Pre-Operational Environmental Baseline in a Tight Gas Reservoir: Hydrochemical Assessment and GeoChemical Background of Surface Water in São Francisco Basin (Brazil)

Abstract

The definition of a predominant geogenic source of ions and its respective background ranges are powerful tools for detecting contaminations coming from the unconventional gas industry at earlier stages. Reference data to compare pre- vs post-drilling may enforce the planning, monitoring, and management in favor of unconventional production sustainability in the long-term. In 2022, the Brazilian Government launched a pilot project named "Transparent well" for public consultation, intending to foment its incipient unconventional hydrocarbon industry. In this context, this study presents preliminary results to compound an environmental baseline to structure reference data about water resources in a pre-drilling scenario. Then, a characterization of surface water and a geochemical background proposal of Indaiá and Borrachudo basins, two potential tight gas plays inside the São Francisco Basin, was performed to understand hydrochemical processes and susceptibilities previously the unconventional industry development. Two sampling campaigns collecting 13 samples each were made to characterize the study area in two different scenarios. Chemical analysis was performed using an Inductively Coupled Plasma - Mass Spectrometry. To settle a hydrochemical background and its respective environmental thresholds, the ±2 Median Absolute Deviation method was applied. Piper and Chadha's diagrams were used to define water-type and the major cations/anions dynamics in the environment. Piper diagram shows three distinct predominant water-type in the study area, Group 1 (Ca-Mg-HCO3-type); Group 2(Na-K-Ca-HCO3-type); and Group 3 (Ca-Na-HCO3-type). Groups 1 and 2 reflect a geological control in the hydrochemistry due to an interaction between the surface water and interlayered strata of Carbonates and Siltstones from the Bambuí Group whereas Group 3 suggest a seasonal influences in water chemistry. Background values and Upper/Lower environmental thresholds for nineteen elements in two different seasons were also proposed for the study area. Reference data to compare pre- vs post-drilling may enforce the planning, monitoring, and management in favor of unconventional production sustainability in the long-term. Environmental baselines focused on water resources may be a key to the responsible development of the incipient unconventional gas industry in Brazil.