Investigating formation water variability in the Búzios oilfield (Santos Basin) using cluster analysis and hydrogeochemical modeling

Abstract

The characteristics of formation waters play a crucial role in production development projects within Pre-Salt oil fields in the Santos Basin, offshore southeastern Brazil. This study investigated the compositional variations of brines of the Búzios Field (northeast of the Santos Basin), proposing for the first time an extensive hydrochemical mapping of the Pre-Salt aquifers and identifying nuances of connectivity. A comprehensive methodology was employed, including analysis of pressure gradients, cluster analysis, and hydrogeochemical modeling to classify different types of brine and understand their behavior under reservoir conditions. A hydrochemical classification routine was applied to 129 formation water samples, identifying seven distinct clusters using the unsupervised k-means algorithm. This clustering process efficiently differentiated representative, non-representative, and contaminated samples. The hydrochemical composition of the formation waters varied among the four hydraulic compartments identified in the field, with their boundaries defined by permeability barriers. Compared with the central portion of the field (regional aquifer), where salinities predominantly range from 155,000 to 185,000 mg/L of NaCl, enrichment of specific ions was observed at the field's extremities, such as boron (northeast), barium and lithium (north), and bicarbonate (west). These hydrochemical differences lead the brine salinities at the field's extremities to exceed 225,000 mg/L of NaCl. Hydrogeochemical modeling provided insights into the pH levels of each brine under reservoir conditions, showing a decrease of between 23 and 32% compared to surface pH values. The speciation analysis showed positive strontianite and barite saturation indices in certain types of formation water, indicating potential precipitation. These areas are considered critical for inorganic scaling in production equipment.