Interpretation of Nuclear-Magnetic-Resonance Response to Hydrocarbons: Application to Miscible Enhanced-Oil-Recovery Experiments in Shales

Abstract

Summary Estimation of total reserves in shale/gas and shale/oil reservoirs is challenging but critical. Different logging tools and core-evaluation procedures are used to address this challenge. Nuclear magnetic resonance (NMR) plays a vital role in understanding fluid content, rock/fluid interaction, and determination of pore-body-size distributions. Hydrocarbon (HC)-hosting pore systems in shale includes both organic and inorganic pores. Recoverable HCs include bitumen and light HCs. Their relative fractions are strongly dependent on thermal maturity. Regardless of detailed chemical characterization, “bitumen” is simply defined on the basis of mobility in this study. The apparent mobility of HCs depends on fluid composition, solubility, and reservoir temperature. Historically, NMR laboratory-calibration measurements (nominally, 2-MHz) on core are conducted at room temperature (25–35°C). This study highlights the importance of running NMR tests at reservoir temperature. Experiments were performed for both bulk fluids and fluid-saturated rock samples. The results show that, at a specific temperature, NMR responds only to the fraction of HCs present in the liquid phase. For routine NMR measurement, at 31°C, only the relaxation signals of compounds more volatile than C17 are acquired. Thus, the C17+ fraction would be invisible to NMR at room temperature, but perhaps not at reservoir temperature. This is critical to the interpretation of NMR log response within the early oil and condensate windows, in which C17+ can be a major fraction. Thus, engineers can underestimate movable HCs by using routine core data as a basis for interpretation. On the basis of NMR experiments for several oil samples, we observed that the T1–T2 distributions depend on the overall composition of total HCs and effective mobilities. The results also show that, in the case when both light and heavy HC fractions coexist in a single phase, they do not appear as different clusters in a T1–T2 distribution map. NMR parameters were used to monitor the amount, composition, and effective mobility of remaining HCs after each injection and discharging cycle, during miscible enhanced-oil-recovery (EOR) huff ’n’ puff experiments on Eagle Ford samples.