Pore characteristics of the carbonate shoal from fractal perspective

Abstract

The carbonate shoal is a notable reservoir for the hydrocarbon enrichment. Fractal theory has been proven to be a robust analytical tool to indicate the pore irregularity and surface roughness for permeable medium. With the aim to study the pore structure of carbonate shoal from fractal perspective, a series of parallel experiments including nuclear magnetic resonance (NMR), high pressure mercury intrusion (HPMI) and scanning electron microscopy (SEM) were carried out in this paper to calculate the fractal dimensions and the results given by various techniques were compared. The relationships between the fractal dimensions along with the petrophysical properties including the pore structure parameters were analyzed. Based on the fractal dimensions, the tested samples and the different pore types could effectively be distinguished. Classification methods for pore system including macropores (dp＞1000 nm), mesopores (1000 nm＞dp＞100 nm), Ⅱ-micropores (100 nm＞dp＞25 nm) and Ⅰ-micropores (dp＜25 nm) in the tight carbonate shoal reservoir were verified by the HPMI-based fractal dimensions. Fractal curves obtained from the NMR data could be divided into two different segments, which respectively corresponds to the seepage pores and adsorption pores. The SEM-based fractal curves were presented as a straight line. Results showed that micropores in the carbonate shoal made greater contribution to the total fractal dimension, although macropores and mesopores tended to have a relatively higher fractal dimension. Porosity and permeability of the tested samples were mainly governed by the meso- and macropores. Negative linear correlations were identified on the one hand between the HPMI-based total fractal dimensions (DHPMI) and the maximum mercury saturation (SHgmax), and the other hand the fractal dimensions of the macropores (DHPMI-macropores) and the average pore size (rave), respectively. Due to the different sensitivities, no clear correlations were obtained between the fractal dimensions obtained from the HPMI, NMR and SEM data in the whole range of pore size distribution, but DHPMI-macropores showed a positive correlation with DNMR-seepage pores.