Abstract

This paper presents a continuation of our fundamental investigations into the 2D T1-T2 NMR response of saturating fluids in the organic-matter pores of pelletized kerogen isolates. We previously reported that T1-T2 data of heptane-saturated kerogen pellets showed two distinct peaks:

  1. a slow-relaxing peak, interpreted as heptane in the kerogen intergranular pores created during pelletization, and

  2. a fast-relaxing peak with large T1/T2 ratio, interpreted as heptane absorbed in kerogen granules, i.e. in intragranular pores.

In this study, we investigate the influence of bitumen extraction on the T1-T2 data of the heptane-saturated kerogen pellets, and we use supporting data, such as kerogen swelling effects, nitrogen adsorption BET, and UVVIS absorption spectroscopy to enhance the interpretation of the NMR data. We find that for the fast-relaxing peak, the T1 and T2 values remain roughly the same after bitumen extraction, however the porosity decreases, which strongly suggests that the fast-relaxing peak is associated with heptane absorbed in the intragranular pores of bitumen and kerogen. For the slow-relaxing (intergranular) peak, we find that the porosity remains roughly the same after bitumen extraction, however the T1 and T2 values increase due to a decrease in apparent surface relaxivity, which we attribute to (weak) diffusive-coupling effects between the inter and intragranular porosities.

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 downhole logs.

Introduction

In the past decade, as the oil and gas production from unconventional reservoirs increased dramatically, the investigations into organic shale have greatly stimulated both NMR log data interpretation and NMR core analysis (Jiang et al., 2013; Kausik et al., 2016; Reeder et al., 2016; Anand el al., 2017; Tandon et al., 2017; Washburn and Cheng, 2017). Among them, studies focused on kerogen have become more and more popular (Ertas et al., 2006; Chen et al., 2012; Singer et al., 2016, 2017; Zhang and Daigle, 2017). Kerogen, which is defined as solid, insoluble and immobile organic matter, constitutes most of the total organic content (TOC) of organic shale (Durand, 1980), which makes characterizing kerogen essential for formation evaluation.

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