Pore-Scale Investigation of the Electrical Property and Saturation Exponent of Archie’s Law in Hydrate-Bearing Sediments

Abstract

Characterizing the electrical property of hydrate-bearing sediments is essential for hydrate reservoir identification and saturation evaluation. As the major contributor to electrical conductivity, pore water is a key factor in characterizing the electrical properties of hydrate-bearing sediments. The objective of this study is to clarify the effect of hydrates on pore water and the relationship between pore water characteristics and the saturation exponent of Archie’s law in hydrate-bearing sediments. A combination of X-ray computed tomography and resistivity measurement technology is used to derive the three-dimensional spatial structure and resistivity of hydrate-bearing sediments simultaneously, which is helpful to characterize pore water and investigate the saturation exponent of Archie’s law at the micro-scale. The results show that the resistivity of hydrate-bearing sediments is controlled by changes in pore water distribution and connectivity caused by hydrate formation. With the increase of hydrate saturation, pore water connectivity decreases, but the average coordination number and tortuosity increase due to much smaller and more tortuous throats of pore water divided by hydrate particles. It is also found that the saturation exponent of Archie’s law is controlled by the distribution and connectivity of pore water. As the parameters of connected pore water (e.g., porosity, water saturation) decrease, the saturation exponent decreases. At a low hydrate-saturation stage, the saturation exponent of Archie’s law changes obviously due to the complicated pore structure of hydrate-bearing sediments. A new logarithmic relationship between the saturation exponent of Archie’s law and the tortuosity of pore water is proposed which helps to calculate field hydrate saturation using resistivity logging data.

Research Progress on the Electrical Properties of Gas Hydrate‐bearing Sediments

“…With increased hydrate saturation, the conductivity of pore fluid in the system decreased, resulting in an increase in the impedance amplitude. At the same time, the dispersion degree of the impedance amplitude, which is the difference between the modulus of The X-CT images showed that a pore is divided into five pores after hydrate formation, resulting in a greater coordination number and more tortuous paths of pore water (modified from Zhao et al, 2022). measured impedance at low frequency and high frequency, also increased accordingly (Fig.…”

“…Many experiments have shown that the resistivity change pattern of HBS samples during hydrate formation is the integrated result of the ion concentration effect and the pore blocking effect. Zhao et al (2022) observed that with an increase of hydrate saturation, the resistivity of samples decreased to a certain extent, which was speculated to be influenced by the ion concentration in the pore brine, because the hydrate formation process only consumes pure water. Yang (2019) analyzed the resistivity measurements of the methane hydrate formation in brinesaturated pure quartz sand (Fig.…”

“…They found that there was an obvious non-Archie phenomenon, since the saturation index of Archie's formula increased exponentially from 1.57 to 2.48 within the hydrate saturation range of 17.13% -30.72%. Similarly, Zhao et al (2022) combined X-ray CT and the four-electrode method to characterize the three -dimensional spatial structure of HBS and measure the resistivity during hydrate synthesis, and studied the variation rule of the saturation index of Archie's formula at the micro-pore scale. Compared to the initially large paths without hydrate, the electrical current would have passed through a smaller and narrower path after hydrate formation, which led to the increase of HBS resistivity.…”

Review and Outlook of Visual and Geophysical Detections for the Pore-Scale Morphology of Gas Hydrates

“…Several laboratories have developed devices for simultaneous visual observations and geophysical measurements. For example, the Qingdao Institute of Marine Geology, China Geological Survey, has developed a joint X-CT and acoustic/electrical detection device. − Bu et al conducted a joint X-CT and P-wave velocity/resistivity test. They found that the sample’s P-wave velocity did not increase linearly with the increase of hydrate saturation but presented stage characteristics related to the change of the hydrate morphology.…”

Integration of Pore-Scale Visualization and an Ultrasonic Test System of Methane Hydrate-Bearing Sediments

“…Zhang et al developed a new testing assembly that combines micro-X-ray computed tomography scans and low-field nuclear magnetic resonance tests to determine pore-scale characteristics, where the obtained test results can be used to evaluate the pore structure of hydrate-containing sediments [54]. Zhao et al combined X-ray computed tomography and resistivity measurement techniques to simultaneously derive the three-dimensional spatial structure and resistivity of hydratebearing sediments [55]. Similarly, some scientists combined X-CT detection techniques with ultrasonic testing technology, such as the high-pressure multi-property characterization chamber (MPCC) developed by Seol et al Such systems provide insight into the total characteristics of hydrate-bearing sediments in the laboratory, as well as in the field of pressure core technology [56].…”