World First Horizontal Look-Ahead from Ultra-Deep 3D Resistivity Inversion for Proactive Geosteering

Abstract Sensitivity of the electromagnetic (EM) field to resistivity ahead of the bit has been proven for Ultra-Deep Azimuthal Resistivity (UDAR) tools, and is actively being used in near vertical wells along with 1D inversion. Inverting for resistivity ahead in horizontal wells would allow early identification of boundary changes prior to intersection yet has been thought not reliably achievable to-date. Deflection of EM components corresponding to 3D changes in resistivity in high angle and horizontal (HA/HZ) wells poses high level of complexity in analyzing the measured tensor components and inverting these components to geology around and ahead. Consequently, a true 3D inversion is mandated for converting component responses to geological models that can identify features exhibiting resistivity changes around and ahead in horizontal wells. The inverted 3D geological model can guide geo-steering decisions. Deployment of UDAR in clastic channelized geological environment - where erosion, small scale structural deformation and dislocations could be encountered - and implementing true 3D inversion derived from full EM tensor components demonstrated sensitivity to features of approximately 50ft ahead of the transmitter for this environment, turning look-ahead in HA/Hz to a reality. As the transmitter is ~ 9ft from the bit, modeling ~ 40ft ahead of the bit has been achieved. Synthetic and field data were used to validate the 3D inversion and its look-ahead capability. During drilling, discontinuities were identified resulting in the well exiting and entering different sand units. This environment was ideal for testing the look-ahead sensitivity as these formation changes were seen to have a major impact on the EM field. To test look- ahead sensitivity, only data leading up to a major discontinuity was used, such that the horizontal lookahead 3D inversion would show the result before the discontinuity was penetrated. As the exact position of the feature was known, the predicted position of the major geological change from the look-ahead sensitivity could be validated. Two major changes were identified in the region of 50ft ahead of the transmitter. Firstly, a buildup of resistivity to the right-hand side of the wellbore ahead, which could have been used for guidance to azimuthally steer to the right for optimal positioning. Secondly, a discontinuity, perpendicular to the well path which showed a high resistivity sand ahead. In both cases these predictions based on the 3D look-ahead inversion were validated. Horizontal look-ahead is viewed in the industry as the next big step in the development of UDAR technology to guide well placement decisions. Demonstrating this capability with both synthetic and field examples shows this is now possible, allowing changes in resistivity ahead vertically and to the sides to be identified and proactively used.