Verification of Slim Ultra-deep Resistivity Inversions in a Complex Geological Environment

Abstract

Abstract Operators have used ultra-deep azimuthal resistivity tools for geostopping in landing situations and for geosteering and geomapping in horizontal wells for several years. However, their use in slim boreholes, where 4¾-in. tools are required, is a relatively new application of the technology. Operators often require ultra-deep azimuthal resistivity data, even in wells where pressure changes or stability issues require slim borehole sections to reach a target. In these scenarios, 1D inversion canvasses, which display the position of resistivity boundaries above and below the wellbore, and azimuthal resistivity images, which display changes in the resistivity to the side of the wellbore, are valuable as an aid for optimal well placement and for mapping formation and fluid changes away from the wellbore. This paper presents early field results for a new 4¾-in. ultra-deep resistivity tool. Verification of the tool results is vital to demonstrate that design changes to accommodate the smaller diameter do not degrade the results relative to larger tool sizes, and that the tool accurately maps the resistivity profile around the well. The case history presented describes a mature field with a complex geology and a history of production and water injection, resulting in fluid boundaries that show significant displacement from their original positions and a complex morphology. It is, therefore, important to verify the ultra-deep resistivity results with multiple additional data sources, such as seismic and other established logging-while-drilling (LWD) tools. The inversion results show a complex geological structure. Faulting, expected from the seismic data, has resulted in the displacement of formation boundaries, and sharp resistivity boundaries show the influx of water into the reservoir. These significant geological features are evident in the ultra-deep resistivity data. Other data sources, such as seismic, density, and shallow-resistivity data, help to verify the structural features and the fluid boundary positions. Evaluation of azimuthal resistivity images from the ultra-deep tool provides further reservoir understanding, because they can demonstrate that the formations dip across the wellbore and that some of the faults cross the well path at an oblique angle. Even where the subsurface geology is very complex, as in this case study, the 1D ultra-deep resistivity inversion results successfully represented the position of formation and fluid boundaries. In addition, azimuthal resistivity data from the tool provided additional information that leads to greater reservoir understanding. There is no evidence to suggest that the smaller diameter of the 4¾-in. tool has compromised the quality of the data.