Acidizing Brazilian Pre -Salt Carbonates Using Seawater Base Acid System

Abstract In Brazil's pre-salt fields, the demand for large volumes of freshwater for acidizing operations, coupled with logistical constraints, has led to the adoption of seawater in the formulation of stimulation fluids. Although seawater has traditionally been avoided due to concerns about calcium sulfate precipitation and scaling, this study reviews existing literature, Petrobras' operational records (which include 625 stimulations using freshwater and 34 with seawater), service companies' data, and simulation studies conducted by Petrobras' R&D Center (CENPES) to assess the real risk of using seawater in carbonate acidizing. Laboratory tests carried out with service companies indicated no significant differences in performance between formulations with and without seawater under conditions of flow rate (Q > 1 cm³/min) and permeability (k > 10 mD). Simulation and field results suggest that there is no risk of formation damage when permeability values exceed 10 mD and flow capacity surpasses 2,000 mD-m. Additionally, no scaling deposition was observed after carbonate acidizing with seawater. The replacement of freshwater with seawater resulted in logistical benefits (annual savings of 2.31 million gallons of freshwater), operational benefits (a cost reduction of US$1.2 million per year), and environmental benefits (a yearly decrease of 178 tons of CO₂ emissions), as well as enhanced availability of stimulation vessels.