Fractal Analysis of Pore Structure Development of Sandstone: A Nuclear Magnetic Resonance Investigation

Abstract

The pore structure of rock has a great influence on its physical and mechanical properties. Factors such as chemical corrosion and temperature changes affect the pore structure evolution. In this paper, the pore structure of sandstone was investigated under rapid freeze-thaw (F-T) cycles and chemical corrosion. A nuclear magnetic resonance (NMR) testing system is used to study the pore structure of tight sandstone samples immersed in different chemical solutions after 10, 20, and 30 F-T cycles. Permeability is determined by using empirical method. Results found that permeability is strongly affected by the erosion of NaOH and NaCl solutions. The pores in the rock were divided into three categories based on the pore size, i.e., minipores, mesopores, and macropores. The results showed that the amount of mini-pores and mesopores both decreased with an increase in the number of F-T cycles while the amount of macropores increased for groups of NaOH, NaCl, and pure water. No conclusive trend can be found in the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> group. Fractal analysis of the pore structure revealed that no conclusive trend was observed for fractal dimension of mini-pores D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> . Fractal dimension of mesopores D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ranged from 2.79 to 2.93, indicating a medium complexity pore structure of the mesopores. Fractal dimension of macropores D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was over 2.9, implying that the pore structure of the macropores is the most complex. The fractal dimension of the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> spectrum DNMR ranged from 2.55 to 2.77. Correlations between the fractal dimensions and porosity are also presented. Results showed that D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> can be good indicators for the pore size volume of sandstone samples immersed in H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , NaOH and NaCl solutions, while DNMR is a good indicator for the pore size volume of sandstone samples immersed in NaOH solution and pure water.