Petrophysical Formation Evaluation using Logging While Drilling (LWD)
measurements is a new requisite when drilling in carbonates reservoirs offshore
Brazil. These reservoirs are difficult to characterize due to an unusual
mixture of the minerals constituting the matrix and affecting rock texture. As
wells are getting deeper and more expensive, an early identification of the
drilled targets potential is necessary for valuable decisions. Brazil operators
have been especially demanding towards service providers, pushing for
development of suitable services able to positively identify and quantify not
only the presence of hydrocarbons but also their flowing capability.
In addition to the standard gamma ray / resistivity / porosity and
density measurements, three new measurements have proven to be critical to
evaluate complex carbonate formations: Nuclear Magnetic Resonance (NMR),
Spectroscopy and Capture Cross-Section (sigma).
Under appropriate logging conditions, NMR data provides lithology
independent porosity, bound and free fluids fractions, reservoir texture and
permeability. Capture Spectroscopy allows assessment of mineral composition in
terms of calcite, dolomite, quartz and clay fractions, and in addition
highlights presence of other heavier minerals. Finally, sigma allows performing
a volumetric formation evaluation without requiring custom optimization of the
classical exponents used in all forms of resistivity saturation equations.
All these new measurements are inherently statistical and if provided by
wireline after drilling the well they may result in significant usage of rig
time. When acquired simultaneously while drilling they have three very clear
- no extra rig time
- improved statistics due to long formation
exposure (drilling these carbonates is a slow process and rate of penetration
(ROP) rarely exceeds 10 m/hr)
- less invasion effect and better hole
This paper describes the development of two LWD tools performing the
above-mentioned measurements and provides example of their use in both Real
Time and memory mode. The evolution of the services over the past years leading
to present day performance is also demonstrated.
Multi-mineral petrophysical analysis is used for advanced interpretation
and quality control. Response of different tool physics is reconstructed in
order to verify data consistency and guarantee reliability of the results
obtained. Comprehensive answers were obtained and tested against independent
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