Short and Long-Term Fracture Permeability Tests Under Stress on Pre-Salt Low Permeability Carbonates After Acid Treatment or with Proppants

Abstract

For relatively low permeability Pre-salt carbonates reservoirs, the question of progressive closure of the stimulated and acidized fractures is raised since their evolution over time could be the main cause of the production decline. Aiming at quantifying the time-dependent behavior and stress sensitivity of the acidized fracture or propped fracture, a series of fracture permeability tests were carried out in laboratory on tight Pre-salt carbonate samples from various facies of a Brazil offshore well. For creating fracture, direct shear tests were performed on initially intact cylindrical carbonate plugs in a conventional triaxial cell. The fresh fractures are then submitted to acid flow over certain duration before permeability measurement under stress. Long term fracture permeability tests lasting over several months have been carried out and were compared with the short-term tests. Correlation with mechanical properties were also investigated. We compare the performances of propped fracture to acid washed one to formalize recommendations for fracture stimulations in the Pre-salt carbonates. The propped fractures clearly performed well, with negligeable stress-sensitivity on fracture conductivity and no noticeable effect of time over 50 days. Mixed results were found for the acid-washed fractures, in which some showed little stress-sensitivity and little time-effect while others the contrary.