Sensitive New NMR Hybrid T1 Measurements for Gas Shale, Heavy Oil, and Microporosity Characterization

Abstract

Abstract For NMR-based formation evaluation, measurement of the longitudinal relaxation time, T1 is a preferred technique for LWD because it is less vulnerable to vibration as compared to T2 measurements. For wireline NMR logging, interpreting T1 data is simpler than T2 because T1 is not affected by the additional signal decay caused by the molecular diffusion in magnetic field gradients. Moreover, T1/T2 provides additional formation and fluid information than T2 alone. Despite these benefits, the commonly used T1 measurements may suffer from either very long measurement times using the inversion-recovery (IR) data-acquisition method or reduced sensitivity in the short-relaxation time range using the saturation-recovery (SR) method. Therefore, there is an increased interest in developing more efficient T1 measurements that have adequate sensitivity for fast- relaxing components abundant in gas shale, heavy oil, and carbonates with microporosity present. We describe a new approach of using a hybrid saturation-inversion-recovery (HSIR) sequence that takes advantage of the higher sensitivity for the short inversion times in the IR sequence and the efficient data-acquisition feature in the SR sequence. To demonstrate the benefit, we simulated many cases representative to gas shale, heavy oil, and carbonate rocks using the standard SR and new HSIR methods. We used Fréchet distance to quantitatively determine the resemblance between the input models and the inversion results and found that HSIR is consistently better in terms of the fidelity to the model. Core measurements are also presented to support the simulation studies. It is shown that HSIR is able to differentiate signal from organic matter in shale samples from other signals, while SR method is not; it is also demonstrated that HSIR can better separate microporosities in carbonate samples than SR, while using a fraction of the time used in IR method.