Extra-deep reading azimuthal resistivity tools have been deployed in various reservoir settings around the world in recent years in an effort to further improve efficiencies in reservoir development. For many years field development relied on standard and azimuthal propagation resistivity tools with depths of investigation up to approximately 5m, contributing to optimized and pro-active geosteering. While effective at geosteering against adjacent boundaries to maintain position in oil bearing formation, more complex reservoir architectures require data sensing further into the formation to allow a closer correlation with seismic models and provide more complete reservoir mapping.

The first extra-deep propagation resistivity tools were developed by employing lower frequency waves, increasing antenna spacing and eventually adding lower frequency azimuthal signals. The new designs greatly increased the depth of detection and also added directional components. However, due to the greater volume of formation being investigated, the deeper readings bring extra complexity and uncertainty to the interpretation process so that innovative inversion software is required to support the tools and produce results that can be used in real-time.

The inversion method described in this paper for the interpretation of extra-deep azimuthal resistivities employs a-priori constraints and is user-controlled in order to accurately monitor laterally and vertically changing geology. The examples shown here will demonstrate how inversion results based on a full suite of resistivity measurements have brought benefits to reservoir understanding by deriving sandstone thickness, detecting multiple bed boundaries, locating remote sandstones and remote resistivity plus the relative dip between the tool and the formation. The integration of this data results in better constrained reservoir models and an improved field development strategy.

This paper will present the results of wells drilled using extra-deep azimuthal resistivity tools on the Peregrino Field in Brazil. The reservoir comprises complex high energy gravity flows consisting of reservoir units difficult to map due to being below seismic resolution. The sandstones have limited lateral extent and thicknesses ranging from 2m to 25m. Originally developed to improve net sandstone drilled in the Peregrino heavy oil reservoir by allowing a more strategic approach to geosteering, the tool deployment has brought additional benefits in reservoir understanding which impact seismic model interpretation, future well planning, completion strategies and reduce the need of pilot holes.

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