The challenges presented by a field in Ecuador included placing the wells in the cleanest, most permeable portion of the reservoir, in the middle of the structure, without penetrating an overlaying kaolinitic bed or the caprock shale above the reservoir or penetrating the water bearing zone. These objectives could not be optimally achieved with traditional wellbore imaging sensors or with deep non-azimuthal wave resistivity. Wellbore imaging instruments identify the relative dip and azimuth of geological events intersecting the wellbore, but the information is clear only after leaving the reservoir. Non- azimuthal wave resistivity predicts the impending intersection with a reservoir boundary, but does not predict the azimuth of approach. In the first instance, a proper decision can be made only after exiting the reservoir; in the second instance, a proper decision requires other knowledge about the probable direction of the approaching boundary. A newly deployed azimuthal deep resistivity while-drilling sensor produces a vast array of azimuthal measurements. When mapped into an image, deep resistivity behaves for the most part like traditional images in which "smiling" patterns indicate that the wellbore is going up stratigraphically and "frowning" patterns indicate that the wellbore is going down stratigraphically. One exception is the newly discovered phenomenon of the "bright spot" that appears when approaching a low resistivity shale or water bearing interval from a high resistivity reservoir. The bright spot clearly indicates an impending reservoir exit. Because it is keyed to the low side of the well, the bright spot indicates the direction of the required evasive action to remain within the desired interval. This visual indicator is complemented by a novel quantitative measurement, the Geosignal, which features a strong exponential dependence on the distance to the boundary of the reservoir. In the examples shown, the visual information from the bright spot is combined with the quantitative information from the Geosignal measurement to properly guide real-time geosteering decisions.
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