GEOCHEMICAL VARIATION OF PRODUCED WATERS AND OVERLYING GROUNDWATERS AT AN ACTIVE CO2 ENHANCED OIL RECOVERY FIELD

The geochemical variability of produced waters at an active CO₂ EOR field can provide insight into how injected CO₂ affects dissolution and precipitation reactions within a formation. Furthermore, water sampling at such a field explores the effectiveness of using geochemical groundwater signals for monitoring, verification, and assurance of CO₂ storage.

Produced water brines and overlying fresh groundwater samples were taken from an oil field in the Permian Basin’s Central Platform. Six sampling events occurred during a 3.5 year period from June 2013 to November 2016, bracketing the onset of carbon dioxide injection. During each sampling event, produced waters were sampled from the oil bearing Permian Upper San Andres Fm., a dolomite formation that lies ~1675-1735 m below the surface. Overlying groundwaters were also sampled from the Triassic Santa Rosa Fm. (~455 m), which hosts a non-oil bearing brine, and the Miocene Ogallala Fm. (~45-55 m), a ubiquitously used freshwater aquifer. Produced waters and groundwaters were measured for field parameters (pH, TDS, and alkalinity) and for major geochemical analytes (Ca, K, Mg, Na, SO₄ and Cl).

Produced waters displayed significant increases for alkalinity, Ca, Mg, K and Na values relative to the pre-CO₂ injection sampling event. Additionally, field and geochemical measurements suggest that the Upper San Andres Fm. produced waters at this field are distinct from overlying groundwaters; specifically, produced waters have significantly higher values for certain parameters (TDS, alkalinity, Ca, K, Na and Cl). Thus, these parameters could be used to detect leakage of Upper San Andres. Fm. produced waters into overlying groundwaters. Overall, preliminary results suggest that (1) CO₂ injection had an effect on produced water geochemistry, (2) produced water geochemistry is distinct from overlying groundwaters and certain parameters would be sensitive indicators of produced water intrusion into groundwater, and (3) based on measured parameters, no intrusion of produced water into overlying groundwaters was observed.