NMR experimental measurement and analysis of Permian-Triassic carbonate rocks in the Sichuan Basin

doi:10.3969/j.issn.1672-7703.2020.03.015

Abstract

Abstract: There are various types of carbonate reservoirs, generally with low matrix porosity, well developed fractures and cavities, and complex pore structures. Pore structure and physical properties are the key factors for evaluation of reservoirs and their oil and gas-bearing properties. In order to study the pore types of core samples and the distribution laws of pore structure in reservoirs from multiple perspectives, a total of 45 core plugs, sampled from dolomite in the Triassic Leikoupo Formation and from tight limestone in the Lower Permian Maokou Formation in the Sichuan Basin, were tested for conventional porosity and permeability, and also subjected to rock-electrical experiments and Nuclear Magnetic Resonance testing. The results show that, for the dolomite reservoir of the Leikoupo Formation, the overall NMR T2 spectrum of the core samples shows obvious single-peak, double-peak and triple-peak characteristics, indicating that there are triple-porous media in the cores (matrix pores, fractures and dissolution vugs). The pore structure is of strong heterogeneity, and the overall physical properties are poor. NMR measurements of tight limestones show that the T2 spectrum presents a significant ‘double peak’ shape. The reservoir space is matrix intercrystalline pores and dissolution pores. The left peaks of the T2 spectra of different cores are basically stable at about 0.1ms, indicating that the pore size is smaller than that of dolomite in the Leikoupo Formation. The maximum absolute error between NMR permeability inverted by the SDR model and permeability measured by gas measurement is 0.31 mD, which shows good consistency. Meanwhile, the NMR results also reflect shale content to a certain extent. For cores with higher shale content, the front-end signal of the NMR T2 spectrum is strong, the distribution width is large, and NMR porosity is usually larger than gas porosity. The experimental measurement and analysis methods used in this paper are generally applicable to carbonate fractured-vuggy reservoirs throughout the Sichuan Basin.

Key words: carbonate rocks, Nuclear Magnetic Resonance, pore structure, porosity, permeability, SDR model

CLC Number:

Feng Qingfu, Zhai Xiufen, Feng Zhou, Liuzhu Ruizhi, Liu Yingming. NMR experimental measurement and analysis of Permian-Triassic carbonate rocks in the Sichuan Basin[J]. China Petroleum Exploration, 2020, 25(3): 167-174.

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