Advances in Formation Evaluation Independent of Conveyance Method: State of the Art Logging While Drilling & Wireline Petrophysical Analysis in a Carbonate Reservoir Offshore Brazil
Recent wells drilled by an operator offshore Brazil provided the
opportunity to perform a direct comparison of multi-mineral formation
evaluation using as input either traditional Wireline (WL) or Logging While
Drilling (LWD) data. The principal target was an Albian carbonate reservoir of
the Quissamã Formation. This formation has a complex lithology with
variable amounts of dolomitization and presence of quartz and clay. Computing a
correct matrix density and characterizing the rock texture for producibility
estimation is critical.
Initially a 12.25 inch diameter vertical pilot well was drilled with
Synthetic Oil Based Mud (SOBM) and logged using basic LWD tools
(resistivity/density/neutron). A complete WL program followed for a better
understanding of reservoir characteristics. The logging program included
induction, neutron-density, nuclear magnetic resonance (NMR), elemental
spectroscopy, formation pressure measurements and fluid samples. In spite of
unknown formation water salinity it was relatively straight forward to identify
a formation water resistivity value consistent with log responses over the
lower reservoir section. Resistive invasion patterns clearly indicated the
permeable intervals below the free water level, confirmed by the NMR T2
distribution profile. Pressure gradients and fluid samples demonstrated the
validity of the analysis.
In order to explore reservoir connectivity and facies variation at some
distance from the vertical hole the pilot well was side-tracked using an "S"
shape trajectory with a maximum inclination of 55 degrees. The side-track
borehole was drilled with an 8.5 inch bit size with the same type of SOBM,
using a BHA which included rotary steering assembly, multi-function measurement
tool and LWD NMR tool. The LWD measurements allowed the formation evaluation
analysis performed in the pilot well to be replicated.
Capture cross-section (or Sigma) may be sensitive to the invaded zone
due to its shallow depth of investigation, while 2 MHz resistivities read far
beyond. Use of Sigma measurement in place of the WL shallowest induction
resistivity for flushed zone analysis proved to be an effective
A porosity partitioning methodology enabled textural analysis using WL
& LWD NMR data. Rock type classification provided the relative fractions of
micro, meso and macro porosities. Side-track data compared to the pilot well
showed facies changes with additional dolomitization but not the expected
increase in porosity. Lithology patterns were recognized over specific
intervals validating the consistency of the WL and LWD spectroscopy.
LWD data quality benefitted from the improved statistics of the
measurements permitted by the slow drilling rate in carbonates.
The results confirmed that when WL acquisition is either not possible or
not economically viable, LWD measurements under the proper drilling conditions
provide equally effective data for a consistent petrophysical analysis. The LWD
data proved to be reliable and demonstrated the potential to spare extra