Key Insights from Comparing LWD and Core NMR in Heavy Oil Carbonates

Abstract Recent advances in LWD (logging-while-drilling) NMR (nuclear magnetic resonance) have enabled the simultaneous measurement of T1 and T2. These advances bring great benefits to the quality of the acquired NMR data. However, the question often asked is whether LWD T1 and T2 can be used to deliver additional information about hydrocarbon quality. The comparison of LWD and core NMR datasets acquired in the same well provides answers to that question. LWD NMR data was acquired in a deviated well penetrating a carbonate reservoir. NMR T2 analysis served as the basis for hydrocarbon quality evaluation, to derive the heavy-oil volume and compute hydrocarbon viscosity. The LWD acquisition was performed in washdown mode after coring. In parallel, special core analysis, including NMR, rock-eval pyrolysis, tight rock analysis, liquid and core flooding experiments, quantified the heavy hydrocarbon volume in the core and helped characterize its impact on mobility. Laboratory NMR analysis was performed using a 2 MHz instrument. The T1T2 maps from the LWD NMR tool and from the NMR core analysis were compared for each of the core depths. Total core NMR porosity was found to be lower than LWD NMR porosity on several samples due to evaporation of either lighter hydrocarbon or water phase, during the core retrieval process, and this impacted the longest T1 and T2 components typically associated with more volatile fluids. A good agreement was observed between LWD and core NMR T1T2 maps for the mid T1 and T2 range. In the short T2 range, the signature was dominated by the heavy hydrocarbon. Shorter T2 amplitudes on core NMR than LWD NMR were associated with the heavy hydrocarbon signature, due to differences in measurement conditions between the core and reservoir, which impact hydrocarbon viscosity. Core NMR measurements also showed higher T1/T2 ratios at the very short T2 range, highlighting a higher sensitivity to fluid type for heavy-hydrocarbon characterization. The T1/T2 ratios differences can be explained by sampling parameters of the NMR instruments (the lab NMR instrument frequency is an order of magnitude higher than that of the LWD NMR tool). There was a concordant trend between the hydrocarbon viscosity evaluated from the LWD NMR and the core-based tight rock analysis and pyrolysis measurements. For the first time, LWD NMR and core NMR T1T2 maps have been compared for the evaluation of heavy hydrocarbons. This paper brings a unique understanding of the complementary strengths of both measurements, integrating them to provide a comprehensive assessment of the varying quality of the hydrocarbon column assessed in this challenging reservoir.