Interpretation Of Propagation Resistivity Logs In High Angle Wells

The interpretation of propagation resistivity logs at very high relative dip angles is much more challenging than the interpretation of vertical logs. Several effects degrade the usefulness of propagation resistivity data when the borehole is horizontal or the relative dip angle is very high (over 70°). These effects include anisotropy, non-circular invasion, and eccentricity in oddly shaped boreholes. Some of these effects cannot be properly quantified so their effect on the data cannot be fully removed. Other effects can be removed, but the remaining presence of these unquantified effects makes this more difficult. An entirely new strategy is necessary to interpret horizontal and high angle wells, and even then the results will not be as accurate as they are in low angle wells. The first problem with interpretation of high angle wells is the inability to remove the effects of differing vertical resolution (more properly called axial resolution in this case). Lateral changes in the formation itself may have a larger impact on the data than the progress of the tool vertically through the formation (which is very slow at high relative dip angles). As a result, a normal inversion will increase the size of the changes within the formation more than it will reduce the effects of nearby bed boundaries. In addition, high angle wells often use the resistivity data to geosteer the well. Geosteering recognizes the nearby bed boundaries as variables and any attempt to eliminate these variables by inversion techniques is not desirable. Therefore, a strategy which treats the distance to the bed as a variable is required. This strategy must also reduce the effects of anisotropy, anomalous dielectric permittivity, and invasion. All of these effects have to be interpreted simultaneously because they all result in various types of separation of the apparent resistivity curves. If one of the parameters is analyzed individually, the resulting attempt to explain all of the separations with a single effect will cause an error in that parameter. In addition, it will then be impossible to determine the other parameters. In this paper several of these formation effects have been simultaneously inverted to produce "true" formation parameters. This method has been used to interpret some field logs. While the results demonstrate the advantage of this strategy over previous methods, the interpretation is still not as effective as interpretation in low dip angle formations. However, it is still possible to produce accurate resistivity curves at fixed depths of investigation.