A New NMR-Based Method for Wettability Assessment in Mixed-Wet Rocks

Abstract

AbstractNuclear Magnetic Resonance (NMR) measurements can be used for quantifying rock wettability because they are sensitive to the type of fluids in contact with grains. Conventional experimental NMR-based methods for wettability determination often need NMR response for the same rock sample in different wettability states at irreducible water saturation and at residual oil saturation, which is time-consuming and could provide non-unique results. There also exist simulation-based methods for characterizing wettability in mixed-wet rocks. These methods either assume oversimplified pore shapes or require microComputed Tomography (CT) images of partially-saturated mixed-wet rock samples and complex image processing algorithms, which makes them computationally expensive. In this paper we introduce a new NMR-based wettability index for characterizing rock wettability and test its applicability in both carbonate and sandstone rock samples using numerical simulations.The new NMR-based wettability index is a function of measured transverse magnetization (72) of mixed-wet rock, bulk relaxation and saturation of each phase, and simulated T2 distributions for fully water-saturated and fully hydrocarbon-saturated. We then use pore-scale simulation of NMR at different wettability states to verify the applicability and reliability of the newly introduced wettability index in rock samples with different pore structures including carbonate and sandstone samples. The first step in model verification is to obtain pore-scale images of few rock samples in each rock type in the formation. We then perform simulation of NMR T2 response for rocks with different wettability levels by numerically solving Bloch-Torrey equation using finite volume method. Then we generate mixed-wet synthetic samples, simulate the T2 response in the synthetic samples, and use the simulation results as inputs for determining their NMR-based wettability index.The results demonstrate the reliability of the new NMR-derived wettability index. The introduced NMR-based method for wettability assessment allows for determination of wettability at any saturation state in the rock samples. It also eliminates the need for NMR calibration at irreducible water saturation and residual oil saturation and thus, might reduce the overall time required for determining wettability of mixed-wet samples. Successful application of the introduced workflow expedites assessment of wettability index and wettability changes in mixed-wet reservoir rocks.