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Summary With the use of both azimuthal-propagation-resistivity (APR) and main- and cross-component data, the resistivity anisotropy and its dip and azimuth angles of a massive formation (anisotropic shale or laminated sand) can be determined. The accuracy of the determined parameters depends on the amount of available data. The minimum amount of data required is two frequency main components and real and quadrature cross components. The boundary effects will distort the solution eventually; however, the anisotropy-enhanced processing will minimize the effects to extend the algorithm to a certain distance away from a boundary.

Abstract Directional drilling, particularly in conjunction with reservoir navigation (i.e., geosteering), is used to land wells at preplanned points in a reservoir and to optimize the wellbore placement of production sections with respect to reservoir boundaries and fluid contacts, by adjustment of well trajectories in real-time. Geosteering is accomplished and facilitated by the recognition of approaching conductivity contrast boundaries not yet penetrated by the wellbore by the use of deep reading tools and geologic models used to predict the boundaries approach. The decision making process needs to be quick and efficient as drilling progresses whilst decisions and interpretations are being made. Using deep resistivity images, and example templates for interpretation, assists in timely decision making and aid in simplifying what is a complex problem. The efficacy of the deep resistivity image interpretation concepts are illustrated by comparison of motifs observed in field data in complex channel sand environments to synthetic models and numerically modeled images of observed instrument responses. Addition of receiver antennas transverse to the axis of the coaxial antenna array permits acquisition of information on the direction to the boundary or contact. The physics of the method dictate that there is little or no detectable signal from the formation except in the presence of a conductivity contrast boundary within the (relatively large) volume of investigation of the antenna array. Electromagnetic radiation propagates considerable distances into resistive reservoir rocks and fluids, enabling electromagnetic logging-while-drilling instruments to detect such boundaries and estimate their orientation and distances in space relative to the wellbore. Images constructed from a combination of the usual coaxial dipole electromagnetic signal augmented by signals from transversely mounted receiver coils offer a visual interpretation option for the responses of logging-while-drilling propagation instruments. However, methods for interpretation of standard borehole resistivity images cannot be applied directly to the interpretation of deep resistivity images. These deep resistivity images vary with hole inclination, conductivity contrasts, and geologic structure, but usually produce a recognizable pattern, called a motif, that can be diagnostic of the geological_structure in the vicinity of the wellpath. Moreover, the motifs can be organized into a manageable number of themes that aid in their interpretation. Proper interpretation of the themes and motifs not only aids in geosteering decisions, but also enhances conventional formation evaluation by bringing an element of directionality to resistivity measurements that has not been possible in the past. In this paper we present real life examples from a challenging environment in the middle east, where simple motifs can help identify common responses and in future speed and aid interpretation for both the contractor and the client. In addition, the naming of these assists in the training of new Navigators, and clients alike. As the industry expands and the real-time decisions are pushed onto often less experienced personnel, and with the rapid change in technology the industry experiences, making interpretations simpler and more memorable is of key import.

A resistivity tool with a large depth of investigation (greater than 30 meters in ideal conditions) has been designed and used in the North Sea Grane field for over three years. An azimuthal resistivity tool with a depth of detection of about 6 m in ideal conditions has now been added to the bottomhole assembly. When the deeper measurement detects a conductive zone there is no information about the direction to the target because the measurement has azimuthal symmetry. The shallower azimuthal measurement will be able to give the direction to the conductive zone when it comes within the depth of detection of the tool. The target for this effort is a thick reservoir (about 60 meters) that has a long, gradual transition from a resistivity of about 300 ohmmeters at the top of the sand to the water zone of about 0.5 ohmmeters at the bottom. A shale formation of about 1.0 ohmmeter is found both above and below the sand. A deep resistivity tool, a normal propagation resistivity tool, and an azimuthal resistivity tool are all used to place the well in the ideal position to produce the reservoir. The azimuthal tool has no response in a homogeneous formation. The result is a better depth of detection because the signal from the target zone does not have to be removed from a constant background signal. Unfortunately, a gradient is not a homogeneous formation and the result is a background response roughly 10 times the normal noise floor of the tool. However, the shallow measurements of the traditional axial propagation resistivity tool are used to estimate the response of the azimuthal tool to the resistivity gradient. The difference between the estimated response and the actual response is a better indication of a nearby conductive bed. Both synthetic models and actual data are used to show how the combination of the three resistivity tools can geosteer in this complicated environment. In particular, this combination is able to distinguish between a shale body above the tool and a shale encroaching from below.

With the use of both azimuthal propagation resistivity main and cross component data, the resistivity anisotropy and its dip and azimuth angles of a massive formation (anisotropic shale or laminated sand) can be determined. The accuracy of the determined parameters depends on the amount of available data. A minimum amount of data are two frequency main components and real and quadrature cross components. The boundary effects will distort the solution eventually; however, the anisotropy enhanced processing will minimize the effects to extend the algorithm to a certain distance away from a boundary.

We illustrate the use of a new technology for navigating and characterizing various types of oil reservoirs. Real-time images from Azimuthal Propagation Resistivity measurements provide a "map" of the resistivity patterns up to several meters around the wellbore. In addition, recently developed processing and quantitative interpretation techniques help guide the placement of the well and provide a new perspective of the formation. When navigating in gas drive reservoirs, the azimuthal resistivity measurement is used to maintain the wellbore at a prescribed distance above the oil-water contact. With its exponential sensitivity to distance, the measurement is able to detect even small changes in the distance to the oil-water interface. In a few instances, the azimuthal information provided by the real-time deep resistivity images indicates probable coning due to offset well production. Similar principles are applied in high angle drilling of water drive reservoirs. The deep azimuthal information allows the drilling engineer to maintain the wellbore at a prescribed distance immediately below a shale roof. The deep resistivity image from the azimuthal resistivity measurement also makes it easy to distinguish the roof from the occasional approaching shale lens. Whereas shallower reading LWD image logs (e.g. Gamma Ray and Density) only indicate a geological feature proximal to wellbore, the deep reading azimuthal resistivity measurement can provide geologic structure information at the reservoir scale. Visual displays show the subsurface surrounding the wellbore; quantitative algorithms accurately compute the distance, direction, and apparent dip for reservoir related geological events. A new conductivity unit named "Transverse Siemens" is proposed to help quantify the new azimuthal propagation measurement.