A New Method of Estimating Tortuosity and Pore Size in Unconventional Formations Using NMR Restricted Diffusion Measurements
- Xinglin Wang (Rice University) | Philip M. Singer (Rice University) | Zeliang Chen (Rice University) | George J. Hirasaki (Rice University) | Harold J. Vinegar (Vinegar Technologies, LLC)
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- Society of Petrophysicists and Well-Log Analysts
- SPWLA 61st Annual Logging Symposium - Online, 24 June - 29 July, Virtual Online Webinar
- Publication Date
- Document Type
- Conference Paper
- 2020. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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Tortuosity and pore size are two petrophysical properties useful for estimating permeability. Given the small pore systems in unconventional rocks, the free diffusion of the pore fluids is restricted by the pore walls. NMR restricted diffusion measurements of fluids in porous media can provide both tortuosity and pore-size information.
In this study, we focus on a series of low permeability organic-rich chalks with connate water present. The core samples are pressure-saturated with two hydrocarbons, high-pressure methane (C1) and decane (C10). NMR measurements are conducted under pressure to obtain the restricted diffusivity of the hydrocarbon-bearing pore space.
In planning the NMR restricted diffusivity measurements, an optimum series of diffusion-encoding times are chosen for the pulse-field gradient (PFG) pulse sequence to obtain the correlation between the restricted diffusivity (D) and free diffusion length (LD). The Padé fit on D versus LD is then used to estimate the tortuosity and pore-size of the hydrocarbon-filled pore space. The restricted diffusivity of C10 lies in the short diffusionlength regime, which determines the surface-to-volume ratio and pore-body size. The restricted diffusivity of high-pressure C1 lies in the long diffusion-length regime, which determines the tortuosity.
We also report on the accuracy of the tortuosity and poresize using propagation of errors in the Padé fit.
The exploration and production of unconventional reservoirs are attracting more and more attention. One of the most important parameters in producing from an unconventional formation is the permeability, which is typically less than 0.1 mD. Various direct permeability measurements, including the steady-state and unsteadystate methods, have been widely used (Bernabé et al., 2006; Sander et al., 2017; Chen et al., 2020). For predicting permeability (Carman, 1997), it is important to understand the relationship between permeability and other petrophysical properties such as porosity, porebody size, pore-body/pore-throat ratio, and tortuosity.
In this report, restricted diffusion is used to estimate the pore-body size and the diffusive tortuosity. Molecular self-diffusion is restricted by the pore wall, especially in unconventional formations where the pore size is small. As a result, the restricted diffusion contains information about pore-body size and diffusive tortuosity (Mitra et al., 1992; Latour et al., 1993). The NMR technique is widely used to measure the restricted diffusion D of the pore fluid and as a function of diffusion length LD (Latour et al., 1993; Zecca et al., 2018; Chen et al., 2019; Yang et al., 2019). Latour et al., 1993 studied the tortuosity in water-saturated glass beads pack. Zecca et al. 2018 studied the tortuosity in the conventional sandstone formation, and observed the tortuosity limit. Yang et al. 2019 used methane to study conventional carbonate formations with permeabilities from 2 mD to 5,000 mD, and observed the tortuosity limit.
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