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AbstractThe purpose of this study is to analyze the evolution of polarization horns and resistivity anisotropy as invasion progresses in a horizontal well drilled through a turbidite sandstone reservoir. Polarization horns are spikes of resistivity tool response that occur in highly deviated and horizontal wells. Resistivity anisotropy occurs when the resistivity measured in one direction in a formation is different from that measured in another direction.The study area is the Albacora field, Campos basin, offshore Brazil. The reservoir is Upper Albian in age and is characterized by a sequence of massive and laminated sandstones with thin (20-30 cm) zones completed cemented by calcite. The horizontal well was drilled with salt-saturated mud and logged using a logging while drilling (LWD) 2 MHz resistivity tool. Seven time-lapse logs were obtained between 60 and 1120 hours after drilling. The time-lapse measurements show significant changes in both polarization horns and resistivity anisotropy as invasion progressed.Two anisotropic sandstones with different types of layering were identified from offsetting vertical wells and correlated to the horizontal well. The first contains lowporosity calcite-cemented layers in which resistivity "horns", absent in the initial LWD log, developed over time. The second contains low permeability fine-grained layers which result in resistivity anisotropy on the initial LWD log.Methodology included reservoir characterization using cores, well logs, mini-permeameter and petrophysical data from offset vertical wells. The Coates free fluid and Archie equations were used to calculate resistivities from minipermeameter, porosity and rock type data. A geologic and invasion model was created and tested by the iterative use of tool-response modeling.Results show that the peaks in the 2 MHz resistivity logs are polarization horns, which developed during the invasion process because of growing resistivity contrast between calcite-cemented layers and adjacent invaded sandstones. In the non-cemented parts of the reservoir, where low permeability streaks are controlled by textural variation and pore geometry, anisotropy effects occurred but no polarization horns developed.