Insights into Pore Types and Wettability of a Shale Gas Reservoir by Nuclear Magnetic Resonance: Longmaxi Formation, Sichuan Basin, China

摘要

Reservoir properties, such as pore types and wettability, are essential for shale gas reservoir evaluation. However, advanced nuclear magnetic resonance (NMR), which has been routinely used in petrophysical characterization of reservoirs, is barely used to estimate these properties in shale gas reservoirs. In this study, several sets of specially designed NMR measurements, together with total organic carbon (TOC), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and contact angle tests, are used to study the pore types and wettability of Longmaxi shale gas reservoir. Results show that the NMR transversal relaxation time (T-2) spectrum can be used to characterize pore types and wettability of gas shale. Three identified T-2 spectral peaks (0.01-0.4 ms, 0.4-15 ms, and 15 ms) separately correspond to organic pores, inorganic pores, and microfractures, which is consistent with the results of FE-SEM. The T-2 spectra in as-received and water/ oil-imbibitionstates qualitatively prove that the microwettabilities of organic pores, inorganic pores, and microfractures are oil wet, water-wet, and mixed-wet, respectively. In addition, a novel wettability index is defined to reflect the microwettability of pores quantitatively. The dominant minerals and TOC jointly affect the pore wettability index; they show good correlations with the wettability index of inorganic and organic pores, whereas there is no obvious correlation with the wettability index of microfracture. In contact angle measurements, water and oil droplets show different behaviors on the surface of shale specimens, which qualitatively indicates that the macrowettability of Longmaxi gas shale is mixed-wet (both water-wet and oil-wet) and more prone to be oil-wet. After analysis of the theory of characterizing macrowettability by NMR, a new NMR-based model is proposed to characterize the macrowettability quantitatively. In summary, this study proposes novel methods and models to characterize the pore types and wettability, expanding the use of NMR in shale gas reservoirs.