Fractal characteristics of low-permeability gas sandstones based on a new model for mercury intrusion porosimetry

Abstract

Studying pore structure characteristics is essential for understanding and evaluating the storage and seepage properties of low-permeability reservoirs. Fractal geometry has been widely employed to study fluid flow in porous media. Although numerous fractal models have been proposed for determining the fractal dimensions from mercury intrusion curves, these models just considered the fractal dimension for pore space and did not consider the fractal characteristic of tortuous pore length, which has been demonstrated to exist in pore structures and been adopted by many investigators.Herein, a new model for calculating the fractal dimensions from mercury intrusion curves is developed by considering both the fractal dimension for pore space and tortuosity. The new model more accurately characterizes the pore geometries of fractal porous media. The sum of the fractal dimensions for pore space and tortuosity is obtained from the slope of log (SHg) vs. log (Pc), where SHg is the mercury saturation and Pc is the capillary pressure. The fractal characteristics and relations between the sum of fractal dimensions and pore structure parameters were examined. The results revealed that the inflection point divides the fractal curves into two segments with different slopes and thus two fractal dimensions were obtained at different pore size ranges. D1 reflects the seepage properties of pore structures, whereas D2 characterizes the storage capacity. D1 exhibited no correlation with D2. In comparison with D1 and D2, Dsw, which was obtained from the saturation-weighted mean of D1 and D2, exhibited better correlation with pore structure parameters than do D1 and D2. Dsw has a good correlation with rapex (pore throat radius corresponding to the apex of the plots of mercury saturation vs. mercury saturation divided by intrusion pressures), demonstrating that the segmentation feature in fractal curves is related to the intrinsic physical pore structure. The increase of the Dsw is accompanied by an increase in the displacement pressure and a decrease in the pore diameter, porosity, and permeability, thereby exhibiting poor physical properties. Dsw was found to be a more appropriate parameter than D1 or D2 for evaluating the complexity and heterogeneity of pore structures.