Fractal analysis of tight gas sandstones using high-pressure mercury intrusion techniques

Abstract

Pore structure is one of important factors affecting the properties of sedimentary rocks, however, it is difficult to describe microscopic parameters of the pore structure such as pore geometry, pore-size distribution, and pore space topology. Fractal theory is an effective and available method to quantify the complex and irregular pore structure of rocks. Routine rock properties measurements and high-pressure mercury intrusion tests (HPMI) were performed on a suite of the Bashijiqike tight gas sandstone samples to delineate the pore network characteristics of these reservoir rocks. Thin section epifluorescence and scanning electron microscopy (SEM) analyses were used to gain insight into pore geometry and pore size distribution of these sandstones.The results show that the pore system of the sandstones mainly consists of intergranular macropores and intragranular micropores. The HPMI analysis shows that these reservoir rocks have complex, heterogeneous microscopic pore structure. There are clear inflection points on the fractal curves of log (SHg) versus log(Pc), i.e., the fractal curves break into two segments at the capillary pressures corresponding to the apex of the Pittman's hyperbola (plot of the ratio of mercury saturation over capillary pressure against mercury saturation). Fractal dimensions were calculated using the slope of straight part of each curves. Small pores (<rapex) tend to have fractal dimension (Df) values less than 2.5, while large pores (>rapex) were more likely to have Df values larger than 3.0. Df for small pores is strongly correlated with r50 and r35, while for large pores, no obvious relationship exists between Df and the pore structure parameters. Df for small pores could be used to evaluate the microscopic pore structures and heterogeneities of reservoir rocks. Over-simplification of cylinder shape of pore space, the high working pressure and well developed micro-fractures result in the high Df values (>3.0) for large pores (>rapex).