A new parameter for characterizing pore-fracture structure heterogeneity: fractal dimension based on the mercury extrusion curve

Abstract

Pressure mercury intrusion test is (MIP) one of the most commonly used methods to characterize pore-fracture structure. Here, we use the fractal dimension of the mercury intrusion curve to analyze the heterogeneity of pore and fracture distribution. Differing from the intrusive mercury curve, the extrusive curve provides a better representation of the seepage capacity of a reservoir. In this paper, the division method of sample types using both mercury invasive parameters (pore volume, pore volume percentage, porosity, permeability) and extrusive parameters (mercury removal efficiency) is discussed. The fractal dimension values of mercury intrusive and extrusive curves are calculated for all samples using the Menger, Thermodynamics, and Multifractal fractal models. Moreover, the fractal significance of the mercury withdrawal curve is examined. The results are as follows. 1) The samples can be divided into three types based on the mercury removal efficiency and total pore volume. Type A is characterized by lower total pore volume (< 0.08 cm³·g⁻¹) and removal efficiency (< 30%), type B has lower total pore volume (< 0.08 cm³·g⁻¹) and higher removal efficiency (> 30%), and type C has larger total pore volume (> 0.08 cm³·g⁻¹) and higher removal efficiency(> 30%). 2) Mercury removal efficiency does not correlate with the mineral composition or total pore volume, but it does show a clear positive correlation with pore volume in the range of 100 to 1000 nm. Unlike the Menger model, the mercury removal curve analyzed using the thermodynamics and multifractal model shows good fractal characteristics. 3) In contrast to the injective curves, the fractal dimension of mercury removal curves exhibits an obvious linear negative correlation with pore structure parameters and mercury removal efficiency. Moreover, the multifractal dimensions D₀–D₁₀ obtained from the mercury removal curves show a negative correlation with porosity and permeability. This indicates that fractal dimension based on the mercury extrusion curve can be used as a new parameter for characterizing pore-fracture structure heterogeneity.