New Insight into the Petrophysical Characterization of Shales with Different Fluid Saturation States Based on Nuclear Magnetic Resonance Experiments

摘要

Petrophysical properties are important parameters for quantitatively evaluating the production of oil and gas reservoirs. Shale reservoirs have complicated pore structures and solid compositions, which makes reservoir evaluations more difficult than the conventional reservoirs. As a nondestructive technique, nuclear magnetic resonance (NMR) has been widely applied in both laboratory and field explorations of shale reservoirs. Many researchers have measured water-saturated shale samples using NMR to investigate porosities, pore structures, and free fluid saturations. However, because of the presence of organic matter and complex wettabilities, saturating shale samples with different types of fluids may change the obtained results. To provide new insight into the petrophysical characterization of shale, we performed NMR experiments (both one-dimensional and two-dimensional NMR) on marine gas shale samples from the upper Ordovician Wufeng formation-the lower Silurian Longmaxi formation in the Sichuan Basin of China. Shale samples were examined by NMR in three states, including oil-saturated, water-saturated, and oven-dried. Moreover, we performed routine porosity measurements, scanning electron microscopy, and nitrogen (N-2) adsorption experiments to comprehensively study the effect of different fluid saturation states on the petrophysical characterization results. The results indicate that more NMR signals can be detected by saturating shale with water, which is useful for estimating petrophysical properties; however, the T-2 distributions of the water-saturated shale samples provide limited information for gas production. The oil-saturated shale samples show two different shapes of T-2 distributions. Shale samples from low-production wells show one main peak, while samples from high-production wells show distinct triple peaks in the T-2 distributions (approximately at 0.22, 3.3, and 170 ms). These peaks represent the existence of microorganic pores, macroorganic pores, and microfractures. A novel identification graph of hydrogen-bearing compositions of organic-rich shale was put forward by analyzing the results of T-1-T-2 maps. The wettability index calculated with the NMR results was determined to be related to the gas production capacity of a well. The findings of our research studies will be useful for studying the pore type and wettability of shale and evaluating the gas production capacity of the well.