Single- and Multifractal Dimension Variation of the Pore-Fracture System in Tight Sandstone by Using High-Pressure Mercury Intrusive Tests and Its Influence on Porosity-Permeability Variation

Micro-structures of the pore-fracture size distributionof targetsandstone samples are studied through high-pressure mercury intrusion(HPMI) experiments, and the compressibility coefficient is quantitativelydescribed by using overlying pressure porosity-permeabilitytests. Then, four single and multifractal models were used to quantitativelydescribe the fractal characteristics of mercury intrusion curves,and the relationship between different fractal models and pore structureparameters is analyzed. Furthermore, the applicability of fractalmodels in characterizing pore-fracture structures was explored. Theresults are as follows: (1) fractal model results show that fractaldimensions of type A by using Sierpinski (D (S)) and thermodynamics models (D (M)) arelarger than those of type B, and fractal dimensions of type A by usingMenger (D (M)) and multifractal models (D (-10)-D (10)) are smaller than those of type B. (2) The Menger model is usedto describe heterogeneity of smaller pore size distribution, whichis proportional to volume percentage of pores with the diameter smallerthan 100 nm; the thermodynamic model is used to describe heterogeneityof medium pore size distribution, which is proportional to the volumepercentage of pores with a diameter of 100 & SIM;1000 nm; The Sierpinskimodel is used to describe heterogeneity of larger pore size distribution,which is proportional to volume percentage of pores with the diameterlarger than 1000 nm; (3) permeability decreases in the form of powerfunction with the increase of confining pressure, and the compressibilitycoefficient and permeability variation coefficient decrease with theincrease of the compressibility coefficient. There is no significantcorrelation between the compressibility coefficient, permeabilityvariation coefficient, and pore structure parameters.