Anisotropy Sensitivity in Ultra-Deep Azimuthal Resistivity Technologies

Logging While Drilling (LWD) tools can be used to measure formation resistivity from Depths of Investigation (DOI) of inches out to dozens of feet. Using multi-depth, multi-frequency electromagnetic propagation resistivity (EM) tools, resistivity profiles can be inverted to provide distance to resistivity boundaries and petrophysical information. The primary piece of information produced is the horizontal resistivity map (Rh), but that is only one piece of the puzzle. The other pieces are the vertical resistivity (Rv), layer thickness defined by boundary position and the dip of the formation. For resistivity purposes, the dip is defined parallel to the horizontal resistivity (Harigawa, 1996). Interpretation of electrically anisotropic reservoirs from vertical resistivity, Rv, and horizontal resistivity, Rh has been covered extensively in the literature by Klein et al., 1997, Mollison et al., 1999, Shoen et al., 1999, Shray et al., 2001, Clavaud et al., 2005, Cao Minh, 2008 and many others. Historically, wells were drilled vertically and logged with wireline tools. Conventional induction resistivity tools induce current loops that circulate perpendicular to the wellbore, and measure horizontal resistivity. Horizontal wells logged with wireline tools, or conventional EM resistivity tools create current loops that circulate in the vertical direction and measured vertical resistivity. With the addition of wireline triaxial resistivity tools in the late nineties, all three resistivity components were measured simultaneously. Over the past twenty years, LWD EM tools have advanced from a relatively short transmitter-to-receiver spacing with a single tilted antenna, to the current technology architecture which has multiple collars each with several antennas tilted at 45 degrees that are non-collocated. These are often referred to as being pseudo-triaxial and pseudo-collocated as they have some sensitivity to multiple axis around the wellbore. (Seydoux et al, 2014) Their inversions provide maps of horizontal and vertical resistivity and dip at a specific inversion point. Understanding the accuracy and uncertainty of these inversions has mostly focused on the horizontal resistivity because the horizontal resistivity map is what is primarily used for geosteering.