A New Workflow for Joint Interpretation of Electrical Resistivity and NMR Measurements to Simultaneously Estimate Wettability and Water Saturation

ABSTRACT

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 multi-physics 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 (i) to introduce the workflow for joint interpretation of resistivity and NMR measurements to simultaneously estimate wettability and water saturation, and (ii) to verify the reliability of estimates of wettability and water saturation by comparison to experimentally measured contact angles, Amott Indices, and gravimetrically assessed water saturation.

The new workflow for assessing wettability and water saturation combines non-linear resistivity- and NMRbased 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) responses of the rock-fluid system, of the saturating fluids, and of water-wet water-saturated and oil-wet oil-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 non-linear models, which are simultaneously solved to estimate wettability and water saturation for each core sample. We verify the reliability of wettability estimates by comparison to the Amott Index and contact angle measurements, and the water saturation estimates by comparison to the gravimetrically measured water saturation.

We successfully verified the reliability of the new method for 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 multi-modal pore-size distribution uniquely from the interpretation of NMR measurements. The introduced integrated workflow improved the accuracy of water saturation estimates in rocks with complex pore structure. For the wettability ranging from oil-wet to water-wet, we observed an average absolute difference of 0.15 between the experimentally measured Amott Index 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 multi-physics workflow eliminates the non-uniqueness associated with wettability and water saturation estimates obtained from independent interpretation of NMR and resistivity measurements.