A Precise Porosity Measurement Method for Oil-Bearing Micro/Nano Porous Shales Using Low-Field Nuclear Magnetic Resonance (LF-NMR)
Low-field nuclear magnetic resonance (LF-NMR) is a non-invasive and effective method for quantitatively characterizing rocks, and has been used to characterize nanopore structure of shales. However, water (or brine) has commonly been used to saturate samples in the previous LF-NMR tests of shale, resulting in hydration swelling. In this study, LF-NMR analyses of kerosene-saturated shales were performed under a variety of test conditions (i.e., various waiting times and echo times) to establish a precise measurement method for characterizing oil-bearing micro/nano porous shale porosity. Furthermore, helium (He) porosity measurements were used to verify the porosity determined by LF-NMR. Comparing the T 2 spectra pertaining to various waiting times, the optimal waiting time was determined. Subsequently, using He-porosity as a benchmark, the relative errors associated with NMR-determined porosities and He-determined porosities were estimated such that the optimal echo time could be determined. The primary findings of this study include followings: (1) The longer the waiting time, the more completely polarized the protons becomes; the protons in the kerosene within shales evaluated in the present study were approximately completely polarized as the waiting time is 1500 ms. (2) Echo time is a parameter that is critical to improving the accuracy of shale porosity determined by LF-NMR; at longer echo time, nanopores within the shales may not be detected resulting in a smaller NMR porosity, while at much shorter echo time (0.075 ms), the relaxometry data of kerogen can be detected resulting in a higher NMR porosity. An echo time of 0.2 ms is required to minimize relative errors. (3) For the shales in the Dongying depression, the most reasonable waiting time and echo time for LF-NMR porosity tests were 1500 ms and 0.2 ms, respectively.
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Document Type: Research Article
Affiliations: Research Institute of Unconventional Oil and Gas and Renewable Energy, China University of Petroleum (East China), Qingdao, Shandong 266580, P. R. China
Publication date: September 1, 2017
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