Utilization of produced water baseline as a groundwater monitoring tool at a CO2-EOR site in the Permian Basin, Texas, USA

Carbon dioxide (CO₂) enhanced oil recovery (EOR) provides a pathway for economic reuse and storage of CO₂, a greenhouse gas. One challenge with this practice is ensuring CO₂ injection does not result in target reservoir fluids migrating into overlying shallow (<1000 m) groundwater formations. Effective monitoring for leakage from storage formations could involve measuring sensitive chemical indicators in overlying groundwater units and within the producing formation itself for evidence of deviation from an initial state. In this study, produced waters and overlying groundwaters were monitored over a five-year period to evaluate which geochemical signals may be useful to ensure that oilfield produced waters did not impact overlying groundwaters. During this five-year period, a mature carbonate oil reservoir in the Permian Basin transitioned from a waterflooding operation to a water-alternating-gas injection (WAG), in which the formation was flooded with CO₂ and various mixtures of produced water. Significant increases in dissolved inorganic constituents [alkalinity, TDS, Na⁺, Cl⁻, SO₄²⁻] were observed in produced waters following CO₂ injection; however, carbonate reservoir dissolution-precipitation reactions appear to be minimal and injected CO₂ appears to be stored via solubility trapping. Although there are statistically significant geochemical variations following CO₂ injection, applying isometric log-ratios to certain parameters establishes a narrow range for post-CO₂ injection produced waters. This narrow range can be considered a baseline for post-CO₂ injection produced waters; this baseline can be utilized to monitor overlying local groundwaters for produced water intrusion. Additionally, certain parameters [Na⁺, Ca²⁺, K⁺, Cl⁻, alkalinity, and TDS] display large concentration disparities between produced water and overlying groundwaters; these parameters would be sensitive indicators of produced water intrusion into overlying groundwaters.