Page 94 - Well Logging and Formation Evaluation

84 Well Logging and Formation Evaluation

4. The difficulty of obtaining sufficient depth of investigation for the
measurements to be useful for saturation determination.

Most of the early drawbacks with the tool have been overcome.
However, it still suffers from the following limitations:

• The tool response is still severely affected by the presence of any dia-
magnetic or paramagnetic ions such as arise from any iron in the mud
or formation, or manganese/vanadium. These will have a great effect
on the relaxation times of the hydrogen nuclei.
• The tool is typically far more expensive to run than a conventional
logging suite, which will normally be run in addition to it anyway, due
to lack of confidence in the tool and the need to keep consistent with
earlier logs in the field.
• Logging speed is typically 800ft/hr, less than half that of conventional
logging (1800ft/hr). Hence, there is additional rig time, adding to the
total cost.
• There are still temperature limitations and a lack of slimhole tools avail-
able. An LWD (logging while drilling) version of the tool is still in the
test phase.
• The depth of investigation is still far shallower than the deep-reading
resistivity tools.
• Early claims asserted that the tool offered a superior measurement of for-
mation permeability; however, in practice the permeability was derived
from an empirical equation involving the FFI and the T por . In most cases
the “global” calibration parameters were found to be inapplicable, requir-
ing a local calibration in each formation against core data.
• It has not been conclusively demonstrated that the tool offers a more
cost-effective or accurate evaluation in standard simple reservoirs
where conventional techniques and models are being applied.

Where it is hoped that the tool may offer a direct advantage over tra-
ditional techniques lies in the following areas:

• Identification of zones previously missed due to high percentage of
clay-bound water that would nevertheless flow dry hydrocarbons
• A more accurate determination of porosity, particularly in complex
lithologies
• Advanced facies discrimination in formations where conventional logs
are not capable of discrimination