Technical Paper: Evolution of Multi-Mineral Formation Evaluation Using LWD Data in Complex Carbonates Offshore Brazil

Society: IBP
Paper Number: 1218_12
Presentation Date: 2012
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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 advantages: 1) no extra rig time, 2) improved statistics due to long formation exposure (drilling these carbonates is a slow process and rate of penetration (ROP) rarely exceeds 10 m/hr), 3) less invasion effect and better hole condition.

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 measurements.

Technical Paper presented at Rio Oil & Gas Expo and Conference 2012, held between September 17-20, 2012, in Rio de Janeiro.

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