In our Southern North Sea gas reservoirs, we have acquired a number of Nuclear Magnetic Resonance (NMR) logs under various conditions. Nuclear magnetic resonance tools respond primarily to the fluids present in the reservoir. Objectives of NMR logging are: to improve permeability estimation in noncored wells and to quantify fluid volumes. In several wells the log interpretation was calibrated by NMR laboratory analysis on core. We interpreted NMR logs in-house using advanced tools developed by Shell.
The application of NMR logs in low to moderate quality gas reservoirs is widely regarded as challenging. Reasons for this are the low NMR signature of gas due to the combined effect of low porosity and a low hydrogen index.
Key findings of our work are:
If properly used, NMR tools confirm the interpretation by conventional tools and improve on permeability and fluid estimation.
Some of our logging jobs could have provided more information with better planning (of acquisition parameters, tools and mud systems) and more experience available.
Porosity and permeability by NMR are less dependent on core calibration than conventional derivation from porosity tools.
Our in-house interpretation increased accuracy and insight into the reservoir.
In this contribution, we will report our experiences with NMR logging and interpretation and comment on the added value of NMR logging in lower quality gas reservoirs.