Multi-Dimensional Inversion Unlocks “Trapped” Reservoir Reserve in Complex Geological Environment

A common challenge while drilling a horizontal well is to keep it placed inside the sweet spot to reduce drilling risk and optimize the production of the well, for example, placing the well at the desired standoff from the OWC and the top reservoir. The well is planned according to the best geological model, built using the available offset wells and surface seismic interpretation, in a geological zone believed to be structurally safe from major faulting and changes in formation dip. However, while developing a brownfield, the aim is to increase the reservoir exposure and capture bypassed oil, for example, in areas close to or along a fault where we have an increase of both geological and drilling risks. The geological and drilling risks are accentuated by intrinsic uncertainties related to the accuracy of both the seismic and surveying system spatial representation, both in the vertical and lateral plane. Thus, even with the most accurate predrill geological model, drilling a safe geometric well requires its placement hundreds of feet away from prognosed faults. Previous generations of ultradeep azimuthal resistivity (UDAR) logging-while-drilling (LWD) technologies were only able to provide information regarding resistivity contrast associated with lithological or fluid properties changes along the drilling direction with no capabilities to map lateral resistivity changes associated with structural features such as a fault. To optimize the placement of the well in a complex geological environment, where seismic analysis shows potential faulting, the new generation of three-dimensional (3D) UDAR reservoir mapping, which provides real-time 3D volumetric interpretation using one-dimensional (1D) longitudinal and two-dimensional (2D) transverse resistivity inversions was deployed. The 3D UDAR service has a modular design with one transmitter and multiple receivers placed along the bottomhole assembly (BHA) and spaced between the other logging-while-drilling (LWD) or measuring-while-drilling (MWD) tools. The transmitter and receivers’ tilted antennas provide a full directional and 3D characterization of the resistivity tensor acquired; at each transmitter-receiver spacing’s six operating frequencies with sensitivity, a broad range of formation resistivity profiles are also recorded. With 3D UDAR, the full 360° electromagnetic tensor is acquired and transmitted in real time using a new data compression algorithm and inverted with both 1D longitudinal deterministic and 2D transverse pixel-based resistivity inversions. The results are then converted into 3D resistivity volumes, providing a full multidimensional representation of reservoir boundaries and features. Geological targets are multidimensional bodies with properties potentially changing in all three directions. While deploying any UDAR service, it is crucial to understand how the resistivity properties are changing not only spatially but also with respect to the depth of detection (DOD) of the electromagnetic resistivity wave. Geological features at the scale of the UDAR measurements could be approximated and solved using 1D longitudinal inversions or higher-grade multidimensional inversions (i.e., 2D transverse inversions). However, as in this case study, for reservoirs with more complex geology, the 1D inversion algorithms may not resolve the structural features surrounding the UDAR systems; therefore, the introduction of the 3D UDAR service with its recent advances in computing power unlocked the implementation of more complex inversion workflows that provides real-time 3D reservoir mapping based on 2D transverse resistivity inversions. Teamwork between the operator and service company allowed the project to move forward from conventional geosteering techniques using previous UDAR technology, where steering decisions were based on 1D resistivity inversions. With the introduction of multidimensional inversion, 3D reservoir steering was performed in real time to azimuthally geosteer the well respect an approaching fault within the dogleg limits to optimize completion. This case study will show how the 3D UDAR service enabled the strategic geosteering of the well in a complex geological environment, unlocking oil reserves