The deep and hard carbonates found offshore Brazil are being extensively evaluated, and some of them would require fracture stimulation to enable commercial development. Due to the high effective normal stresses expected during the exploitation stage of pre-salt development, acid fractures could have very low conductivity values, depending on formation mechanical properties. Since perform propped fractures is more risky, it is important to create selection criteria to know precisely the fracture technique to be used. There is no agreement in the technical community regarding the survival of an acid fracture at closure stress greater than 5,000 psi. However, the experience that leads to this controversy is mainly based on: (1) wells on land and, (2) few published tests on rocks with mechanical properties compatible with those depths. This article aims to discuss Petrobras’ fracturing selection criteria development. The experimental data currently available for acid fracturing Brazilian deep carbonates will be presented. Laboratory test are performed to verify whether an acid induced fracture can withstand high effective normal stresses, keeping an acceptable conductivity. Key mechanical properties are measured in order to understand acid fracture conductivity behavior under stress. These mechanical properties are correlated with log data to allow the use of laboratory results on the fracture technique decision of a particular well. Field data regarding acid fractures wells will also be addressed.

Petrobras pre-salt discovery is a huge hydrocarbon reserve located below an extensive layer of salt at about 5000 m depth. So far, the wells drilled at this scenario found light oil in highly heterogeneous carbonates reservoirs. The formation permeability varies more than four orders of magnitude. Some fields have most of the permeable zone with values ranging from 1 to 10 mD and this scenario requires stimulation in order to maximize productivity. This article will present the first multi-fractured sub-horizontal well constructed in the pre-salt reservoir. In this stimulation operation 8 acid fractures were created in a sub-horizontal open hole well. This article will show the completion selected that permitted to register the bottomhole data above and below the zone where the fracture was being created. This record allowed the interpretation if the fractures were really created in the zones selected or if there were communication between zones. The article will present the calibration tests results, the fracturing data of all intervals and some insights regarding fracture orientation due to temperature behavior. The reservoir depth and the proximity of the salt layer bring many challenges regarding hydraulic fracturing operations. This operation and other field results have shown minimum in situ stresses in the magnitude of 0.7 psi/ft and fracture propagation pressure that exceeds the estimated overburden. The records of the bottomhole pressure during this operation showed that there were some communication between zones. After the stimulation operation, a formation test was done measuring the well productivity and collecting samples of the reservoir fluid. The samples collected were analyzed and the tracers used during the stimulation were identified, indicating which fracture was contributing to production. The data registered during this operation is a huge lesson learned about doing multi-acid fractures in a carbonate reservoir for all the industry.

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.

Well Containment Analysis – WCA - is a set of integrity analysis criteria in a shut in scenario performed during the well design phase. The installation of a capping and shut in of the well shall not induce hydrocarbon leakages to the seabed. One of the integrity analysis criteria within WCA is a situation in which the well, in a blowout scenario, is closed with the capping, and a fracture is then initiated at a specific casing shoe of the well, propagating through shallow formations. The analysis ensures that the time for fracture propagation to reach the mudline is longer than the time to drill a relief well and have the well controlled. When considering a situation of a fracture propagation in a blowout scenario after capping and controlling the well there are two main possibilities: (I) a shallow reservoir can absorb the hydrocarbon flow from a deeper reservoir, (II) the time required for the fracture propagation to reach the seabed is greater than the time needed to control the well in blowout situation, guaranteeing that there is no fluid broaching to the seabed. When considering the time for fracture propagation, a safety margin is needed due to uncertainties in the geological data. This work aims to present a few case studies where well projects didn’t attend WCA analysis considering P90 pore pressure scenario and needed further analysis to ensure attendance of well integrity. In this situation, studies were done considering fracture propagation beginning after the well closure due to a hypothetic blowout, trying to analyze if hydrocarbons would broach to the mudline, and if so, how much time such an event could occur. Study results show the impact of geology and geomechanics on fracture propagation. There are situations in which the fracture remains confined due to the presence of a shallower reservoir and others where the fracture broaches to the mudline in a short amount of time. This analysis has the potential to turn the concept of well integrity more flexible, allowing the well designer to combine, in a tentative optimal way, a safe and feasible design in more challenging scenarios.