Porosity Characterization of Organic Shale Using NMR Technique: A New Method Considering Shale Skeleton Signal

Abstract

Accurate quantitative characterization of the porosity in shales is important for resource prediction, reservoir properties evaluation, and selection of sweet spots of shale oil and gas. Unlike conventional sandstones, organic shales have micro-nanopores and kerogen and are generally rich in clay minerals during the oil-generation stage. The NMR signals of solid organic matter (kerogen) and clay mineral structural water in shales can also be detected under the conditions of the lower TE (such as 0.1 ms), which undoubtedly leads to a higher porosity than the true value. The T₂ spectrum tested under the conditions of low TE is a comprehensive reflection of larger pore fluids and shale skeleton (kerogen and structural water) of shales, therefore, the methods for optimizing NMR testing parameters are not suitable for characterizing the porosity of shales with high organic matter and clay mineral content, there is a need to explore a new method for the NMR measurement of the porosity of shales.

Nine continental shales in Damintun Sag, BohaiBay Basin, China, were selected and subjected to pyrolysis, X-ray diffraction (XRD), weighing, and NMR tests to improve the accuracy of NMR porosity measurements. According to the NMR T₂ spectrum and T₁-T₂ spectrum of shale in different oil contents (original, extracted, and saturated oil state), we revealed that the signals of shale skeleton account for 15.77%-43.10% (with a mean of 28.87%) of saturated oil shale. A stronger NMR signal intensity of the shale skeleton was observed for higher TOC and clay mineral contents. The ∆T₂ spectrum of oil present in the pores of oil-saturated shale was constructed based on the difference between the oil-saturated shale and extracted shale and combined with the standard equation of n-dodecane to directly evaluate the porosity. The porosity that was calculated based on the ∆T₂ spectrum versus the weighing method had an absolute error of ≤0.7%, and the relative error was <10%. Compared with the previous NMR method, the new method does not require optimization of the NMR parameters and considers the contribution of organic matter and clay minerals in the shale skeleton to NMR signals, which is more suitable for the porosity evaluation of continental shale with high TOC and clay mineral content. The research results are helpful for NMR characterization of the porosity in shales.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019