Investigating the effect of enhanced oil recovery on the noble gas signature of casing gases and produced waters from selected California oil fields

In regions where water resources are scarce and in high demand, it is important to safeguard against contamination of groundwater aquifers by oil-field fluids (water, gas, oil). this context, the geochemical characterisation these critical so that anthropogenic contaminants can be readily identified. The first step characterising pre-development fluid signatures (i.e., those unmodified hydrocarbon resource development) understanding how may have been perturbed production, particularly context enhanced oil recovery (EOR) techniques. Here, we present noble gas isotope data produced from wells several water-stressed California, USA, EOR prevalent. systems, only casing gases typically collected measured for their compositions, even when and/or phases present, due relative ease analyses. However, approach relies on a number assumptions (e.g., equilibrium between phases, water-to-oil ratio (WOR) gas-to-oil (GOR) order reconstruct multiphase subsurface compositions. adopt novel, more rigorous approach, measure both (oil-water-gas mixtures) samples Lost Hills, Fruitvale, North South Belridge (San Joaquin Basin, SJB) Orcutt (Santa Maria Basin) Oil Fields. Using method, able fully characterise distribution within system. We find concentrations agree with phase thus two sample types used essentially interchangeably. identified distinct air-derived elemental ratios 20Ne/36Ar), which elevated compared fluids, conspicuously trend towards air values respect overall concentrations. reservoir 20Ne/36Ar using show strongly correlated (r2 = 0.88–0.98) amount injected ~500 m well. suggest increase resulting injection sensitive volume interacting injectate, effective ratio, composition injectate. Defining injection-modified compositions crucial distinguishing sources hydrocarbons observed proximal groundwaters, quantifying transport mechanisms controlling occurrence.