The use of electromagnetic logging tools while drilling to investigate the space in front of the drill-bit and to predict the geologic strata and/or fluid property ahead has been one of the industry-focuses in recent years. The commercial look-ahead tools have two main designs based on the published tool configurations and data-channel definitions. The first type employs combinations of tilted coils to acquire multiple or all components of electromagnetic fields and presents logs defined in novel expressions as functions of these field components. The second kind uses combination of the voltage measurements from orthogonal coils (coils pointing along and perpendicular to tool-axis) and the propagation resistivity logs from coaxial coils of large coil-spacing.
This work starts with reviewing the tool physics and the mathematic expressions defining the logs. The look-ahead capabilities are then investigated by considering the case where a borehole is perpendicular to the formation boundaries. It is found that the necessary information for forward detection applications is exclusively from the apparent resistivity logs, not the so-called geo-signals or any other logs. We further explore an ideal scenario, i.e., to detect the presence of a good conductor in front of a high resistivity formation to obtain the asymptotic behaviors for the two measurement types. The asymptotic expressions lay the foundation to understanding the intrinsic differences between the two methods.
The insights derived from the theoretical work and the subsequent modeling results for a broad set of models define the response characteristics and the probing capabilities of the look-ahead techniques. It is realized that the look-ahead instrument design can be simplified, such as using only the familiar three-coil coaxial configuration with a suitable long coil-spacing and multiple carefully chosen operating frequencies to achieve a robust detection-distance coverage.
It is worth to point out that the commercially available look-ahead services are mainly applicable to vertical and inclined wells so far. For look-ahead applications in horizontal wells, due to the contribution of the geologic and fluid boundaries lateral to the tool and the inhomogeneities ahead of the drill-bit, there is no reliable method yet to simultaneously solve the large number of unknown parameters associated other than some special situations. The look-ahead evaluation in the horizontal wells must be investigated further by understanding the characteristics of exponential attenuation and geometric spreading of the electromagnetic fields in a conductive formation and the inherent relatively poor resolution through combining with other detection physics and the introduction of proper geologic model constraints.