Excess permeability in the Brazil pre-salt; nonmatrix types, concepts, diagnostic indicators, and reservoir implications

Excess permeability in carbonate reservoirs is typically related to the presence of dual-porosity multiscale nonmatrix features that can result in early water production or breakthrough of injected fluids. Therefore, early recognition of nonmatrix deeply impacts field development strategies. We use publicly available data from the pre-Salt of Brazil to screen for nonmatrix presence and dynamic potential. We integrate static (core and wire-line logs) and dynamic (drilling losses and well tests) data sets using a consistent workflow to investigate the types and distribution of nonmatrix excess permeability away from wellbore control. Observations across multiple data sets reveal a multiscale, nonmatrix pore system that is identifiable and important to include in geologic models when considering reservoir performance. The main nonmatrix components include faults and fractures, pervasive networks of touching vugs, centimeter-scale vugs, and caves. Lost circulation zones (LCZ) encountered while drilling through nonmatrix features suggest they are not only open but connected and with considerable flow capacity. The LCZ dynamic potential is further supported by excess permeability observations (up to two orders of magnitude greater than matrix) from well tests and a set of characteristic inflections in the pressure transients. The greatest uncertainty remains on the magnitude and extent of nonmatrix away from the borehole. However, a process-based approach to nonmatrix characterization in combination with the observations presented here provides an opportunity to define geologically plausible scenarios for nonmatrix types, distribution, and properties. Geologically and dynamically constrained conceptual models can then be used for scenario-based uncertainty analysis in a play with limited long-term production history.