Joint Interpretation of Electrical Resistivity andT2 NMR Measurements To Estimate Wettability and Water Saturation
Authors: Newgord Chelsea , Posenato Garcia Artur , Heidari Zoya 
Published: March 2020

The wettability of rocks can be assessed from interpretation of borehole geophysical measurements, such as electrical resistivity and nuclear magnetic resonance (NMR). These wettability models often require additional inputs (e.g., water saturation, porosity, and pore-geometry- related parameters), which are difficult to obtain independently. Consequently, a multiphysics workflow that integrates resistivity and NMR measurements can reduce the number of input parameters, resulting in a more accurate and robust wettability assessment. The objectives of this paper are to introduce a new workflow for joint interpretation of resistivity and NMR measurements to simultaneously estimate wettability and water saturation, and to verify the reliability of wettability and water-saturation estimates by comparison with experimentally measured contact angles, Amott Indices, and gravimetrically assessed water saturation.

This new workflow for assessing wettability and water saturation combines nonlinear resistivity- and NMR-based rock physics models. The inputs to the resistivity-based wettability model include the resistivity of the rock–fluid system and brine, porosity, and pore-geometry-related parameters. The NMR-based wettability model requires the transverse (T2) response of the rock–fluid system, saturating fluids, and water-wet water-saturated and hydrocarbon-wet hydrocarbon-saturated rocks. To verify the reliability of the new integrated workflow, we perform resistivity and NMR measurements on core samples from different rock types, covering a range of wettability and water-saturation levels. These measurements are inputs to the nonlinear models, which are simultaneously solved to estimate wettability and water saturation for each core sample. We verify the reliability of wettability estimates by comparison with the Amott Index (IA) and contact-angle measurements, and the water-saturation estimates by comparison with the gravimetric water-saturation estimates.

We successfully verified the reliability of the new method for this joint interpretation of resistivity and NMR measurements to estimate wettability and water saturation of limestone and sandstone core samples. For water-saturation levels ranging from irreducible water saturation to residual oil saturation, we observed an average relative error of 11% between the gravimetrically assessed and the model-estimated water saturation. It is challenging to estimate water saturation in rocks with multimodal pore-size distribution uniquely from the interpretation of NMR measurements. One contribution of the introduced workflow is improving the accuracy of water-saturation estimates (in addition to wettability) in rocks with complex pore structure and wettability states. For the wettability ranging from hydrocarbon-wet to water-wet, we observed an average absolute difference of 0.15 between the experimentally measured IA and the model-estimated wettability. These model-estimated wettability values were also consistent with the contact-angle measurements. It should be noted that the new workflow relies on physically meaningful and measurable parameters, which minimizes calibration efforts. Furthermore, the multiphysics workflow eliminates the nonuniqueness associated with wettability and water-saturation estimates obtained from independent interpretation of NMR and resistivity measurements.