Focused Nuclear Magnetic Resonance

Abstract

Borehole NMR has become an important source of petrophysical information, helped by the development of increasingly sophisticated hardware. In order to address the most fundamental challenge to the utility of NMR – namely the inherent non-uniqueness of the inversion – it is necessary to take a holistic approach in which hardware, job planning, inversion and analysis are considered as a whole. It is especially important to understand the role of signal-to-noise, and develop strategies to deal with it, recognizing that physics dictates that some of the most ambitious interpretation objectives may not be compatible with fast logging speeds.

Starting with hardware, a new logging tool has been developed to maximize signal from the formation and minimize noise from the borehole. Negligible borehole signal and high tolerance to tool orientation error result from focusing the static magnetic field within a 90 degree sector. Extremely low coupling between the RF antenna and the borehole allows power consumption to be kept small. This eliminates the need for fluid excluders, even in very saline muds, as well as enhancing reliability and operational flexibility. Control of the static magnetic field uniformity along the sensor together with optimally shaped symmetrical pre-polarization zones allow for motion-artifact-free logs and accelerated polarization of fluids with long T1 components. Tight control of phase and amplitude characteristics of transmit and receive paths ensure acquisitions have negligible levels of systematic echo train distortions. Higher gradients allow improved characterization of heavy oil and other small diffusion coefficient fluid components, and are a factor in addressing the commercial pressures for higher logging speeds in unconventional reservoirs.

The operation of the tool is tightly integrated with the design of the data processing and analysis, and manifests itself in highly flexible activation sequences which are capable of being optimized for particular interpretational needs. This is particularly useful for fluid typing from T2-D and T1-T2 in unconventional reservoirs, noting that the existence of a theoretically optimum sequence is no guarantee that it can be physically realized. This is addressed with a sophisticated job planning tool and laboratory testing of activation sequences. Operationally the sequences may be optimized on a zone-by-zone basis. Subsequent processing uses the acquired data to identify the best trains to pass to the inversion which facilitates the use of new evolving simultaneous joint inversion techniques for better analysis.

Field trials in over 50 wells encompassed diverse borehole and formation environments. Relative to the tool used as a basis for comparison, results demonstrate step-change improvements in data quality. Processed results and interpretations are reviewed relative to data from supporting logs, to highlight the benefits of integrating multiple data sources. The new service significantly improves confidence in the commonly delivered NMR petrophysical properties and contributes to a better understanding of what can be achieved and how to achieve it.