Inversion-free 4D AVA framework for fluid and pressure monitoring in a Brazilian pre-salt carbonate reservoir

This study presents a 4D Amplitude Variation with Angle (AVA) framework to detect and map fluid-saturation and pressure changes in the Tupi Field, a complex pre-salt carbonate reservoir in Brazil's Santos Basin, using only P-wave (PP) seismic data. While conventional 4D full-stack analysis yields a single amplitude-difference response that often requires detailed well-history interpretation, 4D AVA analyzes time-lapse variations in intercept and gradient ( Δ R₀– Δ G), enabling pressure- and fluid-related effects to be distinguished based on characteristic behavioral patterns. The workflow integrates 1D and 3D petro-elastic modeling (PEM), well logs, reservoir simulation, and production data. Synthetic pre-stack seismic data generated under multiple production scenarios are used to construct a classification scheme relating Δ R₀ and Δ G changes to dynamic reservoir processes such as gas or water saturation variation and pressure change. Three-dimensional modeling further accounts for lateral heterogeneities, including faults and permeability barriers. Application to field data identifies increased water saturation near water injectors, gas migration around gas injectors, and localized pressure buildup, with spatial patterns indicating hydraulic connectivity between injectors and producers. Independent dRMS amplitude maps support the inferred flow pathways. This inversion-free, cost-effective approach improves pre-salt reservoir monitoring, reduces interpretation uncertainty, and supports WAG optimization.