Effects of Bitumen Extraction on the 2D NMR Response of Saturated Kerogen Isolates
- Zeliang Chen (Rice University) | Philip M. Singer (Rice University) | George J. Hirasaki (Rice University)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 58th Annual Logging Symposium, 17-21 June, Oklahoma City, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2017. copyright held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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There is mounting evidence that 2D T1-T2 NMR can be successfully used for fluid typing and saturation in organic shale. More specifically, it has been generally observed that in organic-shale cores the T1/T2 ratio is higher for saturating hydrocarbons than for saturating water, which could be exploited as a technique for NMR saturation logging in unconventional organic-shale reservoirs. This paper presents a continuation of a fundamental study into the 2D NMR response of saturating fluids in the organic-matter pores of pelletized kerogen isolates. Working with kerogen isolates avoids any potential complications from saturation history and wettability alterations of as-received organic-shale cores.
Previous reports of T1-T2 data of heptane-saturated kerogen pellets indicate two distinct peaks. One peak has a similar porosity to a random bead-pack, and is interpreted as heptane in the kerogen inter-granular pores created during pelletization. The other peak has a large T1/T2 ratio, and is interpreted as heptane absorbed in kerogen granules. In this study, we look at the influence of bitumen extraction before pelletization and saturation with heptane, and use supporting data such as nitrogen adsorption BET and UV-VIS absorption spectroscopy to enhance the interpretation of the NMR data.
For the fast-relaxing peak, T1 and T2 remain the same after bitumen extraction, however the porosity attributed to this peak decreases. This has two interpretations: (1) heptane absorbed in bitumen has the same T1 and T2 as the bitumen extracted kerogen; (2) the response is due only to heptane absorbed in the bitumen and the bitumen is not entirely removed in the extracted sample.
For the slow-relaxing peak, there is an observed decrease in surface relaxivity for heptane after bitumen extraction. This is attributed to the extraction reducing intra-granular porosity which lowers the diffusivity of heptane into the granule due to greater restriction. This in turn reduces its surface relaxivity of the heptane in the inter-granular pores.
Our findings provide key insight into the role of kerogen and bitumen on the NMR response in organic shale, which can be used to improve fluid typing and saturation estimates from 2D T1-T2 NMR data, both in the lab and from down-hole logs.
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