Pre and post salt carbonate reservoirs are becoming the main source of hydrocarbon production in Brazil. One of the reasons behind this success is the development of many fields located on pre salt areas along the Brazilian shore. Production optimization is vital for sustainability, which requires efficient methods and technology to stimulate these reservoirs. The main cost-effective approach used in Brazil to improve the production in carbonate reservoir areas is matrix acidizing. Stimulation fluids are pumped into the formation below the fracture pressure to create wormholes that can bypass near wellbore damage and increase the drainage radius along the wellbore. This study offers an insight of the most common methods and technology used to perform matrix acidizing on offshore carbonate reservoirs in Brazil. The main aspects of the study are fluids selection, pumping strategies, completion design and pressure behavior analysis. Before 2012, most of the offshore stimulation operations in Brazil were performed on sandstone reservoirs. With the advent of pre salt development this scenario changes drastically leading to a high volume of matrix acidizing on carbonate reservoirs in both post and pre salt zones. The main challenges to designing and executing matrix acidizing on carbonate reservoirs in Brazil are the long reservoir intervals combined with high permeability contrasts, high formation temperature and the need of large fluid capacity to accommodate the required treatment designs. Additionally, super duplex stainless steel metallurgies must be considered for corrosion inhibition protection. To overcome these challenges: first, a proper fluid diversion technique must be employed to distribute the treatment fluid across all targeted zones; next, a proper fluid selection must be made to avoid carbonate face dissolution; and finally, corrosion protection is required for the well bore completion components from the corrosion rates increased by the high temperatures downhole. The lessons learned and best practices captured over the past decade on more than 250 offshore carbonate matrix acidizing operations has led to much success in the post and pre salt zones of Brazil.

Lapa is a pre-salt deep-water field located around 270km off the coast of São Paulo, Brazil at Santos basin. This carbonate reservoir lies in water depths of around 2,100m and can produce good quality light 26° API oil. The stimulation in large carbonate reservoirs is very challenging, and techniques used for Lapa were based on chemical divergence. The development in offshore environments requires proper planning, execution, and monitoring to achieve the desired results and, of course, profitability. The matrix acidizing method was chosen to stimulate all wells of this campaign (2 producers and 2 injectors). This method consists of bypassing formation damage and stimulating the reservoir by creating wormholes via chemical pumping. In the design phase, stimulation operations previously performed at this field were reviewed, analyzed, and optimized. The main changes were regarding the completion strategy without the use of coiled tubing and placement during the completion phase as it could optimize the time and the cost for the project. The volumetric rate (gal/ft) was also reduced and the selection of the main fluid changed after several laboratory analysis and software simulations. The Lapa field requires high fluid volumes due to the length of the intended treatment interval. The assembly of a stimulation plant on a supply vessel from operator fleet (multi-purpose FSV – field support vessel) was the most cost-efficient approach to address the high volumes required as there was no Well Stimulation Vessel (WSV) available "on call" in the Brazilian offshore market at that time. This solution could also optimize the vessel fleet while the vessel was not required for pumping as FSV was also equipped with ROV and was mean to carry subsea planned task. The fluid test strategy was also a key point for this successful project as many tests were performed to make sure that the correct fluid system was selected. During this process, several fluid systems and different formulations were submitted for core flow tests and dual core flow tests to evaluate worm holing efficiency of retarded fluids and diversion performance of Chemical diverters. Compatibility tests were also performed, and a mud cake breaker was developed locally, especially for this project. This paper will bring an overview of all aspects regarding Lapa stimulation project since the conception, fluid system selection, laboratory tests, lessons learned and the potentially future strategy for this field.