Wettability Assessment in Complex Formations Using NMR Measurements: Workflow Development and Experimental Core-Scale Verificiation
- Chelsea Newgord (The University of Texas at Austin) | Saurabh Tandon (The University of Texas at Austin) | Zoya Heidari (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) measurements are typically used for estimating fluid saturation, pore structure, and sometimes wettability assessment. However, interpretation of NMR measurements can be challenging in rocks with both complex pore structures and unknown wettability due to multiple peaks in the transverse (T2) responses. Reliability of the conventional NMR-based wettability models has been verified only for rocks with unimodal pore-size distributions. This limits the application of existing NMR-based models for wettability assessment, especially for complex carbonates with multi-modal pore-size distributions. The objectives of this paper are (i) to develop a new workflow that uses NMR measurements to simultaneously estimate fluid saturation and wettability and (ii) to verify the reliability of the new workflow in rocks with unimodal and multi-modal pore-size distributions at different levels of water and hydrocarbon saturation.
The new workflow requires T2 and Diffusivity-T2 (D-T2) or longitudinal-T2 (T1-T2) NMR measurements of the rock-fluid system, saturating fluids, fully water-saturated water-wet, and fully oil-saturated oil-wet rocks. The outputs of this workflow include water saturation and wettability. To experimentally verify the reliability of the introduced workflow, we chemically alter the wettability of core samples to be water-wet, mixed-wet, or oil-wet. We confirm and quantify the wettability level in each core sample using the Amott Index and contact angle measurements. Using a low-frequency 2-MHz NMR spectrometer, we measure the T2, D-T2, and T1-T2 responses of the rock-fluid system and the aforementioned model inputs. We vary the fluid saturation using a core-flood setup and obtain NMR measurements at different fluid saturations. Finally, we input the NMR measurements to the new workflow to simultaneously estimate water/hydrocarbon saturation and wettability.
We successfully applied the new NMR-based workflow to different types of sandstone and carbonate rocks with unimodal and multi-modal pore-size distributions. The reliability of the new workflow was verified by comparing the fluid saturation estimates with the gravimetric measurements, and the wettability estimates with the Amott Index and contact angle measurements. As the non-wetting fluid was injected into the core samples, the T2 distribution varied non-linearly due to the combined impact of wettability and complex pore structure. The calculated NMR-based wettability index ranged from −0.7 to 0.6 for all the core samples, while the Amott Index ranged from −0.6 to 0.6 indicating oil-wet to water-wet samples. The NMR-based wettability index was consistent with contact angle measurements and had an average absolute difference of 0.18 when compared to the Amott Index. The successful core-scale application of the introduced method is promising for its applications to borehole NMR measurements for depth-by-depth in-situ assessment of wettability, if the model input parameters are reliably assessed, as described in the proposed workflow.
|File Size||1 MB||Number of Pages||15|