Technical Paper: Advances in Formation Evaluation Independent of Conveyance Method: State of the Art Logging While Drilling & Wireline Petrophysical Analysis in a Carbonate Reservoir Offshore Brazil

Society: SPWLA
Paper Number:
Presentation Date: 2012


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 alternative.

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 investigation costs.

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