Geostopping With Ultradeep Electromagnetic Look-Ahead Technology to Mitigate Drilling Risks in Brazilian Presalt Carbonate Reservoir

Ultradeep electromagnetic look-ahead technology has proven to be an effective solution for overcoming drilling challenges posed by magnesian clay-rich intervals within the Barra Velha carbonate formation in Brazil’s presalt offshore Santos Basin. These intervals, composed primarily of kerolite and interstratified clays rich in smectite, contribute to operational risks such as borehole instability, fluid losses, fishing incidents, redrilling, and completion difficulties. When these magnesian clay-rich zones are encountered, they must be fully cased before drilling the permeable zone in the next phase. Failure to set casing at the optimal depth can result in significant non-productive time. In this paper, we present how this Ultra-Deep Azimuthal Resistivity (UDAR) look-ahead technology was applied to detect the top of the high-resistivity carbonate reservoir and enable precise geostopping. Previous applications of this technology in the Santos Basin focused on detecting the transition from the extremely highly resistive evaporite section to lower-resistivity formations. However, in this scenario, an opposite conductivity contrast required a different downhole tool configuration and real-time operational workflows to detect the base of this magnesian clay-rich interval up to 15 meters ahead of the bit. This work presents the technical challenges associated with the presence of magnesian clay-rich carbonates in Barra Velha and the limitations of the conventional geostopping workflow. It also presents technical details of the UDAR look-ahead technology implemented and an overview of the planning that preceded field implementation. It includes the criteria used to enhance detection depth ahead of the bit, such as frequency selection, measures to enhance signal-to-noise ratio, and an additional in-situ receiver antenna calibration. Two case studies, covering planning, execution, and geostopping decision-making, support the proposed technical solution. The successful adoption of this technology broadens its application in Santos Basin presalt operations and sets a precedent for applicability in other regions with similar geological challenges.