Pore-Size-Dependent Fluid Substitution Method for Improved Estimation of NMR Porosity, Permeability, and Relaxation Times
- David Medellin (The University of Texas at Austin) | Ali Eghbali (The University of Texas at Austin) | You Wang (The University of Texas at Austin) | Carlos Torres-Verdín (The University of Texas at Austin)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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Nuclear magnetic resonance (NMR) distributions are used to estimate porosity, pore-size distributions, and permeability in fully water-saturated rocks. Residual hydrocarbon in the invaded zone and oil-based mud invasion impact the NMR response and change its interpretation. These effects can be observed in logging-while-drilling (LWD) and wireline (WL) NMR logs, where the NMR distributions may exhibit appreciable differences even though both were acquired in the same formation. We implement a pore-size-dependent fluid substitution (PSDFS) method to transform a nuclear magnetic resonance distribution acquired with an LWD NMR instrument into an equivalent NMR distribution acquired with a WL NMR instrument. The PSDFS method uses a step-function saturation profile that depends on the pore’s relaxation time value at full water saturation.
We introduce a workflow to estimate the necessary NMR parameters for the PSDFS method and show how to correct the measurements for partial oil saturation to get the T2 distribution of a corresponding fully water saturated reservoir rock. It is found that the PSDFS method improves the correlation between wireline and fluid-substituted LWD T2 distributions.
Nuclear magnetic resonance (NMR) T2 distributions are used to estimate porosity, pore-size distributions, and permeability in fully water-saturated rocks (Kleinberg, 1994; Kenyon, 1997; Wang et al., 2004). However, NMR T2 distributions acquired with downhole tools or laboratory measurements in the presence of hydrocarbon or drilling mud filtrate can exhibit additional and shifted peaks when compared to T2 distributions of fully water saturated rocks. The additional peaks correspond to bulk relaxation times of oil; T2 distribution shifts are due to thin water films (Straley et al., 1995; Hursan et al., 2018). The extra and shifted T2 peaks can not only impact the estimation of total porosity due to differences in hydrogen index but can severely affect permeability estimation as well as interpretation of bound and free water volumes (Chen et al., 2004, 2006). This situation is of concern in wireline logging, where invasion changes the original fluid in place thus giving rise to a mismatch between logging-while-drilling (LWD) and wireline NMR logs.
We apply a pore-size-dependent fluid substitution (PSDFS) method (Medellin et al., 2019) to NMR T2 distributions acquired with LWD instruments and compare them to the corresponding wireline NMR T2 distributions. To that end, a set of LWD T2 distributions is transformed into fluid substituted T2 distributions that more closely match wireline T2 distributions.
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