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 2 SO 4 group. Fractal analysis of the pore structure revealed that no conclusive trend was observed for fractal dimension of mini-pores D 1 . Fractal dimension of mesopores D 2 ranged from 2.79 to 2.93, indicating a medium complexity pore structure of the mesopores. Fractal dimension of macropores D 3 was over 2.9, implying that the pore structure of the macropores is the most complex. The fractal dimension of the T 2 spectrum D NMR ranged from 2.55 to 2.77. Correlations between the fractal dimensions and porosity are also presented. Results showed that D 2 and D 3 can be good indicators for the pore size volume of sandstone samples immersed in H 2 SO 4 , NaOH and NaCl solutions, while D NMR is a good indicator for the pore size volume of sandstone samples immersed in NaOH solution and pure water.INDEX TERMS Pore structure, freeze-thaw, chemical corrosion, nuclear magnetic resonance, fractal analysis.

Research on Strength Prediction Model of Sand-like Material Based on Nuclear Magnetic Resonance and Fractal Theory

“…Many scholars have discussed and analyzed the classification of pore structure in rock and put forward various methods. Referring to research results of the classification of sandstone pore structure [9,39], this study divides internal pores of the material into three types: micropore (pore radius < 0.1 µm), mesopore (0.1 µm < pore radius < 1 um), macroporous (pore radius > 1µm), the specific division is shown in Figure 2. Furthermore, T2 spectral area is an important parameter reflecting the number of pores with different radius, it can quantitatively describe the distribution of pores in the total pores of different structural types within the rock.…”

“…In Formula (10), T is total number of data samples, n is forecast periods number, Y i is the actual value of the variable in the period i, Y ip is the prediction value of the variable in period i. By substituting the data in Tables 4 and 5 into Formula (9) and Formula (10), it turns out that MAPE equals 4.9841%. In conclusion, the expression of sandstone -like material strength prediction model established in this paper has a high correlation coefficient and small MAPE.…”

“…Many scholars have taken NMR as the main research method and carried out experimental studies on the microstructure and mechanical properties of rock materials. Zhang [9] and Deng [10] discussed the micro-pore structure evolution of sandstone under freeze-thaw cycles by NMR analysis. Liu [11] and Jiang [12] carried out the cyclic freeze-thaw test and cyclic dynamic impact test on granite and sandstone, studying the microporosity changes of different rocks under cyclic impact by NMR.…”

NMR Experimental Study on Dynamic Process of Pore Structure and Damage Mechanism of Sandstones with Different Grain Sizes under Acid Erosion

“…As can be seen from Formulas (7)- (12), the chemical reactions in rocks differ according to the mineral composition and cause different types of chemical erosion damage. In this study, quartz was the main mineral in the sandstone samples, accounting for a large proportion of the total mineral contents.…”

“…Fang et al [11] studied the combined effects of chemical erosion and freeze-thaw cycles on the compressive failure characteristics of yellow sandstones. Zhang et al [12] studied pore structure changes in sandstones subjected to rapid freeze-thaw cycles and chemical erosion. Li et al [13] studied the effects of acid solution type and soaking time on the physical and mechanical properties of sandstones.…”