Improving Nmr Log Interpretation to Support Mature Field Development, A Case Study Offshore Abu Dhabi

Abstract

Nuclear Magnetic Resonance logging measurements (NMR) provide detailed information about rock texture and pore distribution. The main objective of this study is to highlight a carbonate reservoir characterization example in a mature field, offshore Abu Dhabi; providing qualitative porosity, permeability and pore type classification in real time (while drilling), to support efficient field development decision making. Different logging while drilling vendors tools (NMR-WD) operate at different concepts; some use the longitudinal relaxation time (T1) measurements, others apply the transverse relaxation time (T2). In this case, a low magnetic field gradient (T2) tool type was deployed in a tight formation horizontal oil producer. The well objective is to expose the maximum reservoir contact (MRC). Primarily, the acquired (NMR) spectrum was used to deliver accurate total porosity, to compute Archie's water saturation. However, delivering a quantitatively reliable permeability become very challenging in the complex carbonate environment subject to study as it was well linked to (NMR) pore size distribution. At first, a standard (T2) cutoff value was applied. The computed (bulk irreducible water – BVI) was too low and hence the permeability was too high, resulting in inaccurate NMR interpretation. Next, a varying T2 cutoff – per zone was applied based on the changing spectrum profile itself. Finally, a Gamma Inversion technique by the service company was introduced to better quantify the different pore types and the corresponding permeability. The (NMR) log analysis was validated with well core data in addition to production logging results. The data was applied to design the well stimulation and completion programs resulting in a healthy oil producer drain added to the asset. Integration of Gamma Ray-resistivity-NMR and borehole image logs helped to consolidate the interpretation findings hence supporting decision making for mature field development.