Abstract
Several light oil and gas reservoirs are present within an extensive Permian clastic sequence in the Middle East. Well logs and core data revealed numerous challenges for the petrophysical description of this formation. Rapidly changing depositional environments and diagenetic effects caused heterogeneities in grain size and sorting within clean sands. Consequently, gamma ray and conventional porosity logs have little sensitivity to rock quality variations. Secondly, an influx of meteoric water into the reservoir rocks decreased formation water salinities which adds uncertainties to the estimation of fluid saturations from resistivity logs. Finally, gas-oil contacts are present in some reservoirs where log-based in-situ hydrocarbon typing is of great practical value. The introduction of nuclear magnetic resonance (NMR) logs has successfully mitigated these issues as evidenced by petrophysical interpretations and formation testing in the area's exploration and delineation wells.
Lateral facies variations and complicated reservoir structures warranted the deployment of NMR technology in horizontal development wells for better well placement and completion optimization with logging while drilling (LWD) NMR as the most preferable option. The NMR logs led geosteering decisions and proactive well planning to singificantly increase reservoir contacts in producer wells. Permeability predicted by the NMR logs is of great value in real-time well placement decisions and completion design including ICD installations. The low-gradient LWD NMR measurement gives rise to a very simple and robust real-time fluid identification thanks to the the good separation of water and oil signals in the NMR T2 spectrum. This fluid quantification, combined with bound fluid analysis, helps determine the well's position in the transition zone by detecting free water.
This paper summarizes the experience with both wireline and LWD NMR technologies in the area. Lessons learned include considerations for deployment, tool activations and NMR interpretation.
