_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 230588, “Acidizing Brazilian Presalt Carbonates Using Seawater Base-Acid System, ” by Alexandre Z. I. Pereira, SPE, Cecelia T. Azevedo, and Dayana N. Silva, SPE, Petrobras, et al. The paper has not been peer-reviewed. _ In Brazil’s presalt 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 traditionally has been avoided because of concerns about calcium sulfate (CaSO4) precipitation and scaling, this study reviews existing literature, the operator’s records, service companies’ data, and simulation studies conducted by Petrobras’ R&D Center (CENPES) to assess the true risk of using seawater in carbonate acidizing. The replacement of freshwater with seawater resulted in logistical, operational, and environmental benefits, as well as enhanced availability of stimulation vessels. Background To assess the use of seawater in acidizing fluids, data were collected from published papers, theses, and scaling simulations conducted by CENPES using commercial software. In the literature, a relationship was established between damage susceptibility, sample permeability, and injection rate in laboratory experiments, demonstrating that higher permeability and flow rates can mitigate the effects of sulfate precipitation. The necessity of using scale inhibitors in seawater-based acid systems for carbonate acidizing also has been indicated. However, from a field-experience perspective, southeastern Brazilian carbonate formations are regarded as highly tolerant to contaminants. When these formations are acidized, they do not show sensitivity to minor precipitation or chemical incompatibility. Considering these observations, this study took a distinct analytical approach to reviewing the literature on the effects of seawater in carbonate acidizing, focusing on the differences between experimental conditions and field observations. A hypothesis exists that negative laboratory results observed in seawater experiments might stem from testing conditions that did not accurately reflect field behavior. Two primary concerns exist regarding the use of seawater in carbonate acidizing: Formation damage caused by CaSO4 precipitation within the formation matrix, and scaling caused by spent acid that is saturated with CaSO4. Since Petrobras had no previous experience with seawater in carbonate acidizing, operators and service companies were consulted to gather insights from operations using this approach. Additionally, numerical simulations performed by CENPES using Scale X software revealed two important findings: low mass precipitation and a low tendency for scaling within the temperature range of most presalt formations. Effect of Dissolved Sulfate The sulfate concentration in seawater typically ranges from 2, 500 to 3, 500 mg/L, a key reason for using freshwater in the preparation of stimulation fluids. The calcium released from the acid reaction with carbonates precipitates as CaSO4. CaSO4 has low solubility, and its precipitation is typically nearly complete. Without application of sophisticated methods, the amount of precipitated material remains relatively small (3. 54 kg/m3 of acid), and the residual sulfate in solution is negligible (0. 7 mg/L), suggesting that the spent acid is unlikely to cause significant scaling problems.
Chris Carpenter (Mon,) studied this question.