 tight sandstone reservoirs of in Qingcheng area were used, the impact of calculation equations, data range and distribution on fractal dimension values, as well as the correlation between fractal dimension and permeability were analyzed. It shows that different calculation equations will result in quite different fractal dimension values. Meanwhile, for a specific equation, different cumulative order of S Hg will result in completely different D f values, the calculation equation need to clarify its cumulative order for S Hg . It is also found that the limitation of mercury injection pressure affects the fractal dimension values, determining whether the fractal curve exhibits unifractal or multifractal characteristics. The fractal curves of different types of samples show that small pore throats, especially nanoscale pore throats with radii less than 100 nm are commonly not fractal, it is due to that the nanoscale pore throats are close in size and have no significant orders of magnitude. Additionally, for multifractal sandstone reservoirs, when the radii intervals of large pore throats are not consistent with the radii intervals of permeability contribution, the correlation between permeability and fractal dimension of large pore throats may be not strong.