Geosteering with a Combination of Extra Deep and Azimuthal Resistivity Tools

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.