Advanced NMR Acquisition Has Improved the Assessment of Hydrocarbons in Place in the Gulf of Mexico

摘要

Traditional NMR-based hydrocarbon-typing in the Gulf of Mexico is built on dual wait time data acquisition, exploiting only the T_1 contrast of the reservoir fluids. This approach has proved itself over and over. However, in some situations, delineation of fluid properties is ambiguous. Ambiguity is especially common when the well is drilled with oil-based mud or when the viscosity of the in-situ hydrocarbons spans a broad range. Multi-frequency NMR tools facilitate the simultaneous acquisition of multi-TW and multi-TE data, leaving less room for ambiguity when identifying reservoir fluids. The continuous and simultaneous measurement of multiple NMR excitations reveals both the TI and diffusive properties of the reservoir fluids, with more detailed and reliable fluid identification as a consequence. Integration of the NMR-derived variations in fluid properties throughout the reservoir with other, conventional log measurements also gives early access to reservoir features, previously not available at the time of logging. Multi-excitation NMR acquisition was identified as a key element in the formation evaluation program of the subject well. The optimal NMR measurement scheme was developed through extensive simulations of anticipated reservoir and fluid characteristics. The robustness of the proposed acquisition schemes was tested by variation of pertinent characteristics over a broad, yet realistic, range. Prior-testing simulations were time and effort well spent, as it greatly improved the effectiveness of the acquisition. Evaluation and interpretation of the NMR data from the subject well revealed trends in fluid properties that were overlooked by traditional and more established methods of log interpretation. In the paper, we discuss the logic of acquisition design and show examples of logs and their interpretation. The results have been corroborated by independent laboratory analysis of fluid samples taken from the well. The added value from these measurements justified a similar acquisition approach for subsequent wells in the Gulf of Mexico.