ICE 2022


Natural open fractures are common in carbonate reservoirs where they often have a large impact on fluid flow. Such fractures have also been reported in the Aptian Barra Velha formation in the pre-salt lacustrine carbonate reservoirs in the Santos Basin offshore Brazil. These fractures contribute to excess permeability in the pre-salt carbonate reservoirs (i.e., well test permeability is significantly larger than core permeability or log derived permeability) in combination with vugs and karstified layers. It is important to address these features to understand the fluid flow in the reservoir during production and injection, and also for well planning since they represent potential loss zones. Purpose of this presentation is to summarize the characteristics of natural open fractures in the presalt reservoirs and to discuss their distribution, occurrence and formation. The fractures have been interpreted in acoustic borehole image logs in 19 wells across the Santos Basin. The data quality is overall good and consistent so that results can be compared between wells. There are several methods for calculating fracture density from borehole image logs, and in this case the P21 fracture density, which is fracture trace length per area, was preferred. The carbonates in the Barra Velha Formation show various degree of silicification, and P21 fracture density show a strong correlation with volume of silica. Fractures are also frequent in fault damage zones, above and below cavities, and they are also associated with presence of vugs. The main orientation of the fractures is NNW-SSE which is in the same direction as the dominating fault trend, and this indicate a structural control on fracture formation. Natural open fractures are rare in high porosity rocks with a shrubby or grainy texture, and also in intervals with a conglomeratic appearance in the borehole image logs.