Recent advances in high-resolution imaging are well perceived by the oilfield industry as an aid to refine reservoir characterization. The availability of high resolution image data in real-time allows for improved decision making and more optimal wellbore placement when navigating through the reservoir. However, the transmission of high resolution image data to surface is limited by the bandwidth offered by mud pulse telemetry systems. This paper describes a new concept for transmitting imaging data to surface in real-time. The concept includes a highly flexible and effective compression scheme that can be used for all currently available and future telemetry systems. Image transmission parameters can be modified via downlink commands at any time without interruption of the drilling process. In this way, image parameters can be optimally adjusted to suit the available telemetry rates and proportion of the bandwith dedicated to image transmission. This flexibility allows customization of the real-time imaging service to optimize the geosteering process and the ability to provide high resolution data over specific zones of interest. The relationship of pixel size, telemetry rate and the real-time image parameters of latency and redundancy are discussed with reference to real-time resistivity images from field measurements transmitted at different telemetry rates and the corresponding memory images. Introduction Wireline, featuring a multi-phase electrical communication line, provides a real-time connection between the downhole sensors and the surface system. Despite the immediate availability of data at surface, measurements are made long after the formation has been drilled. As a consequence, all major oilfield service companies invested heavily in Logging-While-Drilling (LWD) technology, aiming to measure formation parameters shortly after drilling with the rock in as close to pristine condition as possible. The immediacy of the measurement combined with the availability of downhole data at surface in real-time revolutionized wellbore placement. While bulk measurements still represent the vast majority of LWD logging data, a variety of oriented geophysical measurements such as gamma, density, caliper and resistivity data are available, allowing refined structural reservoir analysis. The visualization of such data via image plots simplifies interpretation. The amount of information available increases with image resolution, which is limited by the intrinsic resolution of the geophysical sensor. High resolution resistivity sensors (e.g. [Ritter et al., 2005]) are providing the highest image definition in industry with an effective pixel size of ¼″ × ¼″ in memory. Such highly resolved images are a valuable aid for a wide variety of applications including:–structural & fracture system analysis–thin-bed analysis to determine net pay thickness–sedimentary feature analysis for input to a depositional environment model–core depth calibration, core orientation and an alternative to conventional coring over long intervals. However, the exploitation of LWD data is currently limited by the slow communication through the mud column. The introduction of azimuthal geophysical measurements with the associated exponential increase in data potentially available further increases the gap between achievable and required data rates. While the need for additional downhole memory space can be resolved by introducing more memory space, recent increases in telemetry speeds can not keep up with the required channel bandwidths. For example, a high resolution azimuthal logging services can provide more than one hundred sectored measurements around the borehole. In memory each of these values is represented by 8 bits. A 500 ms acquisition cycle results in at least 2 kbps required to transmit such uncompressed image data sets to surface. Even sophisticated telemetry systems using the drilling mud as the communication medium allow communication speeds of only a few bits per second. Thus, data reduction and compression techniques (e.g. [Li et al., 2001], [Li and Wang, 2005]) are required suited to decrease the amount of downhole data by the magnitude of 10[3].

The relatively recent development of azimuthal resistivity measurements enables proactive geosteering within complex reservoirs. These successful tools are the major contributor to the substantial expansion of horizontal drilling. The tools enable determining the distance (up to 5 m in ideal conditions) and the azimuthal direction to a resistivity boundary. In ideal conditions, the well is inside a high resistivity layer and the shoulder bed is low resistivity, giving geologists warning of approaching adjacent conductive beds. When the tool is in a low resistivity layer, the depth of detection of an adjacent high resistivity layer is much smaller. In these situations, it is often not possible to use the tool for effective geosteering. An extra-deep resistivity tool has been used for several years in Norway and has been introduced in the Peregrino Field in Brazil. It operates at lower frequencies, has large transmitter-receiver spacings and a depth of detection up to 25 m. This tool was deployed in addition to the conventional directional resistivity instrument. The new application in Brazil was supported by inversion software (still in development) to enable possible interpretation of the geology within the tool range. The inversion results provide information that can help identify adjacent reservoir layers while in the target zone and measure the thickness of the reservoir layer being drilled. Examples are presented from one well where the extra-deep resistivity provided early warnings and additional information that helped to steer the well successfully and maximize reservoir coverage. The extra-deep measurements from the tool also provide valuable reservoir understanding and knowledge for future well planning purposes.

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.

The relatively recent development of azimuthal-resistivity measurements enables proactive geosteering within complex reservoirs. The tools enable determining the distance (up to 5 m in ideal conditions) and the azimuthal direction to a resistivity boundary. In ideal conditions, the well is inside a high-resistivity layer and the shoulder bed is low resistivity, giving geologists warning of approaching adjacent conductive beds. When the tool is in a low-resistivity layer, the depth of detection of an adjacent high-resistivity layer is much smaller. In these situations, it is often not possible to use the tool for effective geosteering.An extradeep-resistivity tool has been used for several years in Norway and has been introduced in the Peregrino Field in Brazil. It operates at lower frequencies than the shallower reading tools, has large transmitter/receiver spacings, and a depth of detection up to 25 m. This tool was deployed in addition to the conventional directional-resistivity instrument.The new application in Brazil was supported by inversion software (still in development) to enable possible interpretation of the geology within the tool range. The inversion results provide information that can help identify adjacent reservoir layers while in the target zone and measure the thickness of the reservoir layer being drilled.Examples are presented from one well where the extradeep resistivity provided early warnings and additional information that helped to steer the well successfully and maximize reservoir coverage. The extradeep measurements from the tool also provide valuable reservoir understanding and knowledge for future well-planning purposes.Extradeep-Resistivity Measurements. To illustrate how the extradeep-resistivity measurements complicate visual interpretation, consider a simple model in Fig. 1.

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