Abstract:

In recent years, the exploration and development of shale oil and gas has been increasingly extensive, and many challenges have come with it. Porosity is a fundamental parameter for shale reservoir evaluation and reserve estimation, which provides element data for the determination of sweet spots and the formulation of development plans. Low-field nuclear magnetic resonance (LF-NMR) technology has the non-invasive, non-destructive characteristics and has become a crucial method for shale porosity measurement. But shale has significantly different nuclear magnetic resonance (NMR) response mechanism from that of conventional sandstone and carbonate reservoir rocks. In addition, improper experimental parameters and inversion process can lead to errors or even faults in the measurement of shale porosity by NMR. In this paper, first, T₁-T₂ correlation is adopted to qualitatively identify the hydrogen-containing components of dry and saturated shale. Then, on account of the characteristic that the NMR signal is proportional to the number of spin protons in the measured sample under uniform static magnetic field and constant temperature field, an experimental method is proposed to directly measure the porosity of shale by using the difference of the first amplitude of the NMR free induction decay (FID) signal between formation water saturated shale and dry shale after calibration with standard water sample. The experimental results show that the NMR porosity provided by this method is in good agreement with the weight porosity, and the influence of the background signal of hydrogen-containing components of the matrix is eliminated.

Key words: Organic matter, Shale, Nuclear magnetic resonance, Petrophysics, Porosity determination